Foto personale

Daniele BRUNELLI

Department of Chemical and Geological Sciences

Lugli, Federico; Brunelli, Daniele; Cipriani, Anna; Bosi, Giovanna ( 2017 ) - C4-plant foraging in Northern Italy: stable isotopes, Sr/Ca and Ba/Ca data of human osteological samples from Roccapelago (16th–18th century AD) - ARCHAEOMETRY - pp. da 1119 a 1134 ISSN: 0003-813X [Articolo in rivista (262) - Articolo su rivista]
Abstract

Human osteological samples (n = 23) taken from different anatomical parts of 11 individuals from the early modern (16th–18th century AD) site of Roccapelago (Modena, Italy) were systematically analysed for δ13C, δ15N and trace elements to investigate their diet. δ13C and δ15N correlate and show a high variability between individuals, attesting to the dietary contribution of C4 plants. This is supported by pollen analysis of the burial site samples, which revealed the presence of maize. δ15N correlates with Sr/Ca, suggesting that the main protein source could have been milk and dairy. We therefore interpret the strong correlation between δ13C and δ15N as evidence for C4-plant foraging practice and the exploitation of livestock for meat and milk, combined with possible direct intake of C4 plants. The Roccapelago site represents an important case study to track the evolution of the post-medieval diet and the introduction of maize cultivation in southern Europe, as also attested by historical sources.

Lugli, Federico; Cipriani, Anna; Peretto, Carlo; Mazzucchelli, Maurizio; Brunelli, Daniele ( 2017 ) - In situ high spatial resolution 87Sr/86Sr ratio determination of two Middle Pleistocene (c.a. 580 ka) Stephanorhinus hundsheimensis teeth by LA–MC–ICP–MS - INTERNATIONAL JOURNAL OF MASS SPECTROMETRY - n. volume 412 - pp. da 38 a 48 ISSN: 1387-3806 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Bone and tooth tissues are important biological archives to study eating habits and provenance of ancient humans and animals. By taking advantage of the high spatial resolution offered by the Laser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry (LA–MC–ICPMS) technique, we investigated the 87Sr/86Sr intra-tooth variability of two Stephanorhinus hundsheimensis tooth fragments from the Middle Pleistocene site of Isernia La Pineta. We detected significant Sr isotopes variability within the rhinos tooth enamel (enamel average ±2 sigma: sample RH–IS30 0.70951 ± 0.00014; sample RH–IS260.70976 ± 0.00015) with values higher than the “local”87Sr/86Sr ratio (dentine average ±2 sigma: sample RH–IS30 0.70918 ± 0.00013; sample RH–IS26 0.70934 ± 0.00009). This is likely linked to a different water and food intake with a Sr isotopic signature similar to volcanic soils nearby (Roccamonfina: from 0.7093 to 0.7100; Colli Albani: >0.7100) and supports the idea that the Stephanorhinus hundsheimensis species moved around seasonally. The improvement of non-destructive, accurate and precise analytical methods to decrypt the information hidden within bone and tooth hard tissues of archeological material is crucial to unravel critical questions about evolution, migration and ecology of human and animals. We have successfully took upon this challenge using three matrix-matched reference materials, with variable Sr concentration (c.a. 100–1000 ppm), to correct unresolved interferences arising from LA analyses.

Cipriani, Anna; Brunelli, Daniele ( 2017 ) - Mantle-Crust Isotopic Relationships along Mid Ocean Ridges: Constraints from the Analysis of Time Series - Goldschmidt Abstracts [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

Isotopic relationships between parent mantle and daughter MORBs have been used to reveal the composite nature of the source tracking the missing components in their isotopic fingerprints. An opportunity to address this issue is given by temporal sections of single ridge segments where MORB and residual parent mantle peridotites can be assessed together. The possibility has been offered by the flexured and uplifted lithospheric slab that exposes, on the sea floor along a seafloor spreading flow line, a zero to 26 Ma relatively undisturbed lithospheric section (Vema Lithospheric Section or VLS) generated at the 80 km long Mid Atlantic ridge segment (EMAR) at 11°N. Temporal variations of the Nd isotopic composition of crustal basalts and parental mantle along the VLS reveal a large dispersion of residual isotopic composition with respect to the melt products. Equilibration with partially mixed melts can account for the observed relationships in the mantle rocks. The mean MORB isotopic composition and the average composition of the residues do not match because of the preferential extraction of the low-melting component. The compositional difference in both isotopic and elemental distribution is a function of the average degree of melting of the mantle. This observation can only be justified by progressive melting of composite lithologies where mantle potential temperature and amount of dispersed low-melting lithologies control the relative extent of melting of the mantle host and that of the dispersed heterogeneities resulting in differential fractions of mixed melts in the final products. This observation is confirmed by global correlations in Nd isotopes and chemical indicators of degree of melting from other portions of the mid ocean ridge system.

Brunelli, Daniele; Cipriani, Anna ( 2017 ) - Pyroxenites sow discord between parent mantle and daughter MORB - Goldschmidt Abstracts [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

Mantle rocks and MORBs sampled along the 26 Ma record of the Vema Lithospheric Section or VLS (Mid Atlantic 11°N) reveal a profound divergence between parent and daughter rocks in their compositional evolution. Finding the cause of this discordance requires comparing the evolution through time of the extent of melting with the produced crustal thickness and changes in the chemistry and isotopic composition of the sampled rocks. Mantle rocks show a progressive increase of their degree of melting during time, as if the potential temperature had increased in the last 26 Ma. However, plate kinematics reveal a significant decrease of the spreading rate that should sensibly lower the average degree of melting. MORB inferred degree of melting decreases through time while their isotopic fingerprint becomes more depleted. The observed variations can be reconciled by considering that a variable amount of lowmelting lithologies entered the melting region. The observed decoupling of the degree of melting of mantle and MORB is attributed to the effect of undercooling exerted by lowmelting heterogeneities due to heat diffusion before the host mantle starts melting itself. The observed temporal evolution is matched by a decreasing amount of low-melting, isotopically enriched, lithologies (pyroxenites) dispersed in a DMM-type host mantle. Approaching the present day setting, the amount of pyroxenites has become negligible restoring harmony between parent mantle and MORB daughters in terms of degree of melting and integrated melt production. This observation can be extended to the entire MOR system revealing the sensitivity of the spreading system to the amount of low-melting lithologies dispersed in the depleted mantle host. At limit conditions of the mantle potential temperature, heat diffusion into the low-solidus melting lithology prevents the host mantle from reaching its solidus.

Di Renzoni, Andrea; Ayala, Gianna; Brunelli, Daniele; Levi, Sara Tiziana; Lugli, Stefano; Photos-Jones, Effie; Renzulli, Alberto; Santi, Patrizia ( 2016 ) - Aiding and abetting the archaeological enquiry: geochemical work-in-progress at the site of San Vincenzo, Stromboli, Aeolian Islands, Italy - Proceedings of the 6th Symposium of the Hellenic Society for Archaeometry - British Archaeological Reports Oxford GBR) - n. volume International Series 2780 - pp. da 167 a 174 ISBN: 9781407314303 ISSN: - [Contributo in volume (Capitolo o Saggio) (268) - Capitolo/Saggio]
Abstract

This paper focuses on the site of San Vincenzo, Stromboli, Italy, and the use of the portable X-Ray Fluorescence analyser (p-XRF) in the field, as a fast and efficient means of geochemical data collection and processing, without the need to remove a sample. The purpose of the exercise is to aid the archaeological enquiry and to attempt to tie archaeological deposits and their chronology with the natural bedrock (i. e. scoriae and lapilli). We conclude that throughout the Bronze Age phase of the settlement the chemical make-up of the archaeological deposits is drawn largely from the lapilli-rich deposits which were formed after the end of the Neostromboli period, punctuated with those drawn from the scoriaceous lava that preceded the lapilli phase at the end of the same period. On the other hand, the post-BA deposits are geochemically different, pointing to new eruptive events. Our on-going work aims to systematically assess and compare the information that derives from each of the different disciplines involved – archaeology, geology, geochemistry and volcanology – in an attempt to reveal site formation processes and the anthropogenic activities within.

Paquet, Marine; Cannat, Mathilde; Brunelli, Daniele; Hamelin, Cedric; Humler, Eric ( 2016 ) - Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61 to 67°E) - GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS - n. volume 2016 - pp. da 1 a 55 ISSN: 1525-2027 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The easternmost part of the Southwest Indian Ridge (61°-67°E) is an end-member of the global ridge system in terms of very low magma supply. As such, it is a good laboratory to investigate the effect of melt/mantle interactions on the composition of erupted basalts: for a given volume of erupted basaltic melt, the volume of reacted mantle is potentially greater than at more magmatically robust ridges. We analyzed major, trace element and isotopic compositions in three groups of rocks: plagioclase-bearing ultramafic and gabbroic rocks dredged in nearly amagmatic spreading corridors; basalts from the sparse volcanic cover of these corridors (“ultramafic seafloor basalts”); and basalts dredged from the intervening, more volcanically active domains (“volcanic seafloor basalts”). Ultramafic seafloor basalts have significantly lower CaO and Al2O3 contents at a given MgO than most volcanic seafloor basalts. We propose that both types of basalts are derived from similar parental melts, but that the ultramafic seafloor basalts are more affected by reactions between these parent melts and the mantle rocks in the lithosphere below the ridge. We infer that these reactions occur in the walls of conduits that allow the aggregated melts extracted from the melting mantle to rise through the axial lithosphere and to the eruption sites. The principal effect of these reactions is to enrich the asthenospheric melts in MgO through olivine dissolution. This effect is not expected to be as noticeable, but could still play a role in basalt petrogenesis at more magmatic regions of the global slow-spreading MOR system.

Maia, Marcia; Sichel, Susanna; Briais, Anne; Brunelli, Daniele; Ligi, Marco; Ferreira, Nicolas; Campos, Thomas; Mougel, Bérengère; Brehme, Isa; Hémond, Christophe; Motoki, Akihisa; Moura, Denise; Scalabrin, Carla; Pessanha, Ivo; Alves, Eliane; Ayres, Arthur; Oliveira, Pedro ( 2016 ) - Extreme mantle uplift and exhumation along a transpressive transform fault - NATURE GEOSCIENCE - n. volume 9 - pp. da 619 a 623 ISSN: 1752-0894 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Mantle exhumation at slow-spreading ridges is favoured by extensional tectonics through low-angle detachment faults, and, along transforms, by transtension due to changes in ridge/transform geometry. Less common, exhumation by compressive stresses has been proposed for the large-offset transforms of the equatorial Atlantic. Here we show, using high-resolution bathymetry, seismic and gravity data, that the northern transform fault of the St Paul system has been controlled by compressive deformation since ∼10 million years ago. The long-lived transpression resulted from ridge overlap due to the propagation of the northern Mid-Atlantic Ridge segment into the transform domain, which induced the migration and segmentation of the transform fault creating restraining stepovers. An anticlockwise change in plate motion at ∼11 million years ago initially favoured extension in the left-stepping transform, triggering the formation of a transverse ridge, later uplifted through transpression, forming the St Peter and St Paul islets. Enhanced melt supply at the ridge axis due to the nearby Sierra Leone thermo chemical anomaly is responsible for the robust response of the northern Mid-Atlantic Ridge segment to the kinematic change. The long-lived process at the origin of the compressive stresses is directly linked to the nature of the underlying mantle and not to a change in the far-field stress regime.

Lugli, F.; Cipriani, A.; Mazzucchelli, M.; Sforna, M.C.; Brunelli, D. ( 2016 ) - In situ 87Sr/86Sr LA-MC-ICPMS on biogenic apatites: a matrix-matched standard correction approach - Goldschmidt Conference Abstracts - pp. da 1904 a 1904 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

Strontium isotope ratios are a strong tool to study ancient hominin and animal migrations, hence the increasing need to have a simple, fast and microdestructive analytical technique to obtain accurate and precise 87Sr/86Sr ratios of precious tooth enamel and bone tissue. The traditional analysis by the TIMS or MC-ICPMS tecniques requires sample dissolution; therefore, several LA-MC-ICPMS methods have been developed to prevent sample destruction, particularly for prehistoric human teeth. Instrumental calibration on human enamel is difficult because of the typical low-Sr concentration and analytical interferences. In fact, the methodology for data reduction of in situ Sr isotopes of biogenic apatite is largely debated in the literature [e.g. 1, 2]. While monoatomic interferences (Kr, Rb, REE2+) are routinely corrected, the correction of polyatomic interferences (CaCa, CaAr and 40Ca31P16O) are challenging. In particular, the CaPO molecule strongly interferes on mass 87, hindering the achievement of precise and accurate 87Sr/86Sr ratios. Following on the work of Horstwood et al. (2008), we developed a method based on the concurrent analyses of multiple matrix-matched standard materials. We show how the linear regression of 87Sr/86Sr accuracy vs. 1/88Sr of at least three standards allows correction of this interference. During each analytical session, we analyse our four in-house matrix-matched standards (a human tooth, a bovine tooth, a swine tooth and a shark tooth) covering a wide range of Sr concentrations (from c.a. 100 ppm of the human tooth to the 1000 ppm of the shark tooth). A daily CaPO model is then built to predict the expected accuracy of the analysis. This correction gives an external reproducibility to the 4th decimal digit (e.g. 2σ-human enamel = 0.00047; c.a. 100 ppm) and an accuracy between the 4th and the 5th decimal digit when applied to analyses with a laser spot sizes of 100μm and a linear dynamic ablation pattern. Monitoring of the CaPO molecule formation during analysis is also achieved by performing several high resolution mass scans.

Jones, Richard; Brunelli, Daniele; Cannavò, Valentina; Levi, Sara Tiziana; Vidale, Massimo ( 2016 ) - The Riace bronzes: recent work on the clay cores - Proceedings of the 6th Symposium of the Hellenic Society for Archaeometry - British Archaeological Reports Oxford GBR) - n. volume 2780 - pp. da 21 a 27 ISBN: 9781407314303 ISSN: 0143-3067 [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

New samples of clay cores from the two Riace bronze statues have been analysed chemically, petrographically and by SEM to shed light on their origins. Sources in or around Corinth and Athens are excluded; the Argolid in the Peloponnese remains a possibility, and the Megarid should be considered further on geological grounds.

Lugli, Federico; Cipriani, Anna; Reghizzi, Matteo; Brunelli, Daniele; Benazzi, Stefano ( 2015 ) - In situ strontium isotope analysis on biogenic apatite: the use of Laser Ablation and Multi Collector Inductively Coupled Plasma Mass Spectrometer (LA–MC–ICPMS) in anthropological research - XXI Congress of the Italian Anthropological Association [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

The use of LA–MC–ICPMS in anthropological research is an innovative approach for measuring strontium isotopic ratios of human enamel. This technique, due to its micro-destructivity, allows to exam 87Sr/86Sr on precious human remains, without the drawbacks of the dissolution method. Despite this advantage, the laser ablation technique (LA) is not without flaws, principally related to unavoidable analytical interferences. For these reasons, data obtained with the laser are usually less precise and accurate than data obtained with the classical dissolution method. In particular, problematic interferences are represented by 86Kr formed in the gas, by double-charged REE (Rare Earth Element), by 87Rb, and by other polyatomic isobaric interferences (i.e. 44Ca40Ar, 40Ca44Ca, 40Ca13P16O). In this work, we measured the Sr isotopic ratio of a shark tooth with both dissolution and LA method, to test the precision of the LA method. A shark tooth is a bio-apatite formed in a marine environment, thus its 87Sr/86Sr ratio reflects the modern marine ratio of ~0.7092. Our preliminary laser analyses show that we are able to reproduce the isotopic ratio of our shark tooth obtained by Sr chromatographic separation and HR–MC–ICPMS with a precision to the fourth decimal place and that this ratio reflects the modern sea water value. If confirmed by further studies, our preliminary results suggest that the LA technique is a reliable method to explore hominin movement and migrations.

Federico, Lugli; Anna, Cipriani; Daniele, Brunelli; Giorgio, Gruppioni; Maurizio, Mazzucchelli ( 2015 ) - In situ trace element analysis of human hard tissues by Laser Ablation ICP–MS - TRACE ELEMENTS BETWEEN DEFICIENCY AND TOXICITY: UPDATE AND PERSPECTIVES - pp. 39 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

After the advent of the laser ablation system (LA), the analysis of micro-samples of hard materials has spread to many scientific fields (i.e. geology, engineering, archaeometry, anthropology). The Laser Ablation Coupled with a Mass Spectrometer (as in the LA–ICP–MS) allows to investigate any hard material at the scale of a few µms, for example, defining the elemental compositional profile of bone, teeth and nephroliths, measuring any element with an atomic mass between 2 and 255 amu (i.e. heavy metals as Pb, Cr, Cd, As, etc.). Moreover, one of the main advantages offered by LA, compared to more traditional techniques, is the drastic reduction of analytical time, because the sample is analysed as it is without chemical dissolution or separation of elements. In this work, we have tested and developed a routine to measure all trace elements in human bones and teeth with the LA–ICPMS housed at the University of Modena and Reggio Emilia. We analyse all trace element but we have focused here our attention on Zn and Pb, given their importance and significance in dentistry, human health and forensic pathology studies. We have then analyzed two human teeth, measuring their trace elements compositional profile and drawing conclusions on the health history of these two individuals. Materials and Methods. We analysed two human teeth (molar 1HT–R and premolar 2HT–R) sampled from two post-medieval mummies (Roccapelago, Modena, 16th – 18th cent.). The concentrations of Pb and Zn were measured in situ through a 213 nm Nd:YAG Laser Ablation system coupled to a quadrupole ICP–MS system (Thermo Fisher Scientific® X SeriesII). Before analysis, each sample was cut in half with a diamond low-speed wheel saw, to expose the inner parts of the tooth. Eighteen LA–spots (diameter 100 µm) were analysed along two profiles (n=10 sample 1HT–R; n=8 sample 2HT–R), exploring enamel and dentine. The elemental concentrations of our samples have been corrected through a calibration with NIST 1486 Bone Meal as a multi-element standard. To correct any matrix-related effect, the calcium concentration, measured with an ESEM Quanta 200, has been used as the internal standard. To ensure the accuracy of the method, a two-tailed t-test was performed on two certified elements of the standard. Statistical analysis shows no significant difference between expected and measured values (p<0.05; mean of 3 analyses). Results. The first tooth (1HT–R) shows a Zn concentration ranging from 230 in the outer enamel to 110 ppm in the inner primary dentine (x = 150 ± 38 ppm, 1σ) and, respectively, a Pb concentration ranging from 3 to 1 ppm (x = 2 ± 1 ppm, 1σ). From the outer enamel to the inner dentine, the second tooth (2HT–R) shows a Zn concentration ranging from 301 to 123 ppm (x = 177 ± 66 ppm, 1σ) and a Pb concentration ranging from 6 to 2 ppm (x = 3 ± 1 ppm, 1σ). Conclusions. Our results are in agreement with Pb and Zn profiles described in literature for human teeth (Humphrey et al. 2008). The concentrations of both elements tend to increase from the enamel-dentine junction (EDJ) to the enamel external surface and tend to decrease from the EDJ to the inner dentine. The Pb concentration of both samples is on average lower than the threshold of poisoning reported for human bones (5 ppm; Hess et al. 2013). The Zn concentrations are low, in particular in the outer enamel. As reported by Lynch (2011), this could be related to tooth wear or to the old age of the two individuals (~ 40 yr). The LA–ICP–MS is a non-destructive in–situ analytical technique, fundamental in forensic pathology and human health studies to measure the elemental composition of human hard tissue and in particular, in outlining the history of the tooth by trace element compositional profile from the inner to the outer portions of the tooth.

Zanetti, Alberto; Bertotto, Gustavo W.; Mazzucchelli, Maurizio; Ponce, Alexis D.; Giovanardi, Tommaso; Brunelli, Daniele; Aragón, Eugenio; Bernardi, Mauro I.; Hémond, Christophe ( 2015 ) - Multiple refertilisation of a strongly refractory mantle column in the extra-Andean back-arc (Paso de Indios, Argentina) - MINERALOGIA - n. volume 43 - pp. da 103 a 104 ISSN: 1899-8291 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

In the central part of the Chubut province, close to the village of Paso de Indios, there are several outcrops of Cenozoic basalts carrying spinel-facies mantle xenoliths. In this area, located in the extra-Andean back-arc, basaltic necks and dikes outcrop between 43° 36′ – 43° 50′ S and 68° 53′ – 69° 02′ W, along with remnants of lava flows, being divided in two groups of Paleocene and Eocene age. This volcanism was generated by extensional tectonic related to an episode of transform plate margin that affected the southern sector of South America western margin from the Paleocene to the Oligocene, as the Aluk plate detached and a slab window opened beneath the study area. In this contribution, the petrochemical processes experienced by mantle xenoliths hosted in Eocene basalts belonging to the Matilde lava flow, the Leon volcano and the Chenque dike, are presented and discussed. The studied samples are mainly spinel-facies harzburgites and clinopyroxene(Cpx)-poor lherzolites, with some dunites. The Chenque xenoliths mainly display porphyroclastic to equigranular texture, whereas those from Matilde and Leon volcanoes have coarse-granular to porphyroclastic textures. Estimated equilibrium temperatures based on pyroxenes solvus range from 800 to 940°C. The overall refractory character of the mineral assemblages is matched by the major element mineral compositions, which are mostly Al-poor and Mg-and-Cr-rich. Spinel composition is consistent with melt extraction from 8 to 14% for Chenque and Leon samples, and from 14 to 18% for the Matilde ones. The estimated degree of partial melting rises up to 24% considering the literature spinel data. However, the occurrence of melt related open-system processes is suggested by local trends of positive correlation between Na and Cr# in Cpx, being fully confirmed by the trace element compositions. In particular, the Matilde harzburgites ubiquitously show Cpx with transient U-shaped REE patterns. The LREE fractionation is very strong, with LaN up to 100 and REE patterns minimum in the M-HREE region between 0.1-1 xCI. The HREE level content (LuN down to 1 xCI) is consistent with 20-23% fractional melting of spinel DM. V-to-U-shaped REE patterns are also shown by Chenque lherzolites and harzburgites. Their M/HREE are significantly more fractionated than that expected in residue after spinel-facies basalt removal, thus suggesting an onset of the partial melting process at garnet facies conditions. Other Chenque lherzolites experienced a more pronounced refertilisation process led by LREE-enriched to LREE-depleted melts. The latter gave rise to peculiar, transient LREE-depleted sinusoidal patterns in Cpxs through reaction with the depleted ambient peridotite. A refertilised geochemical composition is also shown by the Leon samples, with harzburgite Cpxs resulting enriched in highly-incompatible elements such as U, Th, Sr and LREE. The data presented in this study, in combination with those from the literature, allow us to conclude that the shallow mantle column beneath Paso de Indios was strongly refractory in origin, being successively affected by multiple events of melt migration. These letter, however, were able to produce only an incomplete refertilisation of the depleted protoliths, which still record geochemical gradients developed during the interaction with both LREEenriched and LREE-depleted migrating melts. These petrochemical features make the Paso de Indios mantle column a unique study case in the Patagonian region, where the composition of the shallow mantle is usually completely overprinted by multiple stages of melt/fluid migration.

Ponce, Alexis D.; Bertotto, Gustavo W.; Zanetti, Alberto; Brunelli, Daniele; Giovanardi, Tommaso; Aragón, Eugenio; Bernardi, Mauro I.; Hémond, Christophe; Mazzucchelli, Maurizio ( 2015 ) - Short-scale variability of the SCLM beneath the extra-Andean back-arc (Paso de Indios, Argentina): Evidence from spinel-facies mantle xenoliths - OPEN GEOSCIENCES - n. volume 7 - pp. da 362 a 385 ISSN: 2391-5447 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Matilde, León and Chenque hills are some of the several occurrences of Cenozoic basalts carrying ultramafic xenoliths in Paso de Indios region, Argentina. The mantle xenoliths from the Chenque and León hills mainly present porphyroclastic texture, whereas the Matilde hill ones have coarse-grained to porphyroclastic textures. The equilibrium temperatures are in the range of 780 to 940°C, indicating a provenance from shallow sectors of the lithospheric mantle column that was subjected to a relatively low heat flux at Cenozoic Era. According to the modal compositions, the mantle columns beneath Matilde and León hills mostly record partial meting events larger than 22%, while less depleted peridotites occur in the Chenque suite (starting from 10% partial melting). Such an observation is confirmed by the partial melting estimates based on Cr#Sp, which vary from 8 to 14% for the selected Chenque samples and from 14 to 18% for the Matilde ones. The common melting trend is overlapped by short scale cross-cutting local trends. Local trends can be generated by open-system processes, such as open-system partial melting and/or post partial-melting metasomatic migration of exotic Na-Cr-rich melts. Petrographic survey evidences the occurrence of two main mineralogical reaction schemes due to channelled and/or pervasive melt extraction/migration. These are: i) pyroxenes dissolution and segregation of new olivine in olivine-rich peridotites, and ii) replacement of primary olivine by orthopyroxene±clinopyroxene in orthopyroxene-rich peridotites. Enhanced pyroxene dissolution is attributed to channelling of silica-undersaturated melts, whereas replacement of primary olivine by orthopyroxene±clinopyroxene points to reaction with silica-saturated melts. Late disequilibrium reactions identified in the xenoliths comprise: the breakdown of orthopyroxene in contact with the host basalt and (rarely) reaction coronae on orthopyroxene, clinopyroxene and spinel linked to glassy veins. Such features are apparently related to the injection of melt, likely during entrainment into the host basalts and ascent to the surface.

Lugli, F.; Brunelli, D.; Cipriani, A.; Bosi, G.; Gruppioni, G. ( 2015 ) - Stable isotope and in situ trace element analyses on human bone tissue (Roccapelago, 16th-18th cent.): preliminary inferences on diachronic change in eating habits and trace element reliability - XXI Congresso AAI (Associazione Antropologica Italiana) - Università di Bologna Bologna ITA) - pp. da 107 a 107 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

We measured trace elements and stable isotopes (C and N on collagen) on 19 samples, taken from different anatomical area of 7 individuals from the medieval site of Roccapelago (Modena): our attempt is to reconstruct the diet of these individuals and discuss the role of trace elements in palaeodiet. The good preservation of these bodies represents a unique chance to work on likely diagenesis-free archaeological human remains, to test the trustworthiness of trace element analyses. In fact, in the last decades, trace element reliability was questioned several times. We particularly focus our attention on magnesium, strontium and zinc. Our data set shows different eating habits for the studied individuals. While collagen of the samples dated at 16th century yields typical values of a C3-plant based diet, collagen of samples dated at 18th cent yields higher values of both δ13C and δ15N, probably linked to an higher trophic level and to a shift toward a C4-environment. Similarly, Sr and Mg show the same increment in individuals of 18th cent, while Zn does not show any significant variation. The latter is quite discussed in the scientific literature and has been commonly used as a marker for a protein-rich diet. Our lack of correlation between Zn and stable isotopes (especially N) confirms the uselessness of this element in palaeodiet reconstruction. Geochemical data are also corroborated by the botanical evidence of maize presence (C4 plant) and by historical sources that attest the presence of maize in Emilia-Romagna during 18th century.

Levi, Sara Tiziana; Cannavò, Valentina; Brunelli, Daniele; Di Renzoni, Andrea ( 2015 ) - Wikipottery [Banca dati (295) - Banca dati]
Abstract

Archaeometric and archaeological catalogue and classification of Central Mediterranean Prehistoric Pottery

Levi, S.T.; Ayala, G.; Bettelli, M.; Brunelli, D.; Cannavò, V.; Di Renzoni, A.; Ferranti, F.; Lugli, S.; Martinelli, M.C.; Mercuri, A.M.; Photos-Jones, E.; Renzulli, A.; Santi, P.; Speranza, F ( 2014 ) - Archaeological and volcanological investigation at Stromboli, Aeolian Islands, Italy - ANTIQUITY - n. volume 88, 342 - pp. da 1 a 1 ISSN: 0003-598X [Articolo in rivista (262) - Articolo su rivista]
Abstract

Preliminary results of the interdisciplinary archaeological excavation of the bronze Age (Capo Grazino) village at Stromboli san Vincenzo

D. Brunelli; A. Verzani; R. Spallanzani; M. Seyler; M. Cannat ( 2014 ) - Asthenospheric processes beneath the ultraslow Smoothseafloor region in the eastern South West Indian Ridge - Abstract book - Società Geologica Italiana Roma ITA) - RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA - n. volume 31, Suppl. 1 - pp. da 384 a 384 ISSN: 2035-8008 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

Mantle melting at ultraslow spreading ridges is constrained by the low potential temperature and thicker-thannormal LID that limits the extent of the melting column. As a result very small amounts of melts are produced inhibiting the formation of a “normal” oceanic lithosphere and leading to a purely tectonic seafloor extension dominated by serpentinization: the recently investigated Smoothseafloor type spreading (Sauter et al., 2013). At depth the reaction of very small amounts of percolating melts and host asthenospheric mantle leaves traces of the melt/rock reactions as incomplete mineral replacement and strongly variable modal distribution at short scale (dm). Enstatitic, and to a less extent diopsidic, pyroxenes appear to be repeatedly dissolved and recrystallized leaving poekilitic pyroxene and spinel leftovers. Melts enriched in incompatible elements are possibly generated in the garnet field then reacting with the rock under near-batch conditions, i.e. at low rates of melt extraction with respect to the actual rock porosity (Brunelli et al., 2014). Prolonged pyroxenes’ dissolution-recrystallization results in enhanced enrichment of the most incompatible elements in the percolating melts that only occasionally are extracted from the system. As a consequence Na (and LREE) countertrends with the melting indicators as mineral Cr# and concentration of the moderately incompatible elements (HREE, HFSE). Accordingly the associated basalts are characterized by a strong Na enrichment and compositional trends separated from those generated in the surrounding regions. Brunelli D., Paganelli E. & Seyler M. 2014. Percolation of enriched melts during incremental open-system melting in the spinel field: A REE approach to abyssal peridotites from the Southwest Indian Ridge. Geochimica et Cosmochimica Acta, 127, 190–203. doi:10.1016/j.gca.2013.11.040 Sauter D., Cannat M., Rouméjon S., Andreani M., Birot D., Bronner A., Searle R. 2013. Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years. Nature Geoscience, 6(4), 1–7. doi:10.1038/ngeo1771.

M. Mazzucchelli; A.D. Ponce; G.W. Bertotto; A. Zanetti; D. Brunelli; T. Giovanardi; E. Aragón; M. Bernardi ( 2014 ) - Evidence for strong depletion, followed by multiple refertilisation, in the mantle column of the extra-Andean backarc (Paso de Indios, Argentina) - Abstract book - Società Geologica Italiana Roma ITA) - RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA - n. volume 31, Suppl. 1 - pp. da 427 a 427 ISSN: 2035-8008 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

In the central part of the Chubut province, close to the town of Paso de Indios, there are several outcrops of Cenozoic basalts carrying spinel-facies ultramafic xenoliths. In this area, located in the extra-Andean back-arc region, basaltic necks and dikes outcrop between 43º 36′ – 43º 50′ S and 68º 53′ – 69º 02′ W, along with remnants of lava flows divided in two groups of Paleocene and Eocene age. This volcanism was generated by extensional tectonic related to a transform plate margin episode that affected the southern South America active margin from the Paleocene to the Oligocene, as the Aluk plate detached and a slab window opened beneath the study area. Here, the petrochemical processes experienced by spinel-facies mantle xenoliths, hosted in Eocene basalts of the Matilde lava flow remnants, the León volcano and the Chenque dike, are presented. The studied samples are mainly spinel-facies harzburgites and clinopyroxene(Cpx)-poor lherzolites, with some dunites. The Chenque xenoliths mainly present porphyroclastic to equigranular texture, whereas those from Matilde and León volcanoes have coarse-grained to porphyroclastic textures. Estimated equilibrium temperatures based on pyroxenes solvus range from 800 to 940°C. The refractory character of the mineral assemblages is matched by the major element mineral compositions, which are mostly Al poor and Mg and Cr rich. Spinel composition is consistent with melt extraction from 8 to 14% (Chenque and León) and 14 to 18% (Matilde). The estimated degree of melting rises up to 24% considering the literature spinel data. However, the occurrence of melt-related open-system processes is suggested by local trends of positive correlation between Na and Cr# in Cpx, being fully confirmed by the trace element compositions. Cpxs from a harzburgitic sample from León volcano show composition rich in U, Th, Sr and LREE. The Matilde harzburgites ubiquitously show Cpx with transient U-shaped REE patterns. The LREE fractionation is very strong, with LaN up to 100 and minimum at the M-HREE region between 0.1-1xCI. The HREE level content (LuN down to 1) is consistent with 20-23% fractional melting of spinel DM. V-to-U-shaped REE patterns are also shown by Chenque lherzolites and harzburgites. Their MHREE are more fractionated than that expected in residue after spinel facies basal removal, thus suggesting an onset of the partial melting at garnet facies conditions. Other Chenque lherzolites seem to result from a more extensive refertilisation processes led by LREE-enriched to LREE-depleted melts. The latter gave rise to transient LREE-depleted sinusoidal patterns through reaction with the depleted ambient peridotite. It is, thus, concluded that the shallow mantle column beneath Paso de Indios records an incomplete refertilisation of strongly depleted protoliths. This represents a unique example for the Patagonian region, where the mantle is usually completely overprinted by multiple stages of melt/fluid migration.

E. Barbieri; D. Brunelli; A. Cipriani; M. Mazzucchelli ( 2014 ) - Melting of Plume Residue beneath the Afar: Implications for Axial Basalts Geochemistry - Abstract book - Socià Geolgica Italiana Roma ITA) - RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA - n. volume 31, Suppl. 1 - pp. da 489 a 489 ISSN: 2035-8008 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The Afar is the place to investigate both the evolution of the lithosphere from continental break up to incipient seafloor spreading and the interaction between rifting processes and a mantle plume. The plume has been evoked as one of the main factors involved in the development of the Afar, but its persistence beneath the depression is still a matter of debate. Recent studies have shown the lack of a well developed plume structure beneath the Afar (Hammond et al., 2013), thus suggesting its partial exhaustion. Geophysical investigations hint at an uppermost mantle dominated by broad asthenospheric upwelling (Rychert et al., 2012; Hammond et al., 2013) affected by decompression melting, feeding the magma chambers stored within both the crust and mantle, and the Afar plume magmatism. However, modern basalts erupted along the northern Afar show a strong enrichment in incompatible and trace elements that partially disagree with a shallow depleted mantle reservoir. Recent geochemical analyses indicate that part of the mantle melting process, still occurs at greater depths (> 80 km) (Ferguson et al., 2013) and several authors suggest the presence of focused diapiric upwelling (Hammond et al., 2013), which probably enhances the melting at greater depths. EMPA and LA-ICP-MS were used to investigate the composition of modern lavas sampled from the Erta Ale Chain and the Asal region in 2011 and 2013. The plume markers are recognizable but with less intensity compared to the Oligocene High Ti lavas, according to the lower activity of the Afar hot spot. Our results suggest a hybrid source characterized by two main reservoirs: an enriched mantle melting in the grt field and a shallow depleted mantle. We elaborated a numerical model that predicts the composition of the axial basalts through the mixing of melts obtained by melting of these theoretical reservoirs. We propose for the Afar region the presence of isolated volumes of enriched material genetically related to the remains of the plume. These bodies generate enriched melts that pollute the liquids obtained by the surrounding asthenosphere before they reach the surface. Isotopic investigations are in progress and will help to better define the involvement of each reservoir. Ferguson D.J., Maclennan J., Bastow I.D., Pyle D.M., Jones S.M., Keir D., Blundy J.D., Plank T. & Yirgu G. 2013. Melting during Late-Stage Rifting in Afar Is Hot and Deep. Nature 499 (7456) (July 4), 70–3. Rychert C.A., Hammond J.O.S., Harmon N., Kendall J.M., Keir D., Ebinger C.J., Bastow I. D., Ayele A., Belachew M., & Stuart G. 2012. Volcanism in the Afar Rift Sustained by Decompression Melting with Minimal Plume Influence. Nature Geoscience 5 (6), 406–409. Hammond J.O.S., Kendall J.M., Stuart G.W., Ebinger C.J., Bastow I.D., Keir D., Ayele A., Belachew M., Goitom B., Ogubazghi G. & Wright T.J. 2013. Mantle Upwelling and Initiation of Rift Segmentation beneath the Afar Depression. Geology 41(6), 635–638.

D. Brunelli; A. Verzani; R. Spallanzani R.; M. Seyler; M. Cannat ( 2014 ) - Multistage asthenospheric melt/rock reaction in the ultraslow eastern SWIR mantle - Abstract book - Società Geologica Italiana Roma ITA) - RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA - n. volume 31, Suppl. 1 - pp. da 383 a 383 ISSN: 2035-8008 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

Very small amounts of melt are produced during mantle upwelling beneath the ultraslow spreading South West Indian Ridge. Sectors of this Oceanic Ridge are characterized by nearly amagmatic spreading with rare limited eruptions of basalts spotting a mantle-derived serpentinitic crust. A large peridotite dataset was recovered during the Smoothseafloor French expedition leaded by D. Sauter and M. Cannat in 2005 (Sauter et al., 2013). Mantle-derived rocks show a significant modal variability from the sample to the dredge scale with frequent occurrences of millimetric to centimetric spinel-bearing pyroxenitic veins. Mantle residua record a multistage reactional history between small amount of transient melts and variably depleted mantle parcels. Incomplete mineral replacements are widespread showing that both pyroxenes are repeatedly dissolved and recrystallized leaving poekilitic pyroxene and spinel textures. Reacting conditions are modelled assuming an incremental open-system melting model under variable critical porosity/F ratios (Seyler et al., 2011; Brunelli et al., 2014). Incoming melts result to be generated by low degrees of melting in the garnet field then reacting with the rock under near-batch conditions, i.e. at low rates of melt extraction with respect to the actual rock porosity. As a consequence Na (and LREE) countertrends with melting indicators as mineral Cr# and concentration of the moderately incompatible elements (HREE, HFSE). This results in rotation of the REE patterns around a pivot element instead of showing progressive depletion as expected after suboceanic mantle decompression. Brunelli D., Paganelli E. & Seyler, M. 2014. Percolation of enriched melts during incremental open-system melting in the spinel field: A REE approach to abyssal peridotites from the Southwest Indian Ridge. Geoch. et Cosmoch. Acta, 127, 190–203. doi:10.1016/j.gca.2013.11.040. Sauter D., Cannat M., Searle R. 2013. Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years. Nature Geosci., 6(4), 1–7. doi:10.1038/ngeo1771. Seyler M., Brunelli D., Toplis M. J. & Mével C. (2011). Multiscale chemical heterogeneities beneath the eastern Southwest Indian Ridge (52°E-68°E): Trace element compositions of along-axis dredged peridotites. Geochem. Geophys. Geosyst., 12, Q0AC15. doi:10.1029/2011gc003585.

Brunelli, Daniele; Paganelli, Emanuele; Seyler, M. ( 2014 ) - Percolation of enriched melts during incremental open-system melting in the spinel field: A REE approach to abyssal peridotites from the Southwest Indian Ridge - GEOCHIMICA ET COSMOCHIMICA ACTA - n. volume 127 - pp. da 190 a 203 ISSN: 0016-7037 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The effects of melting in an open-system scenario are here explored looking to the rare earth element (REE) distribution in mantle residues. We consider a peridotite matrix equilibrated in the spinel field accounting for melt inflow during partial melting. The fertility of the source, inflowing melt composition and melt addition rate as well as the effects of varying the critical mass porosity in an incremental scenario are tested. When a relatively enriched melt enters the system, residual clinopyroxene REE normalized patterns apparently rotate around a light to intermediate REE due to concomitant increase of the light REEs and decrease of the heavy REEs. This effect is enhanced when the critical mass porosity is large with respect to the degree of melting. In these cases the system approaches batch more than fractional melting behaviour because the liquid is preferentially retained in the matrix. This geometry is suggestive of melt accumulation at depth. Four sample suites from the Southwest Indian Ridge are considered. Spinel field equilibrated clinopyroxenes in lherzolites and harzburgites show dredge-scale REE compositional trends that crosscut model fractional melting trajectories. Observed local trends correspond to rotations of the REE patterns attesting for near-batch episodes in the subridge melting history and infiltration of enriched liquids whose composition resemble that of garnet field-generated melts. © 2013 Elsevier Ltd.

Renzulli A; Bettelli M.; Brunelli D.; Cannavò V.; Coltelli M.; Di Renzoni A.; Ferranti F.; Levi S.T.; Martinelli M.C.; Martini M.; Maspero F.; Rosi M.; Santi P.; Speranza F. ( 2013 ) - Archaeology meets Volcanology: an integrated study to date and enhance understanding of the past human settlements at Stromboli, - A4. Volcanoes and Society - Geoitalia Pisa ITA) - n. volume 3 - pp. da 49 a 49 ISBN: 0000000000 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

An ongoing archaeological investigation at Stromboli shows a complex pattern of human occupation of the north eastern sector of the island (San Vincenzo plateau), which allowed an excellent strategic control of the southern Tyrrhenian sea. This interdisciplinary project focus on the stratigraphic sequences, formation processes, artifacts, raw materials and circulation networks. We aim at building a chronological framework integrating archeological, volcanological and historical studies with several methods of absolute chronology (14C, archaeomagnetic and cross-datings). First human occupation of Stromboli is attested by the presence of some prehistoric shards of the Spatarella facies, dated 3500 BC (late Neolithic-early Eneolithic). More abundant remnants of this phase are recorded in the opposite side of the island (Ginostra). Main archaeological evidences are related to a Bronze Age village characterized by rounded huts with stone walls (organized on several terraces) built upon lavas and scoriae dated by archeomagnetism at 6.2 ka ago. It is worth to note these lavas are directly covered by the pyroclastics of the “Secche di Lazzaro” eruption occurred, at ca. 6 ka ago. Artifacts belong to the Capo Graziano facies (Early to Middle Bronze Age 1-2): decorated and undecorated handmade burnished pottery and some imports from the Aegean. The 14C datings of more than 20 layers show a range between 2290- 1475 BC (2 sigma cal. age) attesting for a long lasting human occupation. Radiocarbon datings also show that the settlement of San Vincenzo started at the beginning of Capo Graziano I, with a significant presence during the early periods. The typological variability of the local and the Mycenaean pots seems to confirm this chronology. After the Bronze Age period there are no evidence of human activity on Stromboli, whereas other islands of the Aeolian Archipelago were occupied until the end of the Middle Bronze (1300 BC) or Final Bronze (900 BC; Lipari) Age. New life on Stromboli is attested during the classical phases: a Greek necropolis (320-250 BC) and Roman late Imperial age evidence (II-IV AD). A well preserved, up to 10-15 cm thick, purplish to black layered fallout deposit was discovered in the excavation, covering a channel containing green glazed pottery dated XIV-XV century AD. An origin from a Stromboli paroxysm during the XVI century is strongly supported by three 14C dated charcoals (1325-1520 AD, 2 sigma cal. age) from the layers immediately below the ash fallout. Some authors suggested 1558 for a Stromboli paroxysm but this is not strengthened by historic accounts (e.g. Tommaso Fazello).

Barbieri, Emiliano; Cipriani, Anna; Brunelli, Daniele; Paganelli, Emanuele ( 2013 ) - Barbieri E, Cipriani A, Brunelli D & Paganelli E (2013) Mineralogical Magazine, 77(5) 655 - MINERALOGICAL MAGAZINE - n. volume 77(5) - pp. da 655 a 655 ISSN: 1471-8022 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The Danakil region is a modern example of rifting located atop a mantle plume. Along the rift system, a large number of shield volcanoes erupted large volumes of tholeiitic magmas with a wide compositional range, generally enriched in incompatible and trace elements, reflecting the source heterogeneity and the variability of melting processes that contributed to their generation. The Danakil lavas represent the combined product of continental rifting and ascending mantle plume processes. Major and trace elements were analyzed in modern lavas sampled from the Erta Ale Chain and the Asal region and compared to literature data. Although highly enriched in trace elements, our lavas are significantly different when compared to the Oligocene main lava suites generated in the earliest stages of mantle plume activity. Based on La/Sm, Rb/Sr and Zr/Nb ratios and REE abundances they are intermediate between the high-Ti primitive lavas and the low-Ti tholeiitic basalts erupted 30 Ma ago due to the arrival of the plumehead. Trace elements abundances and geochemical modelling indicate that our lavas derive from a “hybrid” source characterized by a great complexity, possibly a metasomatized sublitospheric-mantle component that includes hydrous phases and melting at depths lower than those that generated the Oligocene lavas. The wide compositional range of the Afar lavas suggests that those modern lavas erupted along the rift are not simply the product of melting of a deep mantle plume but derive from a composite source resulting from the interaction between the plume tail and the surrounding sublithospheric mantle previously metasomatized by the plume activity. As a consequence, this very complex and heterogeneous source undergoes extremely variable melting processes as testified by the characteristic chemistry of each volcanic complex. Further geochemical and isotopic investigations will help to better constrain the signature and contribution of each of the reservoirs and to what extent the mantle is metasomatized by hydrous phases below the Afar region.

D. Brunelli; S.T. Levi; P. Fragnoli; A. Renzull; P. Santi; E. Paganelli; M.C. Martinelli ( 2013 ) - Bronze Age pottery from the Aeolian Islands: definition of Temper Compositional Reference Units by an integrated mineralogical and microchemical approach - APPLIED PHYSICS. A, MATERIALS SCIENCE & PROCESSING - n. volume 339 - pp. da 1 a 9 ISSN: 1432-0630 [Articolo in rivista (262) - Articolo su rivista]
Abstract

An integrated microchemical–petrographic approach is here proposed to discriminate the provenance of archaeological pottery artefacts from distinct production centres. Our study focuses on a statistically significant sampling (n = 186) of volcanic temper-bearing potteries representative of the manufacturing and dispersion among the islands of the Aeolian Archipelago during the Bronze Age. The widespread establishment of new settlements and the abundant recovery of Aeolian-made ceramic in southern Italy attest for the increased vitality of the Archipelago during the Capo Graziano culture (Early Bronze Age–Middle Bronze Age 2; 2300–1430 BC). Potteries from three of the main known ancient communities (Lipari, Filicudi and Stromboli) have been studied integrating old collections and newly excavated material. Volcanic tempers have been first investigated through multivariate analyses of relative abundances of mineral and rock clasts along with petrographic characters. In addition, we performed in-situ mineral chemistry microanalyses by Electron Microprobe and Laser Ablation—Inductively Coupled Plasma Mass Spectrometry to assess major and trace element composition of the most common mineral phases. Four Temper Compositional Reference Units have been recognised based on compositional trends. Two units (AI and AX) are unequivocally distinct by their peculiar trace element enrichment and petrographic composition; they mostly contain samples from the sites of Lipari and Stromboli, respectively. Units AIV and AVIII, restricted to the sites of Filicudi and Stromboli, show distinct petrographic characters but overlapped geochemical fingerprints.

Daniel Sauter; Mathilde Cannat; Stéphane Rouméjon; Muriel Andreani; Dominique Birot; Adrien Bronner; Daniele Brunelli; Julie Carlut; Adélie Delacour; Vivien Guyader; Christopher J. MacLeod; Gianreto Manatschal; Véronique Mendel; Bénédicte Ménez; Valerio Pasini; Etienne Ruellan; Roger Searle ( 2013 ) - Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years - NATURE GEOSCIENCE - n. volume 6 - pp. da 1 a 7 ISSN: 1752-0894 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The global mid-ocean ridge system, where tectonic plates diverge, is traditionally thought of as the largest single volcanic feature on the Earth. Yet, wide expanses of smooth sea floor in the easternmost part of the Southwest Indian Ridge in the Indian Ocean lacks the hummocky morphology that is typical for submarine volcanism.At other slow-spreading ridges, the sea floor can extend by faulting the existing lithosphere, along only one side of the ridge axis. However, the smooth sea floor in the easternmost Southwest Indian Ridge also lacks the corrugated texture created by such faulting. Instead, the sea floor is smooth on both sides of the ridge axis and is thought to be composed of altered mantle-derived rocks. Here we use side-scan sonar to image the sea floor and dredge samples to analyse the composition of two sections of the Southwest Indian Ridge, between 62◦ 05 E and 64◦ 40 E, where the sea floor formed over the past 11 million years. We show that the smooth floor is almost entirely composed of seawater-altered mantle-derived rocks that were brought to the surface by large detachment faults on both sides of the ridge axis. Faulting accommodates almost 100% of plate divergence and the detachment faults have repeatedly flipped polarity.We suggest that this tectonic process could also explain the exhumation of mantle-derived rocks at the magma-poor margins of rifted continents

Valerio Pasini;Daniele Brunelli;Paul Dumas;Christophe Sandt;Joni Frederick;Karim Benzerara;Sylvain Bernard;Bénédicte Ménez ( 2013 ) - Low temperature hydrothermal oil and associated biological precursors in serpentinites from Mid-Ocean Ridge - LITHOS - n. volume 178 - pp. da 84 a 95 ISSN: 0024-4937 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Abstract The origin of light hydrocarbons discovered at serpentinite-hosted mid-ocean hydrothermal fields is generally attributed to the abiogenic reduction of carbon (di)oxide by molecular hydrogen released during the progressive hydration of mantle-derived peridotites. These serpentinization by-products represent a valuable source of carbon and energy and are known to support deep microbial ecosystems unrelated to photosynthesis. In addition, the pool of subsurface organic compounds could also include materials derived from the thermal degradation of biological material. We re-investigate the recently described relics of deep microbial ecosystems hosted in serpentinites of the Mid-Atlantic Ridge (4–6°N) in order to study the ageing and (hydro)thermal degradation of the preserved biomass. An integrated set of high resolution micro-imaging techniques (Scanning Electron Microscopy, High Resolution Transmission Electron Microscopy, Raman and Fourier Transform Infra-Red microspectroscopy, Confocal Laser Scanning Microscopy, and Scanning Transmission X-ray Microscopy at the carbon K-edge) has been applied to map the distribution of the different organic components at the micrometer scale and to characterize their speciation and structure. We show that biologically-derived material, containing aliphatic groups, along with carbonyl and amide functional groups, has experienced hydrothermal degradation and slight aromatization. In addition, aliphatic compounds up to C6–C10 with associated carboxylic functional groups wet the host bastite and the late serpentine veins crosscutting the rock. These compounds represent a light soluble organic fraction expelled after biomass degradation through oxidation and thermal cracking. The detected complex organic matter distribution recalls a typical petroleum system, where fossil organic matter of biological origin maturates, expelling the soluble fraction which then migrates from the source to the reservoir. Ecosystem-hosting serpentinites can thus be seen as source rocks generating a net transfer of hydrocarbons and/or fatty acids issued from oxidative processes and primary cracking reactions, then migrating upward through the serpentine vein network. This finally suggests that deep thermogenic organic compounds of biological origin can be a significant contributor to the organic carbon balance at and far below peridotite-hosted hydrothermal fields.

Marco Ligi;Enrico Bonatti;Marco Cuffaro;Daniele Brunelli ( 2013 ) - Post-Mesozoic Rapid Increase of Seawater Mg/Ca due to Enhanced Mantle-Seawater Interaction - SCIENTIFIC REPORTS - n. volume 3 - pp. da 1 a 8 ISSN: 2045-2322 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The seawater Mg/Ca ratio increased significantly from , 80 Ma to present, as suggested by studies of carbonate veins in oceanic basalts and of fluid inclusions in halite. We show here that reactions of mantle-derived peridotites with seawater along slow spreading mid-ocean ridges contributed to the post-Cretaceous Mg/Ca increase. These reactions can release to modern seawater up to 20% of the yearly Mg river input. However, no significant peridotite-seawater interaction and Mg-release to the ocean occur in fast spreading, East Pacific Rise-type ridges. The Mesozoic Pangean superocean implies a hot fast spreading ridge system. This prevented peridotite-seawater interaction and Mg release to the Mesozoic ocean, but favored hydrothermal Mg capture and Ca release by the basaltic crust, resulting in a low seawater Mg/Ca ratio. Continent dispersal and development of slow spreading ridges allowed Mg release to the ocean by peridotite-seawater reactions, contributing to the increase of the Mg/Ca ratio of post-Mesozoic seawater.

Chiara Boschi;Enrico Bonatti;Marco Ligi;Daniele Brunelli;Anna Cipriani;Luigi Dallai;Massimo D'Orazio;Gretchen L. Früh-Green;Sonia Tonarini;Jaime D. Barnes;Rosa M. Bedini ( 2013 ) - Serpentinization of mantle peridotites along an uplifted lithospheric section, Mid Atlantic Ridge at 11° N - LITHOS - n. volume 178 - pp. da 3 a 23 ISSN: 0024-4937 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Abstract Mantle peridotites from an exposed lithospheric section (Vema Lithospheric Section, VLS), generated during ~ 26 Ma at a ~ 80 km long Mid Atlantic Ridge segment (11° N), have been sampled and studied to understand the evolution of the serpentinization process. The VLS was uplifted due to a 10 Ma transtensional event along the Vema transform. Before the uplift residual mantle rocks were lying beneath a 0.8–1.3 km thick basaltic crustal layer. The major and trace element compositions of the serpentinites, as well as their H, O, Sr, Cl and B isotopic compositions were interpreted based on thermal models of lithospheric spreading from ridge axis. The results suggest that serpentinization occurred mostly near the ridge axis. Serpentinization temperatures, estimated from stable isotopes, are consistent with resetting of the closure temperatures during the tectonic uplift of the lithospheric sliver, reflected by decreasing δ18O and increasing δ11B values. Modeling shows that the thermal influence of the transtensional event affected mainly the region close to the RTI (ridge–transform intersection). Petrological, elemental and isotopic data suggest that, when the ultramafic basal unit of the VLS was uplifted and exposed on the ocean floor, serpentinization became superseded by low temperature water–rock reactions, with Fe–Mn crust formation, which is still progressing, as recorded by δD. Ultramafic mylonites, prevalent in a short stretch of the VLS, show only a partial serpentinization process, together with pervasive contamination by low-temperature Fe–Mn crust.

B. Menez; V. Pasini; D. Brunelli ( 2012 ) - Life in the hydrated suboceanic mantle - NATURE GEOSCIENCE - n. volume 5 - pp. da 133 a 137 ISSN: 1752-0894 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The recesses of the oceanic crust harbour microbes that influence geochemical fluxes between the solid Earth and the hydrosphere1, 2. In the roots of the crust, mantle-derived rocks are progressively hydrated by hydrothermal circulation, a process known as serpentinization. The associated release of molecular hydrogen could provide metabolic energy for microbes3. Phylogenetic analyses of chimneys associated with seafloor hydrothermal systems have provided direct but spatially restricted evidence for the existence of active microbial communities in these hydrated rocks4; indirect evidence comes from isotopic analyses of drill cores5. Here, we examine fully serpentinized peridotites recovered from the Mid-Atlantic Ridge, using Raman microspectroscopy and electron microscopy. We detect high concentrations of organic matter, of two types, intimately associated with serpentine-hosted hydrogarnets. One type contains a complex mixture of aliphatic and aromatic compounds and functional groups such as amides, usually associated with biopolymers such as proteins, lipids and nucleic acids. The other corresponds to dense aggregates of thermally evolved carbonaceous matter, with a weak structural organization, which we attribute to the maturation of carbon compounds present in the other type of organic matter identified. We suggest that the observed endogenic accumulations of organic matter result from past microbial activity within the serpentinized oceanic crust, potentially supported by the by-products of serpentinization. We further suggest that the proposed crustal community mediates elemental fluxes from the Earth’s mantle to the oceans.

M. Andreani; B Menez; A Delacour; V Pasini; AL Auzende; D Brunelli ( 2012 ) - Micro-scale investigation of carbonation process in partially serpentinized peridotites - EGU2012-12598 - AGU Washington USA) - GEOPHYSICAL RESEARCH LETTERS - n. volume 14 - pp. da 1 a 1 ISSN: 0094-8276 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The carbonation of ultramafic rocks is, theoretically, the most efficient reaction to trap CO2 irreversibly in the form of solid carbonates, as predicted by equilibrium thermodynamic calculations. However, the success of industrial or natural carbonation in large ultramafic aquifers or oceanic ultramafic exposures does not only rely on the thermodynamic conditions of chemical reactions, but also on their feedback effects on the reactive surface area and on the local porosity and permeability. In addition, side processes like serpentinization, redox reactions, abiotic catalytic effects, and biological activity, can be expected in such complex natural system. Their occurrence and implications on carbon speciation and carbon transfers during hydrothermal alteration of oceanic peridotites have not been explored yet and requires detailed study of natural and/or experimental carbonation zones. We have combined petrographic and electron microscopy with SIMS, Raman and FTIR microspectroscopy on partially serpentinized peridotites drilled during the IODP leg 304 (30 N, MAR) in order to characterize the mechanisms of peridotite carbonation at the fluid-mineral interface and identify the associated speciation of carbon (inorganic and organic carbon occurrences). We present first results on zones located close to talc-tremolite sheared veins in holes 1309B and D. Associations of carbonates, porous phyllosilicates and oxides are observed in close vicinity of relict olivines that underwent a previous stage of serpentinization. The olivine-carbonate interface is nanoporous which facilitates mass transfer between fluid and mineral. The phyllosilicate identified as saponite results from the metasomatic replacement, during the carbonation stage, of previously formed serpentine. These observations do not favour reaction-induced cracking but rather a transfer-controlled process in an open system. Among the submicrometric dark clusters widely-distributed in saponite and in serpentine, vibrational microspectroscopy reveals the presence of various types of organic compounds that tend to be located close to micrometric sulphides grains. Those results underline the microscale variability of carbon speciation within hydrothermally altered peridotites. The association of reduced carbon phases with the carbonation texture suggests that CO2 conversion may not be limited to solid carbonate formation in natural systems and that biological activity and/or abiotic CO2 reduction, possibly catalyzed by Ni-rich sulphides, can occur contemporaneously. This complex association of reactions has to be unravelled further to determine the respective contribution of abiotic versus biological processes and integrate them in carbon transfers modelling through the oceanic lithosphere.

P. Fragnoli; D. Brunelli; S.T. Levi; A. Renzulli; P. Santi; J.L. Williams ( 2012 ) - Scambi ceramici nei Contesti capo Graziano delle isole Eolie: dati petrografici e petrologici a confronto - Atti del Congresso A.I.Ar. 2012 Modena - Patron Bologna ITA) - pp. da 1 a 10 ISBN: 9788855531665 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

Questo lavoro si pone l’obiettivo di ricostruire la rete interinsulare di scambi ceramici nei contesti Capo Graziano [BA-BMII (2300-1430 a.C.)] dell’arcipelago eoliano. A tal fine sono stati esaminati 274 campioni ceramici provenienti da Lipari, Filicudi e Stromboli e 16 campioni di lave affioranti nelle vicinanze dell’insediamento di San Vincenzo a Stromboli. Le analisi chimiche in situ degli elementi maggiori e in traccia hanno consentito di ipotizzare l’esistenza di produzioni locali sulle tre isole. L’approvvigionamento delle materie prime è diverso nei diversi siti: a Lipari indica l’uso costante della stessa sorgente, mentre si differenzia nel tempo a Stromboli e Filicudi. L’insieme delle osservazioni svolte permette di definire diverse forme di scambio interinsulare.

G. Ayala; D. Brunelli; S.T. Levi; S. Lugli; E. Photos-Jones; F. Sartor; L. Vigliotti ( 2012 ) - Site formation processes and human activity patterms: holistic soil analysis at the prehistoric settlement of San Vincenzo, Stromboli - Atti del Congresso A.I.Ar. 2012 Modena - Patron Bologna ITA) - pp. da 21 a 30 ISBN: 9788855531665 ISSN: - [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

Traditionally archaeological deposits are investigated through the removal of the material culture contained within the matrix of the sediment. Whilst this is the backbone of archaeological investigations to date, the analysis of the sediment from which they came often holds a more nuanced record of what actually went on in the past. Through the application of a suite of sediment analyses, a finer resolution of information can be attained from archaeological contexts and features. At the Bronze Age site of San Vincenzo on Stromboli, from the outset (2009-2011), excavation has been complemented by a systematic sampling programme for sediment analyses (including micromorphology, organic matter, granulometry, magnetic properties, XRF-geochemical analysis). Preliminary analysis has shown that this methodological approach has allowed for the enhanced understanding of site formation processes as well as shown potential for understanding activity patterns across the site.

Sauter D.; M. Cannat; M. Andreani; D. Birot; A. Bronner; D. Brunelli; J. Carlut; A. Delacour; V. Guyader; V. Mendel; B. Ménez; C. MacLeod; V. Pasini; S. Rouméjon; E. Ruellan and R. Searle ( 2011 ) - A 10 Myrs long record of mantle exhumation at the eastern Southwest Indian Ridge - 2011 AGU Fall Meeting [Poster (275) - Poster]
Abstract

Although detachment faulting is thought to be a fundamental, widespread style of accretion of oceanic lithosphere, the kinematic evolution of faulting and the link between deformation and magmatic emplacement are still poorly known. Here we use newly acquired geological and geophysical data from the eastern Southwest Indian Ridge (SWIR) to address this question. The eastern SWIR displays the widest expanses known to date of seafloor with no evidence for a volcanic upper crustal layer. Deep tow TOBI sidescan sonar images and dredged rock samples were collected over two areas with contrasting styles of non volcanic seafloor. In the first survey area (centered at 62°30’E) several broad smooth elongated and symmetric ridges are oriented SW-NE, oblique to the spreading direction (~NS). TOBI images show that the hillsides of these ridges, facing toward and away from the axial valley, correspond to smooth surfaces with mass wasting features and almost no evidence of volcanism. Some of these ridges display highly backscattering fuzzy textures with rounded shape features of unknown origin. Serpentinized mantle-derived peridotites were systematically dredged in those areas. Volcanic textures are only observed at the eastern and western ends of these ridges. The second survey area (centered at 64°35’E) covers the edge between volcanic and non volcanic seafloor. There, ridges are perpendicular to the spreading direction and display mainly an asymmetric shape. TOBI sidescan sonar images show that the inward facing gentle slope of these ridges are smooth and sedimented surfaces covered locally by small sized volcanic patches. Outward facing slopes are steeper and may be covered with volcanic textures. Serpentinized mantle-derived peridotites were also systematically dredged outside these volcanic areas. We interpret the successive non volcanic ridges as the result of large low angle normal faults which exhume mantle rocks alternatively to the northern and to the southern flank of the SWIR. The occurrence of volcanic terrain on top of the detachment faults questions the relationship between shifts in the polarity of these detachment faults and magmatic emplacements.

V. Pasini; B. Ménez; D. Brunelli ( 2011 ) - Cryptoendolithic colonization in the hydrating mantle along mid ocean ridges - . - . . FRA) - MINERALOGICAL MAGAZINE - n. volume 75 - pp. da 2297 a 2355 ISSN: 0026-461X [Abstract in rivista (266) - Abstract in Rivista]
Abstract

abstract

Paganelli E.; Brunelli D.; Seyler M.; Bonatti E.; Cipriani A.; Ligi M. ( 2011 ) - Deep versus shallow melt stagnation in an ultra-slow / ultra-cold ridge segment: the Andrew Bain southern RTI (SWIR) - 2011 AGU Fall Meeting - EOS [Poster (275) - Poster]
Abstract

The Andrew Bain Fracture Zone (ABFZ) represents one of the largest transform faults in the ridge system spanning 750 km in length with a characteristic lens-shape structure. The southern Ridge-Transform Intersection represents the deepest sector of the whole South West Indian Ridge system. During the Italian-Russian expedition S23-AB06, the seafloor in the Southern Ridge Transform Intersection (RTI) has been sampled recovering only ultramafic material in the majority of the dredging sites. The sampled spinel and plagioclase peridotites show hybrid textures, characterized either by deep spinel-field impregnation assemblages (sp+cpx±opx±ol) or by plagioclase-field equilibrated patches and mineral trails (pl+cpx±ol) marked by both crystallization of newly formed plagioclase-field equilibrated trails and formation of plagioclase coronas around spinel. The ones collected from ridge axis show also late gabbroic pockets and veins, variably enriched in clinopyroxene. Overall textures account for important melt percolation/stagnation events occurred in the plagioclase and spinel field. Major and trace element distribution in pyroxenes and spinels from spinel-bearing peridotites overall follow a general melting trend accompanied by a progressive re-equilibration to lower P/T facies at all scales. However, only few samples can be linked to near fractional melting, while the majority of them shows REE pattern and trace element concentrations that cannot be reproduced by fractional melting process. Open-system melting (OSM) better reproduces measured REE patterns. Modeling melting in an open system scenario requires high residual porosity to be accounted for along with generally enriched melts to influx the melting parcel at depth. Melting at high residual porosity suggests a near-batch regime in which enriched melts stagnate in the spinel field. Inhibition of melt segregation during melt/rock interaction asks for a permeability barrier to develop in the region where the mantle potential temperature is suggested to still be high enough to allow partial melting. Energy consumption during garnet breakdown and porosity decrease due to reaction with silica-saturated melt could play a key role in the formation of short scale permeability barriers beneath ABFZ. Alternatively an anomalously thick conductive layer can be responsible of deep inhibition of melting and melt accumulation at depth.

D.K. Blackman; B. Ildefonse; B.E. John; Y. Ohara; D.J. Miller; N. Abe; M. Abratis; E.S. Andal; M. Andreani; S. Awaji; J.S. Beard; D. Brunelli; A.B. Charney; D.M. Christie; J. Collins; A.G. Delacour; H. Delius; M. Drouin; F. Einaudi; J. Escartín; B.R. Frost; G. Früh-Green; P.B. Fryer; J.S. Gee; M. Godard; C.B. Grimes; A. Halfpenny; H-E. Hansen; A.C. Harris; A. Tamura; N.W. Hayman; E. Hellebrand; T. Hirose; J.G. Hirth; S. Ishimaru; K.T.M. Johnson; G.D. Karner; M. Linek; C.J. MacLeod; J. Maeda; O.U. Mason; A.M. McCaig; K. Michibayashi; A. Morris; T. Nakagawa; T. Nozaka; M. Rosner; R.C.; Searle; G. Suhr; M. Tominaga; A. von der Handt; T. Yamasaki; X. Zhao ( 2011 ) - Drilling Constraints on Lithospheric Accretion and Evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N - JOURNAL OF GEOPHYSICAL RESEARCH: SOLID EARTH - n. volume 116 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100–220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45° rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Sauter; D.; M. Cannat; M. Andreani; D. Birot; A. Bronner; D. Brunelli; J. Carlut; A. Delacour; V. Guyader; V. Mendel; B. Ménez; C. MacLeod; V. Pasini; S. Rouméjon; E. Ruellan and R. Searle ( 2011 ) - Invited talk: Mantle exhumation at the Southwest Indian Ridge; preliminary results of the “SMOOTHSEAFLOOR” cruise - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume AGU Fall Meeting 2011 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The eastern Southwest Indian Ridge (SWIR) is among the deepest part of the oceanic ridge system, and it is thus inferred to represent a melt-poor end-member for this system. It displays the widest expanses known to date of seafloor with no evidence for a volcanic upper crustal layer. This nonvolcanic ocean floor has no equivalent at faster spreading ridges and has been called “smooth seafloor” because it occurs in the form of broad ridges up to 2000 m high with a smooth, rounded topography with no resolvable volcanic cones on bathymetric data (Cannat et al., 2006). On R/V Marion Dufresne cruise “SMOOTHSEAFLOOR” in October 2010, we conducted a detailed geological-geophysical survey of such smooth seafloor areas, determining the rock types exposed at these ridges, searching for hydrothermal activity and mapping the volcanic, tectonic and sedimentary structures. 35 dredges and 15 CTDs have been realized and more than 1000 km long TOBI sidescan sonar images and deep towed magnetometer profiles have been collected over two contrasting survey areas up to magnetic anomaly C5n (10 Ma). The main result of our cruise is that serpentinized mantle-derived peridotites were dredged widely throughout the smooth seafloor areas while gabbros and basalts were rare. Mantle rocks were found on moderate slopes (20-35°), facing toward and away from the axial valley, at ridges with symmetric shape, as well as on gentle slopes (<15°) facing the axis at asymmetric ridges whose outward facing steeper slopes may be covered with volcanics. TOBI sidescan sonar images show that these hillsides are ancient large low angle normal faults which were covered locally by small amount of volcanics and dismembered by landslide activity. These results show that mantle exhumation has been the main process which shaped the smooth seafloor areas of the eastern SWIR for the last 10 Myrs.

B. Ménez ; E. Gérard; D. Brunelli; V. Pasini; P. Le Campion; S. Dupraz; M.C. Marinozzi; M. Amor; F. Guyot ( 2011 ) - Keynote: Unraveling microbes-minerals interactions in the deep biosphere - . - . . FRA) - MINERALOGICAL MAGAZINE - n. volume 75 - pp. da 1453 a 1453 ISSN: 0026-461X [Abstract in rivista (266) - Abstract in Rivista]
Abstract

abstract

M. Seyler; D. Brunelli; M. J. Toplis; C. Mével ( 2011 ) - Multiscale chemical heterogeneities beneath the eastern Southwest Indian Ridge (52°E–68°E): Trace element compositions of along-axis dredged peridotites - GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS - n. volume 12 - pp. da 1 a 31 ISSN: 1525-2027 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The Southwest Indian Ridge is characterized by frequent outcrops of mantle rocks in a very slow spreading context. In situ measurements of trace element concentrations in pyroxenes of these rocks, and associated petrogenetic modeling, are reported. Overall, the measured compositions cover the whole range typically observed for abyssal peridotites. The greatest subkilometer-scale compositional variability is observed in the region east of the Melville fracture zone. The best explanation for the observed variability is given by concurrent melting and migration of melts strongly enriched in the most incompatible rare earth elements, such as those produced by a garnet-bearing source, or by refertilization with mixed garnet- and spinel-derived partially aggregated melts. Because the regionally associated basalts bear no “garnet signature” in their chemical compositions, we conclude that the residual mantle preserves the signature of a mantle source component that does not appear in the erupted magmas. Comparison between along-axis variations of basalt isotopic compositions and peridotite chemical compositions suggests that local isotopic enrichments displayed by some basalts can be associated with the “garnet signature” in the peridotite and that our sampling represents only a fraction of the global variability of the subaxial mantle. To the west of the Melville fracture zone, samples are more depleted and homogeneous at dredge scale. In addition to containing enriched components, petrologic modeling indicates that the peridotitic mantle beneath the entire section underwent (previous?) partial melting in the garnet stability field before melting at lower pressures.

Barbieri E.; Brunelli D.; Hellebrand E.; Johnson K. T.; Paganelli E. ( 2011 ) - Partial assimilation during crystallization of a (gabbroic) pluton: the gabbronoritic zone of the Atlantis core complex - 2011 AGU Fall Meeting - EOS [Poster (275) - Poster]
Abstract

The lower oceanic crust largely consists of cumulates, representing crystallization products from melt that erupt on the ocean floor. Removal of interstitial melt from the cumulate minerals is believed to be efficient, driven by compaction. Reaction between percolating melts and cumulate minerals can result in micron-to-meter-scale variations in the phase assemblage and composition. Here we report on the results of a highly detailed petrological study of a sequence of lower crustal gabbroic rocks, using textural and chemical criteria to reconstruct the complex interplay between reactive melts and a highly crystallized gabbroic mush. The exceptionally well-preserved gabbroic sequence was recovered during IODP Legs 304 and 305 at the Atlantis Massif. These gabbros are characterized by high-frequency magmatic refilling of a mush zone, as attested by the cm-m thick succession of magmatic layers, with sharp to diffuse contact. The main gabbroic body (between 300 and 1100 mbsf ) is bounded by two layers of olivine rich troctolites interpreted as pervasively fluxed mantle rocks. Closely spaced sampling of the intervals 930-980 and 1130-1190 mbsf reveal evolved gabbroic lithologies closely interspersed with more primitive members. Olivine and plagioclase consumption is accompanied by progressive crystallization of clinopyroxene (cpx) and later orthopyroxene (opx). Both initially appear as interstitial grains, followed by poikilitic and granophyric textures. The onset of reactive percolation is often associated with deformation, recorded as kink bands in relict olivine and plagioclase and wavy mineral contacts wet by thin cpx layers. As a result, plagioclase and olivine chadacrysts in cpx and opx have resorbed appearance (rounded deformed chadacrysts in large undeformed oikocrysts). Typical chemical signatures of this incomplete resorption are high Ni in pyroxene-hosted low-Fo olivines. Resorbed plagioclases have higher REE and positive Sr correlation with Ba attesting to a progressive equilibration with evolved melts. Simple fractional accumulation in a closed system is unable to fit the observed cpx and opx trends. Large deviations from linear correlations require an influx of primitive material in an ongoing differentiating system. Based on the observed consumption of the primitive assemblage we calculate high rates of assimilation of primitive cumulates. However, the assimilation ratio estimated from cpx trace element contents is much higher than that required to fit the opx trends. These observations attest for a multistage evolution of the sampled rocks: the process starts as near-fractional crystallization, reaching high degrees of crystallinity before melt percolation induces olivine and plagioclase resorption, increasing porosity and crystallizing clinopyroxene. The subsequent crystallization of opx is characterized by a decrease of the assimilation rate and porosity reduction.As such reactive signatures have been reported from other oceanic gabbros, AFC processes in the lower oceanic crust are possibly widespread and may strongly contribute to the compositional systematic observed in global MORB.

Cipriani, A.; Brunelli, D.; Seyler, M.; Paganelli, E.; Barbieri, E. ( 2011 ) - REE pattern rotation in an open system melting: case studies from slow and ultraslow spreading ridges - EOS - n. volume EOS, Trans. AGU, Fall Meet. Suppl., [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The REE compositional space provides a reliable means to recognize the degree of depletion and melt rock reactional events undergone by a parcel of mantle. We model residual clinopyroxene compositions from slow (MAR) and ultraslow (SWIR) sectors of the Mid Ocean Ridges. REE ratios (e.g. SmN/YbN vs. YbN) show compositional trends crosscutting the expected partial melting trends at the typical kilometer length scale. In the REE compositional space they appear as pattern rotations around a mid-point. Open-system melting modeling reveals the intensity of the rotation and the position of the pivot element, depending mainly on the ratio between input/output melt flux and on the enrichment of the percolating melt with respect to the depleted screen. Variations of the residual porosity of the system (Ø) with respect to the degree of melting F result in variations of the nature of the melting process. At low Ø/F the process behaves as near-fractional while at high Ø/F the process behaves as near batch. In an open melting system scenario the effect of an enriched melt fluxing a melting portion can be strongly enhanced by melt stagnation i.e. approaching a near-batch process with low melt output. Model trends present strong enrichment of SmN/YbN at decreasing YbN values. At high Ø/F even YbN increases along with SmN/YbN. These trends well fit measured countertrends at the km lengthscale, i.e. a typical dredge lengthscale. Our observations suggest that in some portions of the melting region the vertical porosity profile can vary resulting in a variability of the nature of the melting process from near-fractional (when extraction prevails) to near batch (when stagnation prevails). Percolation of enriched melts through a melting mantle parcel results in REE pattern rotation whose intensity and midpoint depends on Ø/F, mixing factors and obviously on the composition of the melt itself. Our preliminary results suggest that, first, melt batches generated deep in the mantle are transported out of equilibrium to shallower portions of the melting region and redistributed to the rock porosity. Second, that porosity barriers are present at depth resulting in melt accumulation and stagnation in the spinel facies of the melting region.

Brunelli D.; Seyler M.; Paganelli E.; Barbieri E. ( 2011 ) - Short wavelength vertical fluctuations of the melting regime in the suboceanic melting region - 2011 AGU Fall Meeting - EOS [Poster (275) - Poster]
Abstract

Modelling of mantle residua cpx REE patterns allow recognizing short wavelenght vertical variability of the porosity regime of a melting region differing from that deriving after melt focusing processes ultimately leading to dunitic channelling of the mantle section. A trace element detailed study of residual clinopyroxenes from the ultraslow eastern SWIR section shows compositional trends crosscutting the expected partial melting trends at the typical kilometre lenghtscale. In the REE compositional space these trends appear as pattern rotations around a mid-point. Open-system melting modelling reveals the intensity of the rotation and the position of the pivot element depending mainly on the ratio between input/output melt flux and on the enrichment of the percolating melt with respect to the depleted screen. We derived two important indications on the process: first some regions experience near-batch melting, i.e. melt accumulation with very low output melt flux, in regions soon after the grt/sp transition. This observations suggest permeability barriers to occur in the main melting region possibly due to porosity consumption by melt/rock reaction enhanced by grt breakdown energy competition. Consequently melt stagnation processes similar to those described for the plagioclase facies may act in the spinel field portion of the melting region. Second: we attest the presence of enriched melts delivered to the spinel field region. These melts may derive from very low degrees of melting in the garnet field of a DMM source or by consumption of an enriched heterogeneity with a lower melting point than the surrounding mantle.

E. Aragon; F. D‘Eramo; A. Castro; L. Pinotti; D. Brunelli; O. Rabbia; G. Rivalenti; R. Varela; W. Spakman; M. Demartis; C.E. Cavarozzi; Y.E. Aguilera; M. Mazzucchelli; A. Ribot ( 2011 ) - Tectono-magmatic response to major convergence changes in the North Patagonian suprasubduction system; the Paleogene subduction-transcurrent plate margin transition - TECTONOPHYSICS - n. volume 59 - pp. da 218 a 237 ISSN: 0040-1951 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The southern and central Andes reflect significant along-strike differences of tectonic activity, including shortening, alternating flat-to-normal subduction styles and magmatism. In northern Patagonia, the subduction/supra-subduction system, fore arc, arc and back arc basins developed in an extensional setting during the Paleogene. This was accompanied by landward migration of calcalkalic magmatism which changed to synextensional bimodal volcanism of rhyolitic ignimbrites and interbedded tholeiitic and alkalic basalts. These Paleogene events occurred during a time when the Farallon-Aluk active ridge reached the South American plate, and the Farallon plate subduction was interrupted. They represent a new tectonic regime, characterized by a transcurrent plate margin. The presence in the back arc of a rigid lithosphericblock of 100,000 km2 represented by the North Patagonian Massif focused the rotation of the coastal blocks. This resulted in the development of two Paleogene extensional regions to the north and south, respectively, of the Massif and replaced the former back arc. Plate rearrangement caused by the inauguration of the Nazca plate and its regime of orthogonal subduction at the beginning of the Miocene, re-established typical calc-alkaline arc magmatism at the former upper Cretaceous arc locus. Present seismic activity in the subducted plate andtomographic modeling of p-wave velocity anomalies in the upper mantle also suggest the presence of a subduction gap that lasted for most of the Paleogene in northern Patagonia.

Ligi, Marco; Bonatti, Enrico; Brunelli, Daniele; Cipriani, Anna; Ottolini, Luisa ( 2011 ) - Water in Mid Ocean Ridge Basalts: Some Like it Hot, Some Like it Cold [Monografia o trattato scientifico (276) - Monografia/Trattato scientifico]
Abstract

The presence in the Earth’s mantle of even small amounts of water and other volatiles has major effects: first, it lowers drastically mantle’s viscosity, thereby facilitating convection and plate tectonics; second, it lowers the melting temperature of the rising mantle affecting the formation of the oceanic crust. H2O concentration in oceanic basalts stays below 0.2 wt% except for basalts sampled near “hot spots” that contain significantly more H2O than normal MORB, implying that their mantle plume sources are unusually H2O-rich. Basalts sampled in the Equatorial Atlantic close to the Romanche transform, a thermal minimum in the Ridge system, have a H2O content that increases as the ridge is cooled approaching the transform offset. These basalts are Na-rich, being generated by low degrees of melting of the mantle, and contain unusually high ratios of light versus heavy rare earth elements implying the presence of garnet in the melting region. H2O enrichment is due not to an unusually H2O-rich mantle source, but to a low extent of melting of the upwelling mantle, confined to a deep wet melting region. Numerical models predict that this wet melting process takes place mostly in the mantle zone of stability of garnet. This prediction is verified by the geochemistry of our basalts showing that garnet must indeed have been present in their mantle source. Thus, oceanic basalts are H2O-rich not only near “hot spots”, but also at “cold spots”.

Brunelli, Daniele; Seyler, M. ( 2010 ) - Asthenospheric percolation of alkaline melts beneath the St. Paul region (Central Atlantic Ocean) - EARTH AND PLANETARY SCIENCE LETTERS - n. volume 289 - pp. da 393 a 405 ISSN: 0012-821X [Articolo in rivista (262) - Articolo su rivista]
Abstract

Two peridotite suites collected by submersible in the equatorial Atlantic Ocean (Hekinian et al., 2000) were studied for textures, modes, and in situ major and trace element compositions in pyroxenes. Dive SP12 runs along the immersed flank of the St. Peter and Paul Rocks islets where amphibole-bearing, ultramafic mylonites enriched in alkalies and incompatible elements are exposed (Roden et al., 1984), whereas dive SP03 sampled a small intra-transform spreading centre situated about 370 km east of the St. Peter and Paul Rocks. Both suites are characterized by undeformed, coarse-grained granular textures typical of abyssal peridotites, derived from residual mantle after ~15 % melting of a DMM source, starting in the garnet stability field. Trace element modelling, textures and lack of mineral zoning indicate that the residual peridotites were percolated, reacted and refertilized by ~2.6 % partially aggregated melts in the uppermost level of the melting region. This relatively large amount of refertilization is in agreement with the cold and thick lithosphere inferred by previous studies. Freezing of trapped melts occurred as the peridotite entered the conductive layer, resulting in late-stage crystallization of olivine, clinopyroxene, spinel, ± plagioclase. Chondrite-normalized REE patterns in clinopyroxenes from SP03 indicate that they last equilibrated with (ultra-) depleted partial melts. In contrast, REE concentrations in clinopyroxenes from SP12 display U and S shaped LREE-enriched patterns and the calculated compositions of the impregnating melts span the compositional range of the regional basalts, which vary from normal MORB to alkali basalt sometimes modified by chromatographic fractionation with no, or very limited, mineral reaction. Thus the mylonitic band forming the St. Peter and St. Paul Rocks ridge is not a fragment of subcontinental lithospheric mantle left behind during the opening of the Central Atlantic, nor the source of the alkaline basalts as previously suggested. Rather, dive SP12 sampled residual peridotites of normal MORB mantle that were located close to channels transporting alkali-basalts. Reacted melts escaping from these channels, infiltrated, and locally equilibrated with, the peridotite matrix by ion exchange reactions. Relicts of the source of the alkaline basalts were not sampled but our study suggests that it was a component of the MORB mantle underlying the St. Paul region.

E. Paganelli; D. Brunelli; M. Seyler; E. Bonatti; A. Cipriani; M. Ligi ( 2010 ) - Heterogeneous melting and refertilization of a mantle parcel in a cold spot: Andrew Bain fracure zone (SWIR) - 89th SIMP Journal of Abstracts [Poster (275) - Poster]
Abstract

abstract

D. Brunelli; M. Seyler; C. Mével ( 2010 ) - Mantle heterogeneities beneath the eastern SWIR - SIMP 89 - SIMP Ferrara ITA) - PLINIUS - n. volume 89 SIMP [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The eastern termination of the South West Indian Ridge (SWIR) is characterized by very slow spreading rates, reduced magma production and a peculiar accretionary tectonics[1]. Oblique spreading sectors separate orthogonal segments characterized by an anomalous abundance of variably extended core complexes that accommodate spreading through low angle, long-lived, detachment faulting. High ridge depths, along with abundant mantle exposures at the seafloor and reduced crustal thickness suggest mantle temperatures to be the lowest of the entire South West Indian Ridge [2]. This region is bounded by the Melville fracture zone to the west. Based on basalt isotopic fingerprint it appears that this fracture zone represents a major thermal and compositional boundary [3]. We have carried out a detailed major and trace element study of mantle residua from the two domains east and west of the Melville fracture zone recognizing different compositional domains in the mantle underlying this region accompanied by different melting styles. The western domain reflects the high degree of depletion due to the higher mantle potential temperature. By contrast the domain East of the Melville fracture zone is characterized by mantle that undergone low degrees of partial melting mirrored by the overall lower crustal thickness associated to large obliquity of the spreading axis. These mantle batches show a large interaction with variably aggregated melts. A marked garnet signature and incompatible element enrichment is locally present interpreted as source refertilization by garnet-derived melts. Basalts with compositions close to these melts have been collected in the region but they are not spatially associated to the enriched mantle. These basalts also show the enriched-most isotopic signature suggesting that melting of enriched heterogeneities and significant garnet field refertilization are two aspects of the same process. However these basalts are chemically depleted nearly similar to isotopically depleted basalts from other sectors of the Indian Ridge. This observation requires enriched melts to be generated by large extent of melting of the generating lithology without an important contribution from the surrounding mantle. The prevailing contribution of the enriched lithologies can be explained by the lower solidus but also by a probable more depleted character of the surrounding mantle resulting in a selective extraction of the first. The scattered presence of mantle rocks too depleted for resulting from the melting process beneath the region east of Melville confirms the presence of relict domains of ancient depletions. The entire range of basalt compositions from this stretch of SWIR overlaps the calculated range of the melts in equilibrium with the peridotites strongly suggesting that the variability of the mantle composition is likely similar at local (dredge) and regional scales. References. [1] Cannat, M., Sauter D., Mendel V., Ruellan E., Okino K., Escartin J., Combier V., Baala M., (2006). Geology 34(7), 605-608. [2] Cannat, M., Rommevaux-Jestin C., Sauter D., Deplus C., Mendel V., (1999). Journal of Geophysical Research 104(B10), 22825-22843. [3] Meyzen C.M., Toplis M.J., Humler E., Ludden J.N., Catherine M., (2003) Nature 731-733., 100-110;

A. Cipriani; E. Bonatti; M. Seyler; H. Brueckner; D. Brunelli; L. Dallai; S. Hemming; M. Ligi; L. Ottolini; B. Turrin ( 2009 ) - A 19 to 17 Ma amagmatic extension event at the Mid-Atlantic Ridge: ultramafic mylonites from the Vema Lithospheric Section - GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS - n. volume 10 - pp. da 1 a 53 ISSN: 1525-2027 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A >300 km long lithospheric section (Vema Lithospheric Section or VLS) is exposed south of the Vematransform at 11N in the Atlantic. It is oriented along a seafloor spreading flow line and represents 26 Maof accretion at a single 80 km long segment (EMAR) of the Mid-Atlantic Ridge. The basal part of the VLSexposes a mantle unit made mostly of relatively undeformed coarse-grained/porphyroclastic peridotitesthat were sampled at close intervals. Strongly deformed mylonitic peridotites were found at 14 contiguoussites within a 80 km stretch (4.7 Ma interval); they are dominant in a time interval of 1.4 Ma, fromcrustal ages of 16.8 to 18.2 Ma (mylonitic stretch). Some of the mylonites are ‘‘dry,’’ showing anhydroushigh-T deformation, but most contain amphibole. The mylonitic peridotites tend to be less depleted thanthe porphyroclastic peridotites on the basis of mineral major and trace elements composition, suggestingthat the mylonites parent was a subridge mantle that underwent a relatively low degree of melting. The Sr,Nd, and O isotopic composition of the amphiboles is MORB-like and suggests either that seawater did notcontribute to their isotopic signature or that their isotopic ratios re-equilibrated during fluid circulation inthe upper mantle. Four 40Ar/39Ar ages, on three amphiboles separated from the peridotites, are close tocrustal ages predicted from magnetic anomalies, confirming that the amphiboles formed close to ridge axis.We propose that crustal accretion at the EMAR segment has been mostly symmetrical for the 26 Ma of itsrecorded history, except for the 1.4 Ma interval of prevalent ultramafic mylonites (mylonitic stretch) thatmay record a period of quasi-amagmatic asymmetric accretion of oceanic lithosphere close to the ridge–Vema transform intersection, possibly with development of detachment faults. This interval maycorrespond to a thermal minimum of the subridge upwelling mantle, marking the transition from a periodof decreasing to one of increasing mantle melting below the EMAR segment.

Brunelli D.; Seyler M. ( 2009 ) - Asthenospheric percolation of alkaline melts beneath the St. Paul Region (Central Atlantic) - Geoitalia VII Forum Italiano di Scienze della Terra - . . ITA) - EPITOME - n. volume 03 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

abstract

Brunelli D.; Seyler M. ( 2009 ) - Asthenospheric percolation of alkaline melts beneath the St. Paul Region (Central Atlantic) - Alpine Ophiolites and Modern Analogues - AAVV Parma ITA) - n. volume 1 - pp. da 1 a 1 ISBN: 0000000000 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

abstract

Ponce A.; Bertotto G.; Mazzucchelli M.; Brunelli D. ( 2009 ) - Basaltos del cerro del Mojón, centro-oeste de la provincia de Río Negro, descripción petrográfica y geoquímica - X Jornadas Pampeanas de Ciencias Naturales - CN Santa Rosa ARG) - n. volume 10 - pp. da 1 a 1 ISBN: 0000000000 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

abstract

MAZZUCCHELLI M.; RIVALENTI G.; BRUNELLI D.; ZANETTI A.; BOARI E. ( 2009 ) - Dunites in the Balmuccia Peridotite Massif (Western Italian Alps): their origin by focused percolation of pyroxenite-derived melt. - GEOPHYSICAL RESEARCH ABSTRACTS - n. volume 11 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

In the Balmuccia Massif, an Alpine peridotite thought to represent part of the subcontinental mantle, a 50 m thick and 150 m long dunite body, which occurs as a subconcordant, tabular structure, has been recently recognised. The contacts with the host spinel-facies depleted lherzolite are sharp. The dunite body is composed of spinel-rich dunite containing centimetre-size lenses of relict Cr-diopside websterite, spinel-poor granoblastic dunite and virtually monomineralic Cr-spinel layers exhibiting flow structures. Orthopyroxene is a minor, relict phase in all the lithologies; clinopyroxene is intergranular and amphibole is a minor accessory phase. Overall the dunite body is fairly refractory (Fo in olivine: 90.7-93.8). Strontium and neodymium isotope ratios of clinopyroxene separates from the dunitic body resemble those of a Cr-diopside websterite suite that forms a series of dykes cutting the main peridotite host. It is proposed that the dunites were generated in a part of the mantle veined byearly Cr-diopside websterites by a three-stage process involving partial melting of pyroxenite, infiltration of the pyroxenite-derived melt into the depleted lherzolite and its consequent open-system partial melting and focused flow of the resultant partial melts leading to the production of reactive dunite channels through both peridotite and pyroxenite. This process has been simulated using pMELTS assuming that the pyroxenite partially melts at 1.5 GPa and focused melt transport occurs at pressures greater than 0.7 GPa. The results show that, depending on the focusing factor assumed, dunite can form from peridotite at P < 1.2 GPa and from pyroxenite at P < 1.1 GPa, in both cases over a large pressure range. The model accounts for specific characteristics of the dunite, such as its refractory composition, the presence of orthopyroxene relics, the occurrence of relict websterite lenses in the spinel-rich dunites and the flow structures in the Cr-spinel layers. The proposed mechanism allows dunite formation to occur well within the spinel stability field, and therefore at greater depth than dunites in ophiolites,which generally formed within the plagioclase stability field. The aggregated model melts extracted from the segments where dunite forms are high-Mg alkali basalts resembling, after olivine fractionation, the compositions of enriched-type mid-ocean ridge basalt from slow- and ultraslow-spreading ocean ridges.

MAZZUCCHELLI M.; RIVALENTI G.; BRUNELLI D.; ZANETTI A.; BOARI E. ( 2009 ) - Formation of highly-refractory dunite by focused percolation of pyroxenite-derived melt in the Balmuccia peridotite massif (Italy) - JOURNAL OF PETROLOGY - n. volume 50 - pp. da 1205 a 1233 ISSN: 0022-3530 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A 50 m thick and 150 m long dunite body occurs as a subconcordant, tabular structure in the Balmuccia Massif, an Alpine peridotite thought to represent part of the subcontinental mantle. The contacts with the host spinel-facies depleted lherzolite are sharp. The dunite body is composed of spinel-rich dunite containing centimetre-size lenses of relict Cr-diopside websterite, spinel-poor granoblastic dunite and virtually monomineralic Cr-spinel layers exhibiting flow structures. Orthopyroxene is a minor, relict phase in all the lithologies; clinopyroxene is intergranular and amphibole is a minor accessory phase. Overall the dunite body is fairly refractory (Fo in olivine: 90·7–93·8). Strontium and neodymium isotope ratios of clinopyroxene separates from the dunitic body resemble those of a Cr-diopside websterite suite that forms a series of dykes cutting the main peridotite host. It is proposed that the dunites were generated in a part of the mantle veined by early Cr-diopside websterites by a three-stage process involving partial melting of pyroxenite, infiltration of the pyroxenite-derived melt into the depleted lherzolite and its consequent open-system partial melting and focused flow of the resultant partial melts leading to the production of reactive dunite channels through both peridotite and pyroxenite. This process has been simulated using pMELTS assuming that the pyroxenite partially melts at 1·5 GPa and focused melt transport occurs at pressures greater than 0·7 GPa. The results show that, depending on the focusing factor assumed, dunite can form from peridotite at P < 1·2 GPa and from pyroxenite at P < 1·1 GPa, in both cases over a large pressure range. The model accounts for specific characteristics of the dunite, such as its refractory composition, the presence of orthopyroxene relics, the occurrence of relict websterite lenses in the spinel-rich dunites and the flow structures in the Cr-spinel layers. The proposed mechanism allows dunite formation to occur well within the spinel stability field, and therefore at greater depth than dunites in ophiolites, which generally formed within the plagioclase stability field. The aggregated model melts extracted from the segments where dunite forms are high-Mg alkali basalts resembling, after olivine fractionation, the compositions of enriched-type mid-ocean ridge basalt from slow- and ultraslow-spreading ocean ridges.

Maurizio Mazzucchelli; Giorgio Rivalenti; Daniele Brunelli; Alberto Zanetti; Elena Boari ( 2009 ) - Formation of Highly-refractory dunite by focused percolation of pyroxenite.derived melt in the Balmuccia spinel peridotite massif (Italy) - Workshop on Alpine Ophiolites and Modern Analogues, Parma 2009 - pp. da 1 a 1 ISSN: - [Poster (275) - Poster]
Abstract

abstract

GODARD M.; AWAJI S.; HANSEN H.; HELLEBRAND E.; BRUNELLI D.; JOHNSON K.; YAMASAKI T.; MAEDA J.; ABRATIS M.; CHRISTIE D.; KATO Y.; MARIET C.; ROSNER M. ( 2009 ) - Geochemistry of a long in-situ section of intrusive slow-spread oceanic lithosphere: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge) - EARTH AND PLANETARY SCIENCE LETTERS - n. volume 279 - pp. da 110 a 122 ISSN: 0012-821X [Articolo in rivista (262) - Articolo su rivista]
Abstract

IODP Site U1309 was drilled at Atlantis Massif, an oceanic core complex, at 30°N on the Mid-Atlantic Ridge (MAR). We present the results of a bulk rock geochemical study (major and trace elements) carried out on 228 samples representative of the different lithologies sampled at this location.Over 96% of Hole U1309D is made up of gabbroic rocks. Diabases and basalts cross-cut the upper part of the section; they have depleted MORB compositions similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic and geochemical evidence of extensive melt–rock interactions. Gabbroic rocks comprise: olivine-rich troctolites (> 70% modal olivine) and troctolites having high Mg# (82–89), high Ni (up to 2300 ppm) and depleted trace element compositions (Yb 0.06–0.8 ppm); olivine gabbros and gabbros (including gabbronorites) with Mg# of 60–86 and low trace element contents (Yb 0.125–2.5 ppm); and oxide gabbros and leucocratic dykes with low Mg# (< 50), low Ni ( 65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros are amongst the most primitive and depleted oceanic gabbroic rocks. The main geochemical characteristics of Site U1309 gabbroic rocks are consistent with a formation as a cumulate sequence after a common parental MORB melt, although (lack of systematic) downhole variations indicate that the gabbroic series were built by multiple magma injections. In detail, textural and geochemical variations in olivine-rich troctolites and gabbronorites suggest chemical interaction (assimilation?) between the parental melt and the intruded lithosphere.Site U1309 gabbroic rocks do not represent the complementary magmatic product of 30°N volcanics, although they sample the same mantle source. The bulk trace element composition of Site U1309 gabbroic rocks is similar to primitive MORB melt compositions; this implies that there was no large scale removal of melts from this gabbro section. The occurrence of such a large magmatic sequence implies that a high magmatic activity is associated with the formation of Atlantis Massif. Our results suggest that almost all melts feeding this magmatic system stays trapped into the intruded lithosphere.

D. Brunelli; E. Bonatti; A. Cipriani; M. Ligi; M. Seyler; L. Ottolini ( 2009 ) - Invited talk: Scale and nature of the mantle heterogeneities at the Vema Lithospheric Section (Mid Atlantic Ridge 11°N) - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 90(52) [Abstract in rivista (266) - Abstract in Rivista]
Abstract

A 26 Ma-long mantle stretch has been sampled at the Vema Lithospheric Section (VLS) (11° N along the MAR), providing a unique opportunity to analyze inside the mantle heterogeneities and unravel their effect on the melting regime looking at correlations between rock geochemistry and the amount of melt produced (crustal thickness). The geochemistry of mantle residua sampled at the base of the VLS reveals an overall positive correlation with the magmatic crustal thickness, in that suggesting a thermal/compositional control of the melting process. However short (sample-) scale heterogeneities appear when considering the difference between the degree of melting inferred from crustal thickness (integrated extracted melt) and the actual degree of melting calculated per each sample on the base of major and trace elements. Accordingly, the isotopic systematics of the peridotites show a sample scale strong variability, averaging close to the MORB value at the Ma scale. The observed discrepancy reveals that some samples are more depleted than the expected average degree of melting, suggesting the presence of mantle parcels that have undergone an ancient depletion event prior the upwelling and incorporation into the subridge melting region. Less depleted parcels are also present. These blobs are possibly the result of spinel-field refertilization at the end of the melting region. However, an intriguing inverse correlation between isotopic and major element systematics suggests that even such heterogeneities could have formed beneath the melting region possibly by local metasomatism that heterogeneously affected the deep mantle. Pyroxenite-derived melts may have reacted and refertilized some mantle portions then driving the subsequent melting process in the main melting region. This process can explain the paradox that the isotopically most depleted samples (those less metasomatised) are also those that record the lowest degree of melting beneath the ridge axis. A major, first-order, heterogeneity appears in the composition of the residual phases allowing to recognize a source discontinuity that cannot be related to the subridge melting event. Mantle samples older than 18.5 Ma have higher cpx Na and Al and lower Ca for a given Mg value than younger samples. Both domains are characterized by a normal mantle depletion trend correlating overall with the produced magmatic crust.

H. Belayouni; D. Brunelli; R. Clocchiatti; A. Di Staso; I.E.A. Hassani; F. Guerrera; S. Kassaa; N. Laridhi Ouazaa; M.M. Martín; F. Serrano; M. Tramontana ( 2009 ) - La Galite Archipelago (Tunisia, North Africa): new geological data and insights for geodynamic evolution of the Maghrebian Chain (African-European plate boundary) - JOURNAL OF AFRICAN EARTH SCIENCES - n. volume 56 - pp. da 15 a 28 ISSN: 1464-343X [Articolo in rivista (262) - Articolo su rivista]
Abstract

The location of the La Galite Archipelago on the Internal/External Zones of the Maghrebian Chain holds strong interest for the reconstruction of the geodynamic evolution of the Mesomediterranean Microplate-Africa Plate Boundary Zone.New stratigraphic and petrographic data on sedimentary successions intruded upon by plutonic rocks enabled a better definition of the palaeogeographic and palaeotectonic evolutionary model of the area during the early-middle Miocene.The lower Miocene sedimentary units (La Galite Flysch and Numidian-like Flysch) belong to the Mauritanian (internal) and Massylian (external) sub-Domains of the Maghrebian Chain, respectively. These deposits are related to a typical syn-orogenic deposition in the Maghrebian Flysch Basin Domain, successively backthrusted above the internal units. The backthrusting age is post-Burdigalian (probably Langhian-Serravallian) and the compressional phase represents the last stage in the building of the accretionary wedge of the Maghrebian orogen. These flysch units may be co-relatable to the similar well-known formations along the Maghrebian and Betic Chains.The emplacement of potassic peraluminous magmatism, caused local metamorphism in the Late Serravallian-Early Tortonian (14–10 Ma), after the last compressional phase (backthrusting), during an extensional tectonic event. This extensional phase is probably due to the opening of a slab break-off in the deep subduction system.La Galite Archipelago represents a portion of the Maghrebian Flysch Basin tectonically emplaced above the southern margin of the “Mesomediterranean Microplate” which separated the Piemontese-Ligurian Ocean from a southern oceanic branch of the Tethys (i.e. the Maghrebian Flysch Basin).The possible presence of an imbricate thrust system between La Galite Archipelago and northern Tunisia may be useful to exclude the petroleum exploration from the deformed sectors of the offshore area considered.

M. Ligi; E. Bonatti; D. Brunelli; A. Cipriani ( 2009 ) - Present day versus Temporal Heterogeneity of the Subridge Mantle in the Central Atlantic - EOS - n. volume 90 - pp. da 1 a 1 ISSN: 0096-3941 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

Studies of MORB and of MORP (Mid Ocean Ridge Peridotite) show that the oceanic upper mantle is heterogeneous. Long and short wavelength chemical variability has been reported from the Mid Atlantic Ridge. The subridge mantle degree of melting, estimated from MORP mineral chemistry of mantle equilibrated spinel, opx and cpx, as well as from MORB glasses, decreases from the Azores swell at ~40ο N to the equator, in parallel to the decrease of crustal thickness inferred from the near zero-age Mantle Bouguer anomaly. The temporal evolution of this portion of a slow-spreading mid ocean ridge can be studied along seafloor spreading flowlines normal to ridge segments. A lithospheric section exposed between 10ο and 11ο N, south of the Vema Fracture Zone is giving us the opportunity to study how generation of lithosphere at a 80-km long ridge segment evolved since 25 Ma. Gravity data, and MORP-MORB chemistry correlations suggest a steady increase in crustal thickness and in mantle degree of melting from 20 Ma to Present, with superimposed 3-4 Ma oscillations. Variations of degree of melting, peridotite temperature of equilibration and crustal thickness are not proportional to spreading rate, suggesting variations in mantle source composition and a non-purely passive lithosphere formation at ridge axis. The combination of the zero-age axial ridge trend with the 20 Ma to Present trend could be explained by a subaxial hot/fertile mantle flow from the Azores swell toward an equatorial “cold” belt. The equatorial belt, characterized by long offset transforms, is magma starved and nearly free of basaltic crust. Small quantities of basalt, generated mostly in the garnet stability region by low degrees of partial melting, could not be expelled from the mantle, and froze to form a “constipated”, crust free lithosphere. Numerical calculations show a strong decrease of crustal production as a ridge approaches a transform, proportional to slip rate and offset length. When the amount of melt produced is small, the capability of melt to aggregate and to mix on the way to the surface is low; therefore, the chemical and isotopic signatures of preaggregated melts can be detected. Therefore, long-offset RTI's are potential areas to detect short-scale mantle heterogeneities through the study of MORB. Accordingly, basalts sampled in the equatorial Atlantic close to the Romanche transform, show an increase in trace element variability within a single dredge haul approaching the RTI, with wavelengths comparable to those observed in suites of melt inclusions. The correlation between MORB trace elem

D. Brunelli; E. Bonatti; A. Cipriani; M. Ligi; M. Seyler; L. Ottolini ( 2009 ) - Scale and nature of the mantle heterogeneities at the Vema Lithospheric Section (Mid Atlantic Ridge 11°N) - EOS - n. volume 90 - pp. da 1 a 1 ISSN: 0096-3941 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

A 26 Ma-long mantle stretch has been sampled at the Vema Lithospheric Section (VLS) (11° N along the MAR), providing a unique opportunity to analyze inside the mantle heterogeneities and unravel their effect on the melting regime looking at correlations between rock geochemistry and the amount of melt produced (crustal thickness). The geochemistry of mantle residua sampled at the base of the VLS reveals an overall positive correlation with the magmatic crustal thickness, in that suggesting a thermal/compositional control of the melting process. However short (sample-) scale heterogeneities appear when considering the difference between the degree of melting inferred from crustal thickness (integrated extracted melt) and the actual degree of melting calculated per each sample on the base of major and trace elements. Accordingly, the isotopic systematics of the peridotites show a sample scale strong variability, averaging close to the MORB value at the Ma scale. The observed discrepancy reveals that some samples are more depleted than the expected average degree of melting, suggesting the presence of mantle parcels that have undergone an ancient depletion event prior the upwelling and incorporation into the subridge melting region. Less depleted parcels are also present. These blobs are possibly the result of spinel-field refertilization at the end of the melting region. However, an intriguing inverse correlation between isotopic and major element systematics suggests that even such heterogeneities could have formed beneath the melting region possibly by local metasomatism that heterogeneously affected the deep mantle. Pyroxenite-derived melts may have reacted and refertilized some mantle portions then driving the subsequent melting process in the main melting region. This process can explain the paradox that the isotopically most depleted samples (those less metasomatised) are also those that record the lowest degree of melting beneath the ridge axis. A major, first-order, heterogeneity appears in the composition of the residual phases allowing to recognize a source discontinuity that cannot be related to the subridge melting event. Mantle samples older than 18.5 Ma have higher cpx Na and Al and lower Ca for a given Mg value than younger samples. Both domains are characterized by a normal mantle depletion trend correlating overall with the produced magmatic crust.

A. Cipriani; E. Bonatti; D. Brunelli; M. Ligi ( 2009 ) - 26 Million Years of Mantle Upwelling Below a Segment of the Mid Atlantic Ridge: the Vema Lithospheric Section Revisited - EARTH AND PLANETARY SCIENCE LETTERS - n. volume 285 - pp. da 87 a 95 ISSN: 0012-821X [Articolo in rivista (262) - Articolo su rivista]
Abstract

Temporal variations of temperature and composition of the mantle upwelling below a 80-km long segment of the Mid Atlantic Ridge were reconstructed from 20 to 4 Ma ago from peridotites sampled along a > 300-km long section of oceanic lithosphere (Vema Lithospheric Section or VLS) exposed south of the Vema transform at 11° N [Bonatti, E., Ligi, M., Brunelli, D., Cipriani, A., Fabretti, P., Ferrante, V., Gasperini, L., Ottolini, L., 2003. Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variation in the formation of oceanic lithosphere, Nature, 423, 499–505]. We extended this time interval from 26 to 2 Ma by sampling mantle ultramafics at 18 new sites along the VLS. Peridotite orthopyroxene, clinopyroxene and spinel chemistry suggest a weak trend of decreasing extent of melting of the mantle from 26 to 18.5 Ma ago with superimposed short-wavelength (~ 4 Ma) oscillations followed by a steady increase of degree of melting from 18.5 to 2 Ma ago, with superimposed 3–4 Ma oscillations. Temporal variations of crustal thickness inferred from the Residual Mantle Bouguer Anomaly calculated from gravity data reveal similar trends. The older (26 to 18.5 Ma) and the younger (18.5 to 2 Ma) mantle suites differ in cpx Na2O content and CaO/Al2O3 ratio, suggesting that not only the thermal regime, but also the composition of the mantle source might have been different in the two suites. The two trends are separated by a ~ 1.4 Ma-long stretch (from 18.2 to 16.8 Ma) where deformed ultramafic mylonites prevail, indicating probably an interval of nearly a-magmatic lithospheric emplacement at ridge axis, corresponding to a thermal minimum. Spatially offset correlation along the VLS of crustal thickness (i.e., quantity of basaltic melt released by the mantle) and mantle peridotite degree of melting led to an estimate of ~ 16.1 mm/a for the solid mantle average velocity of upwelling, a value close to the average half spreading rate for the 26 Ma interval covered by the VLS. However, peridotite clinopyroxene geothermometry shows oscillations superimposed on a decreasing trend of calculated equilibration temperature from 26 to 18.5 Ma ago, followed by a steady increase from 16 to 4 Ma ago, suggesting that the solid mantle upwelling velocity varied through time. These results hint at the existence of ~ 10–20 Ma cycles in the activity of the northern Mid Atlantic Ridge.

Paganelli E.; Brunelli D.; Bonatti E.; Cipriani A.; Ligi M. ( 2008 ) - Multi-stage impregnation of the lithospheric mantle at the Andrew Bain FZ (SWIR) - 2008 AGU Fall Meeting - EOS - n. volume 89(53) [Poster (275) - Poster]
Abstract

The Southern ridge-transform intersection of Andrew Bain Fracture Zone (ABFZ) is interpreted as a "cold spot" in the mid-ocean ridge system being characterized by a negative thermal anomaly in the oceanic upper mantle. The negative thermal anomaly is associated to the cold-edge effect due to the great age contrast of the active ridge segments. During the oceanic expedition AB06-S23, in 2006, (organized by ISMAR-CNR, Bologna, Italy, and co-financed by PRNA, Italy) with the russian R/V N. Strakhov, several samples of abyssal peridotites have been collected. Textures and modal distribution of the samples have been investigated revealing a multistage impregnation history. Deep spinel-field impregnation assemblages (sp+cpx‚±opx‚±ol) are followed by plagioclase-field patches and mineral trails (pl+cpx‚±ol) and late shallow gabbroic pockets and veins. The major elements mineral chemistry reveals compositional trends of low-P/T subsolidus partial- to-complete re-equilibration undergone by the upper mantle during the upwelling beneath the ridge. These samples have experienced variable degrees of melting and reacted with percolating melts of possible different composition. In particular, samples showing the lowest degrees of melting have interacted with MORB-like melts and pyroxenitic-derived melts in the spinel and plagioclase stability fields. The presence of these two kinds of melts might prove the presence of enriched portions scattered in a normal depleted mantle beneath ocean ridges. MELTS-based runs provide constraints to variable extents of pyroxenitic-derived melt interaction with the mantle source and crystallization at variable depth of the products of such an interaction. Supported by MIUR-PRIN Cofin project 2007

Ligi M.; Bonatti E.; Bortoluzzi G.; Brunelli D.; Caratori Tontini F.; Cipriani A.; Cocchi L.; Cuffaro M.; Ferrante V.; Khalil S.; Mitchell N C.; Rasul N.; Schettino A. ( 2008 ) - Sea-floor spreading initiation: constraints from geophysical data of the Thetis Deep, northern Red Sea - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 89 (53) [Abstract in rivista (266) - Abstract in Rivista]
Abstract

A major step in the "Wilson Cycle" is the splitting of a continent and the birth of a new ocean, with the consequent formation of passive plate margins. The transition from a continental to an oceanic rift can be observed today nowhere better than in the Red Sea/Gulf of Aden system. We have carried out during several years a number of expeditions in the axial portion of the Northern Red Sea, in the region where the northernmost nuclei of axial emplacement of oceanic crust can be observed. High resolution multibeam, magnetics, gravity and multichannel seismic reflection surveys from the Thetis Deep revealed rates and modes of initial pulses of sea floor spreading, velocity of S to N axial propagation of the oceanic rift, evolution of initial MORB-type crust and nature of the mantle thermal anomaly that caused the transition from a continental to an oceanic rift. The Thetis deep is made of three en echelon fault-bounded axial basins that are joined together with axial volcanic ridges and a large number of scattered small central volcanoes. The southern basin shows a strong linear magnetic anomaly corresponding to the axial neo-volcanic zone. Two negative symmetric anomalies identified as Matuyama are present in the southernmost part of this basin, suggesting that the emplacement of oceanic crust at this site started roughly 2.5 Ma, with an average half spreading rate of 6 mm/yr. The central sub-basin is also characterized by a strongly magnetic linear neo- volcanic zone that, however, is flanked only by a small, "vanishing" symmetrical negative anomaly suggesting emplacement of oceanic crust not earlier than about 1 Ma. The northern sub-basin does not show a clearly defined linear neo-volcanic zone although it displays a strong central magnetization suggesting initial emplacement of oceanic crust < 0.7 Ma. This pattern implies a south to north time progression of the initial emplacement of oceanic crust within the Thetis system, with a propagation rate of about 20 mm/yr. Gravity data inversions constrained by seismic data reveal that the oceanic crust extends from the axial neo-volcanic ridges toward the master faults of the axial depression with crustal thickness ranging from 4 to 6 km. The increasing thickness of basaltic crust toward the edges of the basin together with higher degree of melting, inferred by the geochemistry of the basaltic glasses, and higher central magnetization of the northernmost and youngest basin suggest a pulse of faster spreading rate at the onset of sea-floor spreading.

SUHR G.; HELLEBRAND E.; JOHNSON K.T.M.; BRUNELLI D. ( 2008 ) - Stacked gabbro units and intervening mantle: A detailed look at a section of IODP Leg 305, Hole U1309D - GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS - n. volume 9 - pp. da 1 a 31 ISSN: 1525-2027 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Hole U1309D (Integrated Ocean Drilling Program (IODP) Legs 304/305) penetrated 1415 m into the seafloor of the Atlantis Massif, an oceanic core complex at 30°N, Mid-Atlantic Ridge. More than 96% of all recovered rocks are gabbroic. On the basis of a mineral chemical overview, we suggest that between ≤800 and 1100 m below sea floor (mbsf), a magmatic unit occurs, ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part. Below 1235 mbsf, massive gabbronorites/oxide gabbros were drilled and they may represent the roof of an underlying magmatic unit. The focus here is on the zone where both units interact and screens, totaling 50 m, of a microstructurally distinct, olivine-rich troctolite occur. We argue that the olivine-rich troctolite is a former mantle rock which was converted to a crust-mantle transition zone dunite at the base of the upper magmatic unit. Later, as melts derived from the lower magmatic unit percolated through it, it was equilibrated to a more evolved chemistry and transformed to a fine-grained, olivine-rich troctolite. Our main arguments against a possible cumulate nature of the olivine-rich troctolite are the lack of a systematic downhole trend in compatible elements within the olivine-rich troctolite, its distinctly fine-grained microstructure, the high Cr content of cpx, and its Ni-rich olivine composition. The high NiO for a given Mg/(Mg + Fe) in the olivine-rich troctolite can be modeled by simple equilibration of relict mantle olivine with a mildly evolved melt. Evidence for the percolation of evolved melts through the olivine-rich troctolites are Ti-rich, interstitial pyroxenes and, as inclusions in Cr-spinel, highly evolved amphiboles and orthopyroxenes plus the occurrence of millimeter-scale noritic veins. The percolation by evolved melts would also be the major difference to otherwise conceptually similar rocks from the ophiolitic crust-mantle transition zone.

Suhr G.; Johnson K T M.; Hellebrand E.; Brunelli D. ( 2008 ) - Stacked Gabbro Units Convert Intervening Mantle to Troctolite in Hole U1309D, IODP Expedition 304/305. - 2008 AGU Fall Meeting - EOS - n. volume 89(53) [Poster (275) - Poster]
Abstract

IODP Hole U1309D (Exp. 304/305) penetrated 1415 m into the seafloor of the Atlantis Massif, an oceanic core complex at 30N, Mid-Atlantic Ridge. More than 96% of all recovered rocks are gabbroic. Based on petrographic and mineral chemical data, we suggest that between <800 and 1100 meters below seafloor (mbsf) a magmatic unit occurs, ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part. Below 1235 mbsf, massive gabbronorites/oxide gabbros were drilled, which may represent the roof of a separate underlying magmatic unit. The focus of this study is on the zone where these units interact and screens of a microstructurally distinct, 50 m thick olivine-rich troctolite occur. We argue that the olivine-rich troctolite is a former mantle rock which was converted to a crust-mantle transition zone dunite at the base of the upper magmatic unit. Later, as melts derived from the lower magmatic unit percolated through it, it was equilibrated to a more evolved chemistry and transformed to a fine grained, olivine-rich troctolite. Our main arguments against a possible cumulate nature of the olivine-rich troctolite are the lack of any systematic down-hole trend in compatible elements within the olivine-rich troctolite, its distinctly fine-grained microstructure, the high Cr content of cpx, and its Ni-rich olivine composition. The high NiO for a given Mg/(Mg+Fe) in the olivine-rich troctolite can be modeled by simple equilibration of relict mantle olivine with a somewhat evolved melt. Evidence for the percolation of evolved melts through the olivine-rich troctolites are Ti-rich, interstitial pyroxenes, highly evolved amphiboles and orthopyroxenes as inclusions in Cr-spinel plus the occurrence of mm-scale noritic veins. The percolation by evolved melts would also be the major difference to otherwise similar rocks from ophiolitic crust-mantle transition zones.

Ligi Marco; Bonatti Enrico; Brunelli Daniele; Cipriani Anna ( 2007 ) - Accretion at a Mid Atlantic Ridge Segment: a 25 million years long story - VI Forum - FIST Udine ITA) - EPITOME - n. volume 2 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

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Brunelli D; Bonatti E; Cipriani A; Grindlay N R; Ligi M; Paganelli E; Sclater J ( 2007 ) - Crust-Poor Lithosphere at Cold Spots in the Mid Atlantic and SW Indian Ridges - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 88 (52) [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The Equatorial portion of the Mid Atlantic Ridge is thought to reflect a thermal minimum in the subridge structure, with deeper than normal axial topography underlain by high upper mantle seismic velocities revealed by tomography. This stretch of Ridge is intersected by a number of long offset transforms, the longest being the Romanche (offset ~950 km corresponding to ~50 Myr). As the Mid Atlantic Ridge axis approaches the Romanche transform from the south, it gradually deepens; its rift valley disappears, and, starting roughly 50 km from the transform, the basaltic crust becomes patchy and then disappears, leaving mantle ultramafics outcropping on the sea floor. Modelling the "cold edge" effect of the transform on the axial Ridge segment shows that partial melting of the subridge mantle decreases as the transform is approached. Crust-free lithosphere outcrops continuously for several hundred kilometers in a ~30 km wide belt south of the Romanche, indicating that the present-day lack of crustal production has been prevailing for at least 30 million years. The mantle derived serpentinized peridotites are of two types. The majority of the samples show evidence of strong impregnation by basaltic melts. The mineral chemistry of the samples that are free of impregnation suggests that they have undergone a very low degree of melting. These results suggest a quasi-crust-free lithosphere, produce by a mantle that has undergone little or no partial melting, unable to expel the small quantities of melt it generates. The small quantities of basalt produced in this area tend to have alkali affinity and are strongly enriched in H2O. Their REE content show a strong garnet signature, suggesting that they were generated mostly in the garnet peridotite mantle zone (> 20 kbar). This quasi-crust-free impregnated lithosphere probably prevails at cold spots along mid ocean ridges. Peridotites obtained recently from the SW end of the Andrew Bain transform, that offsets the SW Indian Ridge by ~750 km (~50 Myr), are strongly impregnated by basaltic melt, in a situation very similar to that observed near the Romanche. In contrast, peridotites from the NE end of the Andrew Bain transform are not impregnated, and are residual of a significant degree of melting, probably due to the influence of the Marion plume located a few hundred km away.

Godard M; Abratis M; Awaji S; Brunelli D; Christie D; Hansen H; Hellebrand E; Johnson K; Maeda J; Yamasaki T; Kato Y; ( 2007 ) - Geochemistry of a long in-situ section of intrusive slow-spread crust: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge) - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 88 (52) [Abstract in rivista (266) - Abstract in Rivista]
Abstract

IODP Site U1309 was drilled at Atlantis Massif (western rift flank of Mid-Atlantic Ridge (MAR) 30°N; Expeditions 304 and 305), a 1.5-2 Myr old oceanic core complex. The main hole, Hole U1309D, is the second deepest hole drilled into an intrusive slow-spread oceanic lithosphere: it penetrated 1415.5 mbsf (75% recovery). We present here the results of a bulk rock geochemical study (major and trace elements – ICPMS -) carried out on 234 samples representative of the different lithologies sampled at Site U1309. Over 96% of Hole U1309D is made up of gabbroic rocks, cross-cut by late diabases and basaltic dykes in the upper part of the section. Diabases and basalts have depleted MORB compositions (La/Yb ~0.8 and Yb ~ 3.5 ppm) similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic evidence of melt impregnation. They have relatively fertile compositions, similar to MARK peridotites, with Mg# (100xMg/(Mg+Fe)) of 89-90, Ni>2400ppm and Yb 0.03-0.11 ppm. Gabbroic rocks span a wide range of lithologies and geochemical compositions. They comprise olivine-rich troctolites (>70% modal olivine), troctolites, olivine and troctolitic gabbros (5 to 50% modal olivine), gabbros (including gabbronorites) and oxide gabbros (>2% modal Fe-Ti oxides), which represent respectively 5.4 %, 2.7%, 25.5%, 55.7% and 7% of the core recovered at Hole U1309D. Minor felsic ("leucocratic") dikes cross- cutting gabbros were also sampled. Troctolites and olivine-rich troctolites have high Mg# (82-89), high Ni (up to 2300 ppm) and low trace element contents (Yb 0.06-0.8 ppm). They overlap in composition with peridotites sampled at Atlantis Massif and with impregnated peridotites drilled along the MAR (e.g., ODP Site 1271 (MAR 15°20'N)). Gabbros and olivine gabbros have high Mg# (60-86) and low trace element contents (Yb 0.125-2.5 ppm – (La/Yb)CN ~ 0.4-0.7); these gabbros are among the most primitive and depleted yet sampled along slow spreading ridges (e.g., MAR 23°N and 15°20'N and ODP Hole 735B on the Southwest Indian Ridge). Oxide gabbros and leucocratic dykes represent the most evolved end-members of the gabbroic suite with low Mg# (<50), low Ni (~65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros have Eu positive anomalies (Eu/Eu* ~1.4) whereas oxide gabbros and leucocratic dykes have Eu negative anomalies (Eu/Eu* ~0.75). The former are interpreted as cumulates of a common parental magma, while the later precipitated after the differentiated melts. We note that gabbros and gabbronorites overlap in composition, thus precluding precipitation after a significantly more evolved melt for orthopyroxene crystallization. The bulk composition of gabbroic rocks from Hole 1309D shows no Sr or Eu anomalies; although there may have been local separation of melt and solids, there was no large scale removal of melts from this gabbro section. It is somewhat depleted in the most incompatible elements compared to parental MORB with Ba (40%), Nb (40%) and Rare Earth Elements (REE: Ce 70%, Yb 95%), but enriched in compatible and refractory elements such as Mg (120%) or Ni (200%) and Cr (200%). This may indicate strong chemical interaction between the parental melt and the intruded (assimilated?) shallow depleted mantle, probably during the earliest stages of the formation of Site U1309 gabbroic body. Olivine-rich troctolites may represent the ultimate residue of this reaction- accumulation process.

PEYVE A.A.; SKOLOTNEV S.G.; LIGI M.; TURKO N.N.; BONATTI E.; KOLODYAZHNYI S.Y.; CHAMOV N.P.; TSUKANOV N.V.; BARAMYOKOV Y. E.; ESKIN A.E.; GRINDLAY N.; SCLATER J.G.; BRUNELLI D.; PERTSEV A.N.; CIPRIANI A.; BORTOLUZZI G.; MERCURI R.; PAGANELLI E.; MUCCINI F.; TAKEUCHI C.; ZAFFAGNINI F.; DOBROLYUBOVA K.O.; ( 2007 ) - Investigation of the Andrew Bain transform fault zone (African-Antartctic Region) - DOKLADY EARTH SCIENCES - n. volume 416 - pp. da 991 a 998 ISSN: 1028-334X [Articolo in rivista (262) - Articolo su rivista]
Abstract

The Andrew Bain FZ is among the longest oceanicfaults with the active part extending over about 750 km.Since this mid-oceanic ridge (MOR) region is characterizedby very low spreading rates (16 mm/yr), itsactive part is one of the oldest in the whole MOR system. The study of the fault is of great importance forunderstanding the geodynamics and evolution of Circum-Antarctic regions of the World Ocean. Theresults of previous bathymetric and magnetic investigationsand the spatial distribution of earthquakes showedthat the Andrew Bain FZ includes numerous differentsecond-order structures. This feature is typical of continentalstrike-slip fault zones. Such megatransform fault zones can emerge in slowspreading ridges at a relative displacement of the lithosphere, which is thicker and colder than in most transformfaults. The aim of the present work was to substantiatethis suggestion with factual material.

ILDEFONSE B.; BLACKMAN D.; JOHN B.E.; OHARA Y.; MILLER D.J.; MACLEOD C.J.; ABE N.; ABRATIS M.; ANDAL E.S.; ANDRÉANI M.; AWAJI S.; BEARD J.S.; BRUNELLI D.; CHARNEY A.B.; CHRISTIE D.M.; DELACOUR A.G.; DELIUS H.; DROUIN M.; EINAUDI F.; ESCARTIN J.; FROST B.R.; FRYER P.B; GEE J.S.; GODARD M.; GRIMES C.B.; HALFPENNY A.; HANSEN H-E.; HARRIS A.C.; HASEBE A.T.; HAYMAN N.W.; HELLEBRAND E.; HIROSE T.; HIRTH J.G.; ISHIMARU S.; JOHNSON K.T.M.; KARNER G.D.; LINEK M.; MAEDA J.; MASON O.U.; MCCAIG A.M.; MICHIBAYASHI K.; MORRIS A.; NAKAGAWA T.; NOZAKA T.; ROSNER M.; SEARLE R.C.; SUHR G.; TOMINAGA M.; VON DER HANDT A.; YAMASAKI T.; ZHAO X. ( 2007 ) - Oceanic core complexes and crustal accretion at slow-spreading ridges. - GEOLOGY - n. volume 35 - pp. da 623 a 626 ISSN: 0091-7613 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Oceanic core complexes expose gabbroic rocks on the seafl oor via detachment faulting, often associated with serpentinized peridotite. The thickness of these serpentinite units is unknown.Assuming that the steep slopes that typically surround these core complexes provide a cross section through the structure, it has been inferred that serpentinites compose much of thesection to depths of at least several hundred meters. However, deep drilling at oceanic core complexes has recovered gabbroic sequences with virtually no serpentinized peridotite. Wepropose a revised model for oceanic core complex development based on consideration of the rheological differences between gabbro and serpentinized peridotite: emplacement of alarge intrusive gabbro body into a predominantly peridotite host is followed by localization of strain around the margins of the pluton, eventually resulting in an uplifted gabbroic coresurrounded by deformed serpentinite. Oceanic core complexes may therefore refl ect processes associated with relatively enhanced periods of mafic intrusion within overall magma-poorregions of slow- and ultra-slow-spreading ridges.

Cipriani A.; Ligi M; Bonatti E; Brunelli D ( 2007 ) - Solid mantle upwelling rate beneath the Mid Atlantic Ridge - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 88 (52) [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The upwelling velocity of the solid mantle beneath mid ocean ridges affects processes of melting and generation of the oceanic crust, and constrains models of spreading of oceanic plates. Models of passive flow require that the solid mantle rises beneath a ridge at a speed similar to the half spreading rate of the plates. However, increased buoyancy due to melt depletion and melt retention may cause the sub-ridge mantle to rise at a speed much faster than the half spreading rate. In order to constrain these models it is important to estimate the sub- ridge mantle up-welling rate. Such estimates have been attempted in a few cases by measuring in zero-age basalt disequilibria in short lived isotopes produced by the U-decay series. Melt upward migration from the sub-ridge melting zone to the crust is much faster than its parent solid mantle upward flow; therefore, a time lag is created between melt emplacement as basalt in upper lithosphere and emplacement of the parent residual peridotite in the lower lithosphere. We were able to estimate this time lag along a lithospheric sliver, generated at a 80 km long segment (EMAR) of the Mid Atlantic Ridge located between 10° N and 11°N, just south of the Vema transform. The northern edge of this lithospheric sliver exposes crustal and upper mantle units along a 320 km long spreading flowline equivalent to a 26 Myr long time interval. Comparing temporal variations of crustal thickness, inferred from the Residual Mantle Bouguer Anomaly along a flowline starting from the centre of the EMAR segment, to temporal variations of mantle degree of melting, estimated from mineral chemistry of peridotites exposed along the Vema lithospheric section, allowed us to estimate an average solid mantle rising rate of 16.5 mm/yr below the EMAR segment for a time interval from 26 to 2 Ma. This rate is slightly higher than the average half spreading rate of 15.6 mm/yr for the same period. The similarity between average up-welling rate and spreading rate at 11° N on the Mid Atlantic ridge is in line with up-welling estimates obtained by U-series disequilibria, supporting in general a mostly passive mantle flow model. However, temporal variations of spreading rate at 11° N for the last 26 Myr are decoupled from mantle degree of melting and geothermometry estimates, suggesting variations of mantle upwelling rates due to an active component.

RIVALENTI G; MAZZUCCHELLI M; BRUNELLI D; ZANETTI A; TOMMASINI S; BOARI E ( 2007 ) - The Dunites in the Balmuccia peridotite massif. - EPITOME - n. volume 2 - pp. da 112 a 112 ISSN: 1972-1552 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The Balmuccia massif is dominantly lherzolitic. Dunites are a minor lithotype occurring: a) at the contacts of websterite dykes; b) as tabular bodies associated with layers of Cr-rich spinel; c) as large, decametric lenses typically associated with hornblendite pockets and veins. Type a dunites are <20-30 cm thick and were previously studied by Rivalenti et al. (1995, Contrib. Mineral. Petrol. 121, 275-288), who inferred they were related to local peridotite depletion induced by the melt which fractionated the websterites.Type b dunites are up to 14 m thick and olivine is accompanied by clinopyroxene (fading into wehrlite) and Cr-rich spinel. Clinopyroxene clusters occur in specific horizons and they represent previous, disrupted Cr-diopside dykes. Massive Cr-spinel layers, up to ~8 cm thick, are concordant with the dunite foliation and fade into dunite by decreasing modal spinel. These dunites are discordantly cut by later Cr-diopside websterite dykes and have the highest Mg# values in their bulk rock and silicate phase (Mg# range 0.90 – 0.95 and 0.91 – 0.94 in clinopyroxene and olivine, respectively) and the highest Cr# values of spinel (up to 0.55) so far observed at Balmuccia. In the spinel layers and at their contacts the silicate phases are highly zoned, with increase of the Mg# value at their rim. A similar zoning also occurs, but is less extreme, in the dunites farther from spinel layers.By contrast, type c dunites have the lowest Mg# values found in the Balmuccia peridotites (Mg# down to 0.89 and 0.88 in clinopyroxene and olivine, respectively). In these dunites, the minerals accompanying olivine vary approaching hornblendite pockets. Far from pockets (> 15 cm), olivine coexists with clinopyroxene. At a distance of 10-13 cm, clinopyroxene coexists with a “pyribole” (a mineral constituted by a few microns-large lamellae of clinopyroxene and amphibole). At 7-10 cm distance, clinopyroxene disappears and the pyribole coexists with discrete amphibole (kaersutite) crystals. Closer to the pocket, pyribole disappears and only amphibole and spinel (high Ti, low Cr) are present along with olivine. Similar variations occur in thin veins cutting the dunite and ending at the pocket. The pockets, up to 40 cm large, are constituted by giant kaersutitic amphibole, phlogopite, plagioclase, rutile, Al-Mg spinel. Towards the ambient lherzolites, the dunite fades into wehrlite, where orthopyroxene abundance increases outwards and the outer belt is constituted by a granular, undeformed, opx-rich lherzolite.On the basis of petrography, Sr and Nd isotopes and trace element characteristics of clinopyroxene we preliminarily hypothesise that type b dunites are the result of Cr-diopside dykes melting, eventually triggered by percolation of deeper-derived melts, and reactive percolation of these melts into the ambient lherzolite under increasing melt mass conditions. Type c dunites may represent a relatively shallow event (as indicated by the plagioclase stability) of reactive percolation of an hydrous, possibly alkaline, melt under decreasing melt mass.

Rivalenti Giorgio; Mazzucchelli Maurizio; Brunelli Daniele; Zanetti Alberto; Tommasini Simone; Boari Elena ( 2007 ) - The dunites of the Balmuccia peridotitic massif - VI Forum - FIST Udine ITA) - EPITOME - n. volume 2 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

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Brunelli D.; Mével C.; Cannat M.; Meyzen C.; M.; Bezos A. ( 2007 ) - The roots of the magma plumbing system beneath a cold-thick oceanic lithosphere: magma fractionation and mixing along the South West Indian Ridge - VI Forum - FIST Udine ITA) - EPITOME - n. volume 2 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

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M. Ligi; E. Bonatti; N.R. Grindlay; J. Sclater; S. Skolotnev; A. Peyve; G. Bortoluzzi; D. Brunelli; A. Cipriani; R. Mercuri; F. Muccini; E. Paganelli; F. Zaffagnini; C. Takeuki; Y. Baramykov; N. Chamov; S. Erofeev; A. Eskin; S. Kolodyazhnyy; A. Pertsev; V. Semenov; V. Rastorgyev; N. Tsukanov; N. Turko; V. Yefimov and L. Zotov ( 2006 ) - ANDREW BAIN TRANSFORM: MULTIPLE CONTINENTAL-TYPE TRANSFORM BOUNDARIES AT MID-OCEAN RIDGE - Polar Ridges Meeting and Workshop - pp. da 1 a 1 ISSN: - [Poster (275) - Poster]
Abstract

abstract

D. BRUNELLI; SEYLER M; CIPRIANI A; BONATTI E. AND OTTOLINI L ( 2006 ) - Discontinuous melt extraction and weak refertilization of mantle peridotites at the Vema Lithospheric Section (Mid Atlantic Ridge) - JOURNAL OF PETROLOGY - n. volume 47 - pp. da 745 a 771 ISSN: 0022-3530 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Melting processes beneath the Mid-Atlantic Ridge were studiedin residual mantle peridotites sampled from a lithospheric sectionexposed near the Vema Fracture Zone at 11N along the Mid-Atlantic Ridge. Fractional and dynamic melting models were testedbased on clinopyroxene rare earth element and high field strengthelement data. Pure fractional melting (non-modal) cannot accountfor the observed trends, whereas dynamic melting with critical mass porosity <001 fits better the measured values. Observed microtextures suggest weak refertilization with 01–1% quasi-instantaneous or partially aggregated melts trapped during percolation. The composition of the melts is evaluated, together with their provenance, with respect to the garnet–spinel transition. Partial melts appear to be aggregated over short but variable intervals of the melting column.Deep melts (generated within the garnet stability field at the base of the melting column) escape detection, being separated from theresidues by transport inside conduits or fractures. The temporalevolution of the melting process along the exposed section shows a steady increase of mantle temperature from 20 Ma to present.

A. Cipriani; D. Brunelli; M. Ligi; E. Bonatti ( 2006 ) - From magma-engorged to magma starved “constipated” lithosphere: The Mid Atlantic Ridge from 40°N to the Equator - Polar Ridges - INTERRIDGE Pavia ITA) - n. volume 1 - pp. da 1 a 1 ISBN: 0000000000 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

abstract

Ligi M.; Bonatti E.; Bortoluzzi G.; Brunelli D.; Carminati E.; Cipriani A.; Ferrante V.; Redini F.; Barbino G. ( 2006 ) - Initial emplacement of Oceanic Lithosphere in the northern Red Sea - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 87 (52) [Abstract in rivista (266) - Abstract in Rivista]
Abstract

Multibeam, magnetometric and seismic reflection surveys were carried out in 2005 at the Thetis and Nereus axial deeps in the Northern Red Sea, together with bottom rock sampling. The Thetis Deep includes three en- echelon rift valleys that have joint together. Axial neovolcanic zones were observed in each of these rift valleys, together with large number of diffuse small volcanic cones. Both are made of fresh MORB type rocks. The walls of the rift valleys are formed by normal faulting with superimposed salt tectonics. Complex deformation patterns, partly also caused by salt tectonics, were observed at the transfer zone between Thetis and Nereus Deep to the north. We will present some results of this work, and their bearing on ideas on the initial emplacement of the oceanic lithosphere in the Northern Red Sea.

ILDEFONSE B.; BLACKMAN D.; JOHN B.E.; OHARA Y.; MILLER D.J.; MACLEOD C.J.; ABE N.; ABRATIS M.; ANDAL E.S.; ANDRÉANI M.; AWAJI S.; BEARD J.S.; BRUNELLI D.; CHARNEY A.B.; CHRISTIE D.M.; DELACOUR A.G.; DELIUS H.; DROUIN M.; EINAUDI F.; ESCARTIN J.; FROST B.R.; FRYER P.B; GEE J.S.; GODARD M.; GRIMES C.B.; HALFPENNY A.; HANSEN H-E.; HARRIS A.C.; HASEBE A.T.; HAYMAN N.W.; HELLEBRAND E.; HIROSE T.; HIRTH J.G.; ISHIMARU S.; JOHNSON K.T.M.; KARNER G.D.; LINEK M.; MAEDA J.; MASON O.U.; MCCAIG A.M.; MICHIBAYASHI K.; MORRIS A.; NAKAGAWA T.; NOZAKA T.; ROSNER M.; SEARLE R.C.; SUHR G.; TOMINAGA M.; VON DER HANDT A.; YAMASAKI T.; ZHAO X.; ( 2006 ) - IODP Expeditions 304 & 305 Characterize the Lithology,Structure, and Alteration of an Oceanic Core Complex - Springer Verlag Germany:Tiergartenstrasse 17, D 69121 Heidelberg Germany:011 49 6221 3450, EMAIL: g.braun@springer.de, INTERNET: http://www.springer.de, Fax: 011 49 6221 345229 ) - SCIENTIFIC DRILLING - n. volume 3 - pp. da 4 a 11 ISSN: 0934-4365 [Articolo in rivista (262) - Articolo su rivista]
Abstract

More than forty years after the Mohole Project (Bascom,1961), the goal of drilling a complete section through in situoceanic crust remains unachieved. Deep Sea Drilling Project– Ocean Drilling Program (DSDP-ODP) Hole 504B withinthe eastern Pacifi c (Alt et al., 1993) is the deepest hole everdrilled into ocean crust (2111 mbsf), but it failed to reachlower crustal plutonic rocks below the pillow basalts andsheeted dikes. IODP Expeditions 309 and 312 eventuallyrecovered the long-sought transition from sheeted dikes intounderlying gabbros by drilling into very fast-spreadingPacifi c crust (Wilson et al., 2006). The lithology and structureof oceanic crust produced at slow-spreading ridges areheterogeneous (e.g., Cannat et al., 1997) and offer uniquedrilling access to lower crust and upper mantle rocks. AfterODP Hole 735B penetrated 1500 m of gabbro at the SouthwestIndian Ridge (Dick et al., 2000), IODP Expeditions 304 and305 recently recovered just over 1400 m of little-deformed,gabbroic lower crust from a tectonic window along the slowspreading Mid-Atlantic Ridge.

ILDEFONSE B.; BLACKMAN D.; JOHN B.E.; OHARA Y.; MILLER D.J.; MACLEOD C.J.; ABE N.; ABRATIS M.; ANDAL E.S.; ANDRÉANI M.; AWAJI S.; BEARD J.S.; BRUNELLI D.; CHARNEY A.B.; CHRISTIE D.M.; DELACOUR A.G.; DELIUS H.; DROUIN M.; EINAUDI F.; ESCARTIN J.; FROST B.R.; FRYER P.B; GEE J.S.; GODARD M.; GRIMES C.B.; HALFPENNY A.; HANSEN H-E.; HARRIS A.C.; HASEBE A.T.; HAYMAN N.W.; HELLEBRAND E.; HIROSE T.; HIRTH J.G.; ISHIMARU S.; JOHNSON K.T.M.; KARNER G.D.; LINEK M.; MAEDA J.; MASON O.U.; MCCAIG A.M.; MICHIBAYASHI K.; MORRIS A.; NAKAGAWA T.; NOZAKA T.; ROSNER M.; SEARLE R.C.; SUHR G.; TOMINAGA M.; VON DER HANDT A.; YAMASAKI T.; ZHAO X. ( 2006 ) - Proceedings of the Integrated Ocean Drilling Program volume 304/305 - Integrated Ocean Drilling Program Management International, Inc. College Station USA) - pp. da 1 a 257 ISBN: 19301014; 19301022 | 19301022 ISSN: - [Monografia o trattato scientifico (276) - Monografia/Trattato scientifico]
Abstract

Integrated Ocean Drilling Program Expeditions 304 and 305 composed a two-expedition program at Atlantis Massif, Mid-Atlantic Ridge 30°N, designed to investigate the processes that control oceanic core complex formation and the exposure of ultramafic rocks in young oceanic lithosphere. Geophysical interpretations of unaltered mantle rock occurring <1 km below the seafloor suggested we could drill through an alteration front in ultramafic rocks and obtain fresh mantle peridotite. In order to accomplish these objectives, we drilled at two sites: one in the footwall through an exposed detachment fault and one in the hanging wall. Drilling the fractured basalt in the hanging wall was thwarted by difficulties, whereas drilling in the footwall was very successful. The main hole at Site U1309 penetrated 1415.5 meters below seafloor, and recovery averaged 75%. Igneous rocks recovered from the hole are the most primitive ever cored in slow-spreading ocean lithosphere and provide an exceptional record of magmatic and tectonic accommodation to extension in this environment. The core recovered was dominantly crustal rock types: basalt (~3%) and gabbroic (~91%). A series of olivine-rich rocks (~5%; dunites, wehrlites, troctolites), grouped as olivine-rich troctolites, part of which likely represent primitive cumulates, are interlayered with gabbroic rocks. A few thin mantle peridotite intervals are recognized in the upper 180 m of the section. Overall, the section is moderately altered at conditions ranging from granulite to zeolite facies. The rocks record initial alteration at low strains under granulite- and amphibolite-facies conditions and subsequent, more fully developed, lower temperature alteration (greenschist facies). Fault zone(s) comprising the detachment system must be highly localized to within tens of meters of the present-day seafloor. The existence of a fault at the top of the domal surface is supported by fragments of brecciated talc-tremolite fault schist and fractured metadiabase recovered in Hole U1309B and during the series of shallow cores drilled. Extensive amphibolite facies deformation is lacking, and high-strain ductile shear zones are rare. The absence of a thick, high-temperature ductile deformation zone in the footwall, and the apparent tectonic history (less rotation in the upper 180 m and variable rotations between several distinct, few-hundred meter sections downhole) suggested by paleomagnetic inclination measurements indicate complexity in structural evolution that differs from the simplest models of deeprooted detachment faults, predicted to be associated with high- temperature deformation, and with constant or monotonically varying footwall rotation with depth. Another challenge is that the central dome is clearly not an uplifted dominantly upper mantle section, as had been inferred prior to drilling. The exposures of peridotite along the southern wall of Atlantis Massif, the geophysical results suggesting at least portions of the dome contain fresh olivine-rich rock, and the downhole variability at Site U1309 all likely indicate significant lateral heterogeneity over short distances across the footwall.

D. Brunelli; R. Clocchiatti; A. Cipriani; M. Ligi; E. Paganelli; L. Ottolini and E. Bonatti ( 2006 ) - Snapshots fo the magma plumbing system from melt inclusions and residual peridotites of the Vema Lithospheric Section (MAR 11°N) - Polar Ridges - INTERRIDGE Pavia ITA) - n. volume 1 - pp. da 1 a 1 ISBN: 0000000000 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

abstract

E. Bonatti; G. Bortoluzzi; D. Brunelli; V. Ferrante; M. Ligi; M. Lopez Correa; F. Redini; A. Cipriani; G. Barbino; E. Carminati; A. Calafato; N.C. Mitchell; B. Sichler; M. Schmidt; N. Rasul; S.N. Al-Nomani; F. Bahreth; R.K. Farawati; S.M. Samir; A. Shawky and Y.M Al-Hazmi ( 2006 ) - TRANSITION FROM A CONTINENTAL TO AN OCEANIC RIFT IN THE NORTHERN RED SEA - Polar Ridges Meeting and Workshop - pp. da 1 a 1 ISSN: - [Poster (275) - Poster]
Abstract

abstract

Cipriani A.; Bonatti E.; Brunelli D.; Ligi M. ( 2006 ) - 26 Million Years of mantle upwelling below a segment of the Mid Atlantic Ridge: the Vema Lithosperic Section revisited - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 87 (52) [Abstract in rivista (266) - Abstract in Rivista]
Abstract

An uplifted, relatively undeformed sliver of oceanic lithospheric (Vema Lithospheric Section or VLS), exposing a >26 Myr record of lithosphere generation at a segment of the Mid Atlantic Ridge (EMAR segment, 10°-11°N, central Atlantic), provides the ideal setting to tackle the problem of temporal variations in the processes of creation of oceanic lithosphere at a slow-spreading ridge. A first study, that combined the temporal variations of the mineral chemistry of mantle peridotites sampled along the VLS and gravity measured along a flow line starting from the center of the EMAR segment, detected a steady increase of crustal thickness and of mantle degree of melting from ~20 Ma to 4 Ma before present. Additional sampling was carried out in 2005 along the VLS, extending the coverage of mantle peridotites from a lithospheric age of 26 Ma to 2 Ma. The 26 Ma to 18.5 Ma interval shows short wavelength (~4 Myr) variations of the mantle degree of melting, superimposed on a weak trend of decreasing degree of melting. In the 18.5 to 2 Ma interval a steady increase of degree of melting was observed, with superimposed 3-4 Myr oscillations. Temporal variations of RMBA and crustal thickness inferred from gravity data reveal similar trends. The older (26 to 18.5 Ma) and the younger (18.5 to 2Ma) peridotite suites differ in clinopyroxene CaO/Al2O3 ratio, suggesting that not only the thermal regime, but also the composition of the mantle source might be different in the two suites. Peridotite two-pyroxene geothermometry shows a decreasing trend of the calculated equilibration temperature from 26 Ma to 18.5 Ma ago, followed by a steady increase from 16 Ma to 2 Ma ago, suggesting that the solid mantle upwelling speed varied through time. A ~2 Myr long interval with strongly deformed ultramafic mylonites as the dominant rock type lies between the two opposite trends, as if marking a change in mantle thermal regime. It may reflect a thermal minimum in the subridge upwelling mantle, with nearly a-magmatic emplacement of the lithosphere resulting in strong deformation of the mantle rocks. When upwelling of hot and/or fertile mantle resumes a new cycle starts with injection of igneous crust.

E. BONATTI; D. BRUNELLI; W. R. BUCK; A. CIPRIANI; P. FABRETTI; V. FERRANTE; L. GASPERINI AND M. LIGI ( 2005 ) - Flexural uplift of a lithospheric slab near the Vema Transform (Central Atlantic): timing and mechanisms - EARTH AND PLANETARY SCIENCE LETTERS - n. volume 240 - pp. da 642 a 655 ISSN: 0012-821X [Articolo in rivista (262) - Articolo su rivista]
Abstract

The Vema Transverse Ridge (VTR) is a prominent, long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the Vema transform at 118 N in the Atlantic. It rises abruptly about 140 km from the axis of theMid-Atlantic Ridge (MAR) in ~10 Myr old crust and runs continuously up to ~25 Myr old crust. It reaches over 3 km above the predicted lithospheric thermal contraction level. It is absent in crust younger than 10 Myr; thus, the uplift of the VTR must haveended roughly 10 Ma. The VTR is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that wasgenerated at an 80 km long MAR segment. Based on satellite gravimetry imagery this MAR segment was born roughly 50 Ma andincreased its length at an average rate of 1.6 mm/yr. Multibeam data show that the MAR-parallel sea floor fabric south of the VTRshifts its orientation by 58 to 108 clockwise in ~11–12 Myr old crust, indicating a change at that time of the orientation of the MARaxis and of the position of the Euler rotation pole. This change caused extension normal to the transform, followed between 12 and 10 Ma by flexure of the edge of the lithospheric slab, uplift of the VTR at a rate of 2 to 4 mm/yr, and exposure of a lithosphericsection (Vema Lithospheric Section or VLS) at the northern edge of the slab, parallel to the Vema transform. Ages of pelagiccarbonates encrusting ultramafic rocks sampled at the base of the VLS at different distances from the MAR axis suggest that theentire VTR rose vertically as a single block within the active transform offset. A 50 km long portion of the crest of the VTR roseabove sea level, subsided, was truncated at sea level and covered by a carbonate platform. Subaerial and submarine erosion hasgradually removed material from the top of the VTR and has modified its slopes. Spreading half rate of the crust south of thetransform decreased from 17.2 mm/yr between 26 and 19 Ma to ~16.9 mm/yr between 19 and ~10 Ma, to ~13.6 mm/yr from 10 Ma to present. The slowing down of spreading occurred close in time to the change in ridge/transform geometry, suggesting that the two events are related. A numerical model relates lithospheric flexure to extension normal to the transform, suggesting that theextent of the uplift depends on the thickness of the brittle layer, consistent with the observed greater uplift of the older lithospherealong the VTR.

Sides R.; Holness M.; Brunelli D. ( 2005 ) - Liesegang structures formed during interstitial solidification of crystal mushes: the peridotite plugs of Rum - VMGS annual meeting - VMSG london GBR) - n. volume 2005 - pp. da 1 a 1 ISBN: 0000000000 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

abstract

MAZZUCCHELLI M.; RIVALENTI G.; BRUNELLI D.; PAGLIOLI S; ZANETTI A. ( 2005 ) - Melt percolation and reaction in the Balmuccia peridotite. - EPITOME - n. volume 1 - pp. da 301 a 302 ISBN: 978 ISSN: 1972-1552 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

abs. 10.1474, 01-1081.The Balmuccia lherzolite massif is traditionally considered as a document of the continental lithospheric mantle variably depleted by melting and melt extraction processes. Interaction with percolating melts were only described in the peridotite, over a maximum thickness of a few decimetres, at the contact with websteritic dikes belonging to the Cr-diopside and to the Al-Augite suites, which are the result of focused melt flow and fractionation into fractures or ductile extension zones. Recent detailed mapping of the peridotite heterogeneity, however, revealed the existence of peridotite domains which have petrochemical features ascribable to reactive porous flow of melts of various compositions. These domains can be distinguished into three types:1) regions constituted by cpx-bearing dunite or wehrlite, which contains chromitite horizons and pods up to 7 cm thick. The dunite-wehrlite bands are subconcordant to slightly discordant with the lherzolite foliation, reaching a thickness of about 6 metres and fade into zones with porphyroblastic opx. The process producing chromitite and dunite-wehrlite predated the emplacement of the oldest Cr-diopside dikes. The infiltrated melts, estimated from clinopyroxene trace element characteristics, had alkali-tholeiitic transitional affinity. 2) regions of porphyroclastic lherzolite-harzburgite, in which the porphyroclasts are represented by up to 1.5 cm large orthopyroxenes. These domains, about 10 metres large, are apparently concordant with the protogranular foliated lherzolites. The process producing the porphyroclast was broadly synchronous with the emplacement of the Cr-diopside dykes. These porphyroclastic lherzolites and harzburgites, however, contains boudinaged lenses of Cr-diopside websterite and orthopyroxenite, on their turn crossed by later Cr-diopside dykes. Texturally evident substitution of olivine by orthopyroxene indicates that the percolating melts were silica saturated.3) Regions of dunite, up to 20 metres large, containing pockets and veins constituted by large amphibole crystals, plagioclase, spinel and rutile. Dunite fades into lherzolite containing orthopyroxene porphyroblasts, suggesting the infiltration of hydrous basalt and of SiO2-saturated melts. Field relationships reveals that the process producing dunite post-dates the emplacement of all the Cr-diopside dykes and the early Al-Augite dykes.Studies on the processes, nature of the infiltrated melt and its variations during flow are in progress.

Brunelli D.; M. Seyler; A. Cipriani; L. Ottolini; E. Bonatti ( 2005 ) - Weak residual mantle refertilization by partial melts beneath the Mid Atlantic Ridge - Geoitalia 5° Forum - FIST Udine ITA) - EPITOME - n. volume 1 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

abstract

Brunelli D. ; M. Seyler; A. Cipriani; L. Ottolini; E. Bonatti ( 2004 ) - Discontinuous/episodic partial melt extraction from the melting region beneath the Mid Atlantic Ridge - . - Soc Fran Min Pet Paris FRA) - BULLETIN DE LIAISON DE LA SOCIÉTÉ FRANÇAISE DE MINÉRALOGIE ET DE CRISTALLOGRAPHIE - n. volume 16,3 - pp. da 1 a 1 ISSN: 0999-5870 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

abstract

Ligi M.; Bonatti E.; Brunelli D.; Buck R.W.; Cipriani A. ( 2004 ) - Flexural uplift of a lithospheric slab near the Vema transform (Central Atlantic): timing and mechanism - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 85 (47) [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The Vema Transverse Ridge (VTR) is a prominent, long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the Vema transform at 11ø N in the Atlantic. It rises abruptly about 137 km from the axis of the Mid Atlantic Ridge (MAR) in ~10 My old crust and runs continuously up to ~ 25 My old crust. It reaches over 3 km above the predicted lithospheric thermal contraction level. It is absent in crust younger than 10 My; thus, the uplift of the VTR must have ended roughly 10 My. The VTR is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere. Multibeam data show that the MAR-parallel seafloor fabric south of the VTR shifts its orientation by 5ø to 10ø clockwise in ~ 11-12 My old crust, indicating a change at that time of the orientation of the MAR axis and of the position of the Euler rotation pole. This change caused extension normal to the transform, followed between 12 and 10 My ago by flexure of the edge of the lithospheric slab, uplift of the VTR at a rate of 2 to 4 mm/yr, and exposure of a lithospheric section (Vema Lithospheric Section or VLS) at the northern edge of the slab, parallel to the Vema transform. Ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the VLS at different distances from the MAR axis suggest that the entire VTR rose vertically as a single block within the active transform offset. Erosion has gradually removed material from the top of the VTR and has modified its slopes. A numerical model relates lithospheric flexure to extension normal to the transform, suggesting that the extent of the uplift depends on the thickness of the brittle layer, consistent with the observed topography of the VTR. Spreading half rate of the crust south of the transform decreased from 17.2 mm/yr between 26 and 19 My ago to ~16.9 mm/yr between 19 and ~10 My ago, to ~13.6 mm/yr from 10 My ago to present. The slowing down of spreading occurred close in time to the change in ridge/transfo

CIPRIANI A; BRUEKNER H; BONATTI E; D. BRUNELLI ( 2004 ) - Oceanic crust generated by elusive parents: Sr/Nd isotopes in basalt-peridotite pairs from the Mid-Atlantic Ridge - GEOLOGY - n. volume 32 - pp. da 657 a 660 ISSN: 0091-7613 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Given that oceanic basalts form by partial melting of mantle peridotites that rise below mid-ocean ridges, peridotite and basalt should have identical Sr and Nd isotope ratios.We tested this concept on parallel sets of peridotites and basalts sampled from an exposed section of lithosphere representing 20 m.y. of accretion at the Mid-Atlantic Ridge. The 143Nd/144Nd ratios of the basaltic glasses stay constant, whereas those of the peridotitic clinopyroxenes extend both higher and lower than the basalt ratios, suggesting that the constant isotopic composition of the basalts results from mixing of melts released by peridotitesfrom a broad region of the subridge mantle. The degree of melting undergone by the peridotites correlates inversely with their 143Nd/144Nd ratios. Small-scale isotopic heterogeneity of the peridotites may result from variable premelting metasomatism in the mantle, mostly during periodic, dynamic, subridge upwelling, possibly superimposed upon time-integrated radioactive decay of ancient heterogeneities.

Cipriani A.; Bonatti E.; Brunelli D.; Ligi M.; Ottolini L. ( 2004 ) - Small scale mantle heterogeneities sampled through oceanic basalts from ridge-transform intersections - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 85 (47) [Abstract in rivista (266) - Abstract in Rivista]
Abstract

Studies of MORB have shown that the oceanic upper mantle is heterogeneuos. The nature, distribution and scale of these heterogeneities are still a matter of debate. Short-wavelength chemical variability has been observed along mid-ocean ridges. Melt inclusions show isotopic and chemical variabilty within a single basalt sample and even within a single olivine grain. The distance along axis affected by the ridge-transform intersection (RTI) cold edge effect is proportional to slip rate and offset length. Numerical calculations show a strong decrease of crustal production as a ridge approaches a transform. When the amount of melt produced is small, the capability of melt to aggregate and to mix in magma chambers on the way to the surface is low; therefore, the chemical and isotopic signatures of preaggregated melts can be detected. These predictions make long-offset RTI's potential areas to detect chemical variability in the mantle through the study of MORB. Basalts sampled in the equatorial Atlantic along a ridge segment south of the Romanche RTI (ERRS), show an increase in trace element variability within a single dredge haul, with wavelengths comparable to that observed in suites of melt inclusions. This variability implies that melt transport beneath the ERRS is inefficient at mixing melts and suggests melt transport in channelized systems that limit interaction between melt and solid. The correlation between trace element and isotopic enrichement suggests that most of the chemical variability is probably due to source heterogeneity. Melt transport preserves correlations present in the source, suggesting channels nucleation in the deepest part of the molten region just within melting heterogeneities. Correlations will be probably destroyed if channel networks cross source heterogeneities. Volatile-rich low degree partial melts, generated at depth from enriched peridotite or from pyroxenite lumps, react with and metasomatize

DORAZIO M; BOSCHI C; D. BRUNELLI ( 2004 ) - Talc-rich hydrothermal rocks from the St. Paul and the Conrad fracture zones in the Atlantic Ocean. - EUROPEAN JOURNAL OF MINERALOGY - n. volume 16 - pp. da 73 a 83 ISSN: 0935-1221 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Talc-rich rocks covered by Fe-Mn coatings were recovered from the St. Paul F.Z. (00°37’S-25°34’W, equatorialAtlantic) and Conrad F.Z. (55°29’S-02°05’W, American-Antarctic Ridge). In both occurrences, the talc-rich rocks are associatedwith serpentinized peridotites, gabbroic rocks and minor basalts. The two rocks have very similar trace element, particularly rareearth element, distributions. The St. Paul F.Z. samples are breccias consisting of angular clasts of botryoidal/colloform talc in asubordinate foraminiferal ooze sediment. These breccias probably formed by the collapse of fragile structures formed by theprecipitation of talc at hydrothermal vents. Talc formed when seawater mixed with hydrothermal fluids from a mafic-ultramaficreaction zone. The talc-rich hydrothermal rock found at the Conrad F.Z. shows evidence of a replacement origin. We suggest theprotolith was a gabbroic rock that underwent multi-stage hydrothermal alteration, possibly in a shear zone.These two occurrences represent an evidence of off-axis ocean floor hydrothermal activity, and the study of similar, apparentlyminor, products collected by dredging could be used to reveal the presence of hydrothermal systems in such impervious settings.

D. BRUNELLI; BONATTI E; CIPRIANI A; OTTOLINI L ( 2003 ) - Mantle peridotites from the Bouvet Triple Junction Region, South Atlantic. - TERRA NOVA - n. volume 15 - pp. da 194 a 203 ISSN: 0954-4879 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The Bouvet Triple Junction (BTJ) region in the South Atlantic,where the African, South American and Antarctica plates meet,is affected by several topographic ⁄ melting anomalies. Causesof these anomalies were investigated through a study ofmantle-derived serpentinized peridotites sampled from threesites in the BTJ region: (1) the Inner Corner High at theintersection of the America Antarctic Ridge (AAR) with theConrad transform; (2) the south wall of the Bouvet transform(South West Indian Ridge, SWIR); and (3) the eastern BouvetSWIR Transform Intersection. The degree of melting undergone by these rocks was estimated from relic mineral major- and trace-element composition. Geochemical profiles from residual peridotites and associated basalts show a > 1000-km-wide melting anomaly centred on the Bouvet and Spiess topographic anomalies.

BONATTI E; LIGI M; D. BRUNELLI; CIPRIANI A; FABRETTI P; FERRANTE V; GASPERINI L; OTTOLINI L ( 2003 ) - Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere. - NATURE - n. volume 423 - pp. da 499 a 505 ISSN: 0028-0836 [Articolo in rivista (262) - Articolo su rivista]
Abstract

A 20-Myr record of creation of oceanic lithosphere at a segment of the central Mid-Atlantic-Ridge is exposed along an upliftedsliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry ofthe exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of ,3–4 Myrsuperimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustalthickness indicates that the solid mantle is upwelling at an average rate of ,25mmyr21, but this appears to vary through time.Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axissegmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadilyhotter over the past 20 Myr, possibly owing to north–south mantle flow.The oceanic lithosphere covers two-thirds of our planet: understanding how it forms and evolves is a major challenge in the Earth sciences. It is generally agreed that the oceanic lithosphere forms along mid-ocean ridges, where mantle material upwells and undergoes decompression and partial melting. The melt rises rapidly and freezes, producing the crust, while the peridotitic residue forms the lithospheric mantle. Mid-ocean-ridge topography, structure and composition indicate that near zero age processes of lithosphere formation vary along ridge axis1–3. Less is known, however, of how these processes vary through time, a question important for our understanding of how ocean basins evolve. Variations through time of the thermal regime and/or composition of a mid-ocean ridge should be recorded in lithosphere lying at increasing distances from the ridge axis along sea-floor spreading flow lines. However, older lithosphere is normally covered by sediment and not easily accessibleto high-resolution observation and sampling.An uplifted sliver of oceanic lithosphere (Fig. 1), exposing in thecentral Atlantic an ,20-Myr-long record of creation of lithosphereat a segment of the Mid-Atlantic Ridge (MAR), gave us theopportunity to investigate the temporal variability in the formationof lithosphere, to estimate the upwelling velocity of the mantlebelow the ridge, and to determine whether passive or dynamicmodels of creation of oceanic lithosphere prevail at slow-spreadingridges. A sliver of exposed oceanic lithosphere A major topographic anomaly, the Vema transverse ridge, runs onthe southern side of the Vema transform, which offsets the MAR by 310 km at ,118N (Fig. 1). Its exposed northern scarp (that is, the southern wall of the transform valley) reaches in height up to 4 km above the valley floor (Fig. 2). Submersible4, multibeam5 and seismic reflection data, and bottom samples (Fig. 1) revealed that the northern side of the Vema transverse ridge exposes a ,3–4-km thick, relatively complete and tectonically undeformed upperlithospheric

Bonatti E.; Ligi M.; Brunelli D.; Cipriani A.; Gasperini L. ( 2002 ) - Is the Mid-Atlantic Ridge becoming hotter with time? - 2002 AGU Fall Meeting - EOS - n. volume 83(47) [Poster (275) - Poster]
Abstract

More than 20 million years of oceanic lithosphere accretion history at a segment of the Mid-Atlantic Ridge are recorded in the Vema Lithospheric Section (VLS), a 300 km long flexured and uplifted sliver of lithosphere exposed near the Vema Fracture Zone in the Central Atlantic. Systematic sampling of the basal mantle ultramafic unit and of crustal basalts along the VLS together with geophysical surveys gave us the opportunity to study temporal changes in the processes of generation of the oceanic lithosphere at a ridge axis. The degree of melting of the mantle upwelling below the ridge axis, estimated from the chemistry of mantle-equilibrated mineral phases in the peridotites, as well as crustal thickness, inferred from shipboard and satellite gravity data, both show ~3-4 my long oscillations superimposed on long-range steady increases with time. Based on basaltic glasses elemental and isotopic chemistry, we assume the composition of the source stayed nearly constant. The steady increase with time of mantle degree of melting and of crustal thickness suggests that the mantle rising beneath the MAR became gradually hotter during the last 20 million years, even though the spreading half rate slowed significantly during this time. We offer two explanations for the increase in mantle temperature with time at the Mid-Atlantic Ridge. A first possibility, of local significance, calls for gradual lengthening of the eastern MAR segment where the VLS was created, leading to an increasing degree of melting below center of the segment as it lengthens, due to the a decreasing influence of the "cold edge effect" from the Vema transform. The second, of broader significance, calls for a gradual increase of mantle potential temperature along a significant portion of the northern MAR during the last 20 million years, resulting in an increase of melt production despite decreasing spreading rates. This second hypothesis is supported by an increase of crustal thickness towards ridge axis observed at several other locations in the northern MAR. The chemistry of basaltic glasses, collected along the VLS above the peridotites, suggests that no deep plume source is involved in the steady heating of the Ridge.

BONATTI E; D. BRUNELLI; FABRETTI P; LIGI M; PORTARO R.A; SEYLER M ( 2001 ) - Steady-state creation of crust-free lithosphere at cold spots in mid-ocean ridges. - GEOLOGY - n. volume 11 - pp. da 979 a 982 ISSN: 0091-7613 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Mid-ocean ridges create oceanic lithosphere consisting normally of basaltic crust a few kilometers thick overlying a peridotitic mantle. However, lithosphere free of basaltic crust formed during the past ;30 m.y. at an ;50-km-long stretch of Mid-Atlantic Ridge southof the Romanche Fracture Zone, giving rise to a .500-km-long strip of ocean floor exposing mostly mantle peridotites that have undergone an unusually low (#5%) degree of melting, mixed with peridotites that reacted with a small fraction of basaltic melt. Thislithosphere contains ,10% of scattered gabbroic pockets, representing melt frozen above 25 km depth within a relatively cold subaxial lithosphere. Numerical modeling excludesdry melting below this crust-free lithosphere, because of the cooling effect of the longoffset Romanche transform combined with a regional mantle thermal minimum; however, modeling allows a limited extent of hydrous melting. This unusual lithosphere, unable to expel the melt fraction, characterizes cold spots along mid-ocean ridges.

Cipriani A.; Brunelli D.; Brueckner H.K.; Bonatti E. ( 2001 ) - Temporal variations in the mantle source of MORB near the Vema fracture zone (Central Atlantic): Nd and Sr isotopes in peridotite and basaltic glasses - 2001 AGU Fall Meeting - EOS [Poster (275) - Poster]
Abstract

Sr-Nd-Pb isotopic ratios of zero age basalts sampled along Mid-Ocean Ridges (MOR) have demonstrated that the mantle is heterogeneous at a regional scale. However, how the mantle evolves through time below a single segment of MOR it is still matter of debate. Peridotites and basaltic glasses were collected along a lithospheric section uplifted and exposed on the southern side of the Vema transform (10$^{\circ}$ North, Atlantic Ocean) along a seafloor spreading flow line for a stretch of almost 200 km (corresponding to roughly 10 my). This set of samples offers a unique opportunity to detect changes through time of the mantle signature in a segment of Mid Atlantic Ridge, by analyzing radiogenic isotopes in the clinopyroxenes (cpx) from peridotites and glasses from the overlying basalts. Work is in progress; initial Sr and Nd measurements from cpxs within peridotites indicate several things. First, the cpxs display "depleted" mantle signatures. Second, there is a considerable variation of the isotopic ratios along the exposed section ($^{143}$Nd/$^{144}$Nd varies from 0.51293 to 0.51345, $^{87}$Sr/$^{86}$Sr varies from 0.70228 to 0.70422) and these variations occur over a short time scale (some occur within an interval of one million year). Next, the Sr and Nd ratios are inversely correlated and fall along the mantle array. Finally, cpx Nd ratios are inversely correlated with the Cr/Al ratio of the spinel and ortopyroxene (opx) from the peridotites while Sr ratios are positively correlated. Thus, the chemically most depleted peridotite with high Cr/Al ratios show the most enriched isotopic signatures, a pattern that has also been observed in alpine-type peridotites and peridotite nodules and that is generally interpreted as metasomatism by enriched fluids affecting depleted peridotite more extensively than less depleted peridotite. This may indicate that the temporal variations in the extent of melting detected by Cr/Al ratio in spinel and opx (Bonatti et al., Variations with age of mantle ultramafic composition near the Vema Fracture Zone, Central Atlantic. EOS, Vol.79, No.45, F919) are related to rapid changes in the degree of depletion of the upwelling mantle sources and that the degree of depletion of these mantle sources is an inherited feature from earlier processes rather than the result of melting at the MOR.

Bonatti E.; Brunelli D.; Fabretti P.; Ligi M.; Portaro RA.; Seyler M. ( 2000 ) - Crust-Free “Constipated” Lithosphere created at cold spots in Mid Ocean Ridges - 2000 AGU Fall Meeting - EOS - n. volume 81(48) [Poster (275) - Poster]
Abstract

A ~50 km stretch of the MAR south of its intersection with the Romanche transform lacks a basaltic crust and exposes mantle ultramafics. We tested the hypothesis that the absence of a basaltic crust reflects a temporary phase of a-magmatic extension by sampling the ocean floor along ridge-parallel profiles taken at increasing distances from the MAR axis, in lithosphere of increasing age. The basaltic crust is nearly absent in a ~50 Km wide strip of ocean floor on the southern side of the Romanche FZ extending from ridge axis for over 400 km along a sea floor spreading flow line. Assuming a spreading half rate of 1.7 cm/yr, crust-free lithosphere formed and spread for roughly 30 m.y., excluding for this area a model of alternating cycles of igneous injection and a-magmatic extension at ridge axis. A ~30 m.y. steady-state formation and spreading of lithosphere that lacks a basaltic crust can be explained by the effect of a regional mantle thermal minimum combined with the thermal effect of a long-offset, slow-slip transform. The basaltic crust-free lithosphere created at the MAR close to the Romanche FZ consists of "normal" mid ocean ridge peridotites (N-MORP) that, however, have undergone an unusually low (<5%) degree of melting, mixed with "metasomatic" peridotites (M-MORP) that have reacted with a small fraction of basaltic melt. This lithosphere contains <10% of scattered gabbroic pockets. These melt impregnations and gabbroic pockets represent melt produced by small degree of melting, frozen within the top ~20 Km of the relatively cold subaxial lithosphere. This unusual lithosphere, unable to expel the melt fraction, ("constipated lithosphere") should be common in "cold spots" along mid ocean ridges.

Ligi M.; Bonatti E.; Gasperini L.; Brunelli D.; Fabretti P. ( 2000 ) - Rapid uplift of a lithospheric Sliver Near the Vema FZ (Central Atlantic) due to change in the pole of rotation - 2000 AGU Fall Meeting - EOS [Poster (275) - Poster]
Abstract

The Vema F.Z. offsets the Mid Atlantic Ridge by about 320 km at 10ø45' N. The southern flank of the transform valley is bounded by a prominent transverse ridge (VTR), reaching a minimum depth of ~500 m. Several lines of evidence indicate that the north facing wall of the VTR exposes a complete, relatively undisturbed section of uplifted oceanic lithosphere. The length of the VTR is ~300 km, similar to the length of offset of the transform. The VTR starts abruptly about 140 km from the MAR axis, in crust roughly 10 m.y. old, assuming an half spreading rate of 1.5 cm/y. This age marks the end of the uplift phase of the VTR. The time of initiation of uplift is marked probably by an abrupt change of orientation of sea floor morphostructural fabric, from parallel to the present ridge axis to ~5§ to 10§ oblique. This change of orientation took place about 12 Ma, and is related to a change in the position of the pole of rotation that caused extension across the transform and triggered flexure of a lithospheric slab and uplift of the transverse ridge. The ~4 km uplift of the VTR took place in roughly 2 m.y. with an average rate of uplift of 2 mm/y. Uplift rates appear to have been similar throughout the 300 km length of the VTR. The flexure and rapid uplift did not cause major internal deformation of the lithospheric slab.

Bonatti E.; Gasperini L.; Ligi M.; Brunelli D.; Cipriani A.; Fabretti P. ( 1999 ) - La struttura termica del mantello oceanico e la creazione della litosfera oceanica nell’Atlantico centrale e meridionale - Abstract Geoitalia - SIMP Pisa ITA) - PLINIUS - n. volume 22 - pp. da 1 a 1 ISSN: 1120-317X [Abstract in rivista (266) - Abstract in Rivista]
Abstract

abstract

Bonatti, E.; Brunelli, Daniele; Cipriani, Anna; Ottolini, L.; Seyler, M. ( 1999 ) - Spatial and temporal heterogeneity of the oceanic mantle in the Central Atlantic - Ofioliti, 24:72 - n. volume 24 - pp. 72 ISSN: - [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

Studies of Mid Ocean Ridge Peridotites (MORP) sampled from the Kane (24°N) to the Chain (2°S) transforms in the Central Atlantic reveal upper mantle domains with different composition and subjected to different thermal conditions and degrees of melting along this stretch of mid ocean ridge. These different domains include thermal minima (“cold spots”), such as observed in the equatorial area, and thermal maxima (mini-“hot spots”). Zero-age topography, gravimetry and seismic tomography support the concept of along axis mantle thermal and/or compositional heterogeneity. Close spaced sampling profiles of mantle-derived peridotites were obtained along seafloor spreading flow lines at two Latitude (0° and 11°N). These profiles indicate significant temporal variations in the composition and thermal structure of the oceanic mantle, and of processes of lithosphere creation at ridge axis.

Brunelli D.; Cipriani A.; Ottolini L.; Vaggelli G.; Santo A.P.; Bonatti E. ( 1999 ) - The Bouvet melt anomaly in the South Atlantic: geochemistry and influence on the Triple Junction Geometry - Abstract Geoitalia - SIMP Pisa ITA) - PLINIUS - n. volume 22 - pp. da 1 a 1 ISSN: 1120-317X [Abstract in rivista (266) - Abstract in Rivista]
Abstract

abstract

Gasperini L.; Bonatti E.; Borsetti AM.; Brunelli D.; Capotondi L.; Carrara G.; Cipriani A.; Fabretti P.; Ligi M.; Kastens K. ( 1999 ) - Time constraints on the emplacement of an uplifted sliver of oceanic lithosphere at the Vema transverse ridge (Central Atlantic) - EUG 10 - EUG cambridge GBR) - JOURNAL OF CONFERENCE ABSTRACTS - n. volume 4-1 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The Vema fracture zone is the northernmost of a set of major transform faults which displace the Mid Atlantic Ridge (MAR) at 10°50' N by over 320 km. It is paralleled on its southern flank by a prominent transverse ridge, the summit of which shoals up to 450 m below the sea level near the western ridge transform intersection. The north facing wall of the Vema transverse ridge exposes a relatively complete and undisturbed sliver of upper oceanic lithosphere. Consolidated pelagic limestones locally encrust the exposed igneous rocks, while shallow water carbonate platforms, up to 600 m thick, cap the summit of the transverse ridge. The Vema transverse ridge originated by vertical movements that have uplifted a flexured sliver of oceanic lithosphere. Multichannel seismic reflection profiles taken across the transverse ridge and the transform valley give evidence of recent transpression in the valley, and of extension across the transverse ridge and in the area south of it. Vertical movements might be due to these transpressive and transtensional stresses, related to small changes in spreading direction. Pelagic, as well as shallow water limestones recovered from the transverse ridge, will provide age constraints on the emplacement history of the lithospheric sliver exposed on the transverse ridge.

Brunelli D.; Cipriani A.; Ottolini L.; Bonatti E. ( 1999 ) - Vema Fracture Zone (Central Atlantic): Temporal variations of mantle composition and of accretion processes at ridge axis - Journal of Conference Abstracts - EUG Cambridge GBR) - JOURNAL OF CONFERENCE ABSTRACTS - n. volume 4-1 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

The Vema Fracture Zone in the Central Atlantic offers a unique opportunity for studying how the processes of formation of oceanic lithosphere vary with time. An uplifted, relatively complete and undisturbed section of oceanic upper lithosphere is exposed on the southern wall of the transform valley along a seafloor spreading flow line for a distance of about 300 km corresponding to almost 20 m.y. (if we assume an average spreading half-rate of 15 mm/y). Close spaced sampling (at 6-8 km horizontal intervals) of the basal portion of this exposed lithospheric section, for a stretch of almost 200 km, produced a set of mantle-derived peridotites serpentinized to various degrees. These samples preserve relicts of mantle-equilibrated "primary" phases such as olivine, opx, cpx and spinel and give us the opportunity of studying temporal variations of the composition of the mantle for a ~10 m.y.-long interval of time. The peridotites are mostly protogranular or porphyroclastic harzburgites and lherzolites. In a portion of the lithospheric sliver, corresponding to a 2-m.y. time interval, the ultramafic rocks are strongly deformed with amphibole-bearing mylonites prevailing. Preliminary electron and ion probe data on major and trace element chemistry of the mantle-equilibrated minerals in porphyroclastic peridotites, show significant compositional variations with age. For example, the Cr/Al ratio of spinel and the Al2O3 content of opx vary systematically with distance from ridge axis, suggesting both long- (several m.y.) and short- (< 1 m.y.) wavelength systematic variations in the composition and degree of melting of the upwelling mantle. These variations probably relate to systematic temporal changes of the thermal structure of the mantle below the Mid-Atlantic Ridge. Stretches of the lithospheric sliver where amphibole-bearing ultramafic mylonites are dominant may correspond to intervals of prevalent a-magmatic extension at ridge axis.

Ligi M.; Bonatti E.; Brueckner HK.; Brunelli D.; Cipriani A.; Fabretti P.; Ottolini L ( 1998 ) - Contrasting Ultra-Slow Ridges near the Bouvet Triple Junction in the South Atlantic - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 1 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

Two ultraslow mid ocean ridges, i.e., the westernmost portion of the Southwest Indian Ridge (SWIR) and the easternmost portion of the American-Antarctic Ridge (AAR), meet the southernmost portion of the Mid Atlantic Ridge (MAR) at the Bouvet Triple Junction. Although both the SWIR and the AAR have very slow spreading rates, i.e., 7.5 mm/y and 10 mm/y, respectively, they display strongly contrasting morphostructural and petrological characteristics. The AAR axial zone is deeper than normal (>4000m); some of its segments are oblique, being oriented roughly 45 degrees from the direction of spreading; basalt and peridotite chemistry suggests a relatively low degree of melting of the mantle below the AAR. In contrast, the SWIR axial zone is shallower than normal ($<$2500m); the last segment (Spiess Ridge) is particularly anomalous, reaching ~400 m below sea level. Basalt and peridotite chemistry of this portion of SWIR suggests high extents of melting of the upper mantle. The H2O content of the SWIR basaltic glasses ranges between 0.6% and 1.0% and is significantly higher than in normal MORB. The high extent of melting at the SWIR segments is probably related not only to a mantle thermal anomaly, but also to its high volatiles content. Mantle thermal structure and composition, rather than spreading rate, are the main factors determing the structure of these ridges.

Paola Fabretti; Enrico Bonatti; Alexander Peyve; Daniele Brunelli; Anna Cipriani; Xsenia Dobrolubova; V. Efimov;S. Erofeev; Luca Gasperini; J. Hanley;Marco Ligi; A. Perfiliev; V. Rastorguyev; Y Raznitsin; Galina Savelieva;V. Semjenov; V. Simonov; S. Sokolov; S. Skolotnev; S. Susini; I. Vikentyev; ( 1998 ) - First results of cruise S19 (PRIMAR project): Petrological and structural investigations of the Vema transverse ridge (equatorial Atlantic) - GIORNALE DI GEOLOGIA - n. volume 60 - pp. da 3 a 16 ISSN: 0017-0291 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We carried out in January-March 1998 a geological-geophysical cruise to the Vema fracture zone that offsets by 320 km the Mid Atlantic Ridge in the central Atlantic. This expedition (S19) was part of PRIMAR (Russian-Italian Mid Atlantic Ridge Project). The field work aimed at obtaining geophysical and petrological data from a prominent transverse ridge that runs on the southern side of the transform valley and constitutes a major topographic anomaly relative to the depth/square root of age relationship. Previous work had shown that a relatively undisturbed section of oceanic lithosphere is exposed on the northern side of the transverse ridge for roughly 270 km along a seafloor spreading flow line. Given an average spreading half rate of 16 mm/y, this length corresponds to over 16 My. One of the objectives of our expedition was to sample at close-spaced (~5 km) horizontal intervals the mantle ultramafic basal unit, in order to detect temporal variations of mantle composition and of accretion processes at ridge axis. Preliminary observations on ultramafic rock samples obtained at 35 sites suggest strong temporal variations of mantle structure and composition. Multichannel seismic reflection profiles were carried out in order to understand the processes that uplifted the transverse ridge and exposed the sliver of oceanic lithosphere. Magnetometric profiles were made to better constrain spreading rates.

Bonatti E.; Brunelli D.; Cipriani A.; Fabretti P.; Gasperini L.; Ligi M.; Peyve A.; Savelieva G.; Skolotnev S.; Susini S. ( 1998 ) - Variations with age of ultramafic composition near the Vema fracture zone, Central Atlantic - EOS Trans. AGU - AGU Washington USA) - EOS - n. volume 1 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

A relatively undisturbed sliver of oceanic lithosphere is exposed on the sea floor for a distance of about 300km along a sea floor spreading flow line near the Vema Fracture Zone in the Central Atlantic. Assuming a spreading half-rate of 15mm/y, the exposed lithospheric sliver represents an almost 20 m.y. time interval of accretion at the axis of the Mid-Atlantic Ridge. The basal, mantle ultramafic unit of this lithospheric section was sampled at close-spaced (~6-8 km) intervals for a stretch of almost 200 km, corresponding to roughly 13 m.y. of accretion. A study of these samples, which is in progress, will give us information on temporal variation of the thermal structure and composition of the upper mantle below a segment of the Mid-Atlantic Ridge. The first results, involving the major and trace element composition of mantle-equilibrated minerals; olivine, orthopyroxene, clinopyroxene and spinel in porphyroclastic peridotites suggest both long (several m.y.) and short ($<$ 1m.y.) wavelength systematic variations that reflect temporal variations in the composition and extent of melting of the mantle below the Mid-Atlantic Ridge. Anphibole-bearing ultramafic mylonites are dominant in a stretch of the section roughly corresponding to a 2 m.y. time interval, that may correspond to a period of prevalent a-magmatic extension at ridge axis.

Ligi M.; Bonatti E.; Bortoluzzi G.; Carrara G.; Fabretti P.; Penitenti D.; Terenzoni M.; Brunelli D.; Cipriani A.; Gilod D.; Peyve A.A.; Skolotnev S.; Turko N. ( 1997 ) - Death and transfiguration of a triple junction in the south Atlantic - Geophysical Newsletter XXII EGS General Assembly [Poster (275) - Poster]
Abstract

abstract

Bonatti E.; Gasperini L.; Ligi M.; Carrara G.; Chierici F.; Fabretti P.; Susini S.; Tartarotti P.; Brunelli D.; Cipriani A.; Gilod D.; Peyve A.A.; Skolotnev S.; Turko N. ( 1997 ) - Morphostructural analysis of the western intersection of the mid Atlantic ridge with the Romanche transform (equatorial Atlantic) - Geophysical Newsletter XXII EGS General Assembly [Poster (275) - Poster]
Abstract

abstract

Luca Gasperini; Enrico Bonatti; Daniele Brunelli; Gabriela Carrara; Anna Cipriani; Paola Fabretti; Dolores Gilod; Marco Ligi; Alexander Peyve; Sergey Skolotnev; Sara Susini; Paola Tartarotti; Natasha Turko; ( 1997 ) - New data on the geology of the Romanche F.Z., equatorial Atlantic: PRIMAR-96 cruise report - GIORNALE DI GEOLOGIA - n. volume 59 - pp. da 3 a 18 ISSN: 0017-0291 [Articolo in rivista (262) - Articolo su rivista]
Abstract

An oceanographic expedition was carried out in May and June 1996 with the R/V Gelendzhik to the Romanche Fracture Zone (F.Z.), in the frame of an Italian-Russian joint program for the geological study of the equatorial Atlantic (PRIMAR). We present here a cruise report, with some preliminary data and scientific results. The choice of the ship was determined mostly by the availability of a new, state of the art oceanic multibeam system (SIMRAD EM-120-S). The '96 expedition focused on the Romanche F.Z. western ridge/transform intersection (RTI) and complemented a 1994 expedition that covered the eastern RTI. Multichannel seismic reflection, gravimetric and magnetometric profiles were collected, as well as bottom rock samples. Moreover, we acquired over 10,000 nautical miles of high resolution multibeam bathymetry. With these new data the entire active part of the Romanche transform (> 900 km) has been covered by multibeam morphobathymetry.

Gabriela Carrara; Giovanni Bortoluzzi; Nevio Zitellini; Enrico Bonatti; Daniele Brunelli; Anna Cipriani; Paola Fabretti; Luca Gasperini; Marco Ligi; Daniela Penitenti; P.F. Sciuto; A. Mazarovich; A. Peyve; N. Turko; S. Skolotnev; D. Gilod ( 1997 ) - The Bouvet Triple Junction Region (south Atlantic): A report on two geological expeditions - GIORNALE DI GEOLOGIA - n. volume 59 - pp. da 19 a 33 ISSN: 0017-0291 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The Bouvet Triple Junction is located where the Antarctic, south American and African plates meet in the south Atlantic. Two oceanographic expeditions were carried out in this region in 1994 and 1996 by the Institute of Marine Geology, CNR (Bologna), and the Geology Institute of the Russian Academy of Science (Moscow) under the sponsorship of the Italian Antarctic Research Program (PNRA). The main objectives of these expeditions were to define the structural, morphological and petrographical differences of the three divergent margins that meet in the Bouvet region, and to clarify the evolution in space and time of the geometry of the Triple Junction. Single and multichannel seismic reflection, gravimetric and magnetometric profiles were obtained during the first cruise, with the R/V Akademik Strakhov, in addition to medium resolution multibeam bathymetry of the Bouvet island area and in the southern portion of the Mid Atlantic Ridge. The second cruise, carried out with the R/V Gelendzhik, completed the bathymetric survey of the entire Triple Junction region using an high resolution multibeam system. Seafloor sampling was carried out by dredging in both cruises. Some of the data obtained during these two expeditions are reported in this paper.