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Dipartimento di Scienze Chimiche e Geologiche - Sede Dipartimento di Scienze Chimiche e Geologiche
- Percolation of enriched melts during incremental open-system melting in the spinel field: A REE approach to abyssal peridotites from the Southwest Indian Ridge
[Articolo su rivista]
Brunelli, Daniele; Paganelli, Emanuele; M., Seyler
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.
- Stratigraphy and facies analisys of the Langhian Dağpazarı reef complex (Mut Basin, Southern Turkey)
[Abstract in Atti di Convegno]
Vescogni, Alessandro; Bosellini, Francesca; Cipriani, Anna; Gürler, Gonca; Ilgar, Ayhan; Paganelli, Emanuele; Chiossi, Irene .
During the Early/Middle Miocene, the eastern part of the Mediterranean region registered the narrowing, and the ultimate interruption of the connections with the Indian Ocean. This event represented one of the most important episodes in the Cenozoic evolution of the Mediterranean marine biota. For this reason, the study of Middle Miocene reefal communities from this area can offer an important contribution for the reconstruction of the palaeogeography and palaeoecological settings of the Mediterranean shallow-water, carbonate-producers. The primary aim of the present study is to provide a small-scale, detailed reconstruction of the stratigraphy and facies associations of a Langhian reef complex from the Mut Formation (Mut Basin, Southern Turkey). This study has been based on field observation and microfacies analysis, with a particular emphasis on the characterization of calcareous algae associations (both corallines and green codiacean) and coral fauna. The Dağpazarı reef represents a portion of a small, isolated carbonate platform located 12 Km north from the town of Mut. Its early Langhian age has been determined by 87Sr/86Sr isotope analyses performed on several well preserved oyster samples. Dağpazarı reef is represented by two main stratigraphic sequences separated by a major erosion surface. Only a few meters of lowermost sequences are exposed, while the upper one measures about 72 meters in thickness and is characterized by the superimposition of four different cycles. During the first cycle, the amplitude of the sea transgression allowed the growth of a massive coral framestone on the shelf-edge, associated to well developed slope deposits toward the basin. In particular, concentrations of small rhodoliths with large nuclei and laminar/columnar structure characterize the shallower part of the slope, while in the middle slope the presence of Halimeda bioherms indicates a palaeodepth of about 30/50 meters. The two subsequent cycles show the superimposition, on the shelf-edge, of two similar shallowing-upward sequences, each of them related to a small-scale, transgressive phase. In particular, each sequence exhibits at the base a coralline red algae bindstone, typical of low hydrodynamic conditions, made of thin, laminar/foliose crusts associated to discoidal/irregular rhodoliths. This facies is followed by Halimeda bioherms and biocalcarenite deposits, the latters characterized by the presence of abundant “hooked” and foliose coralline algae crusts associated to gastropods concentrations. The coexistence of the latter features within biocalcarenite deposits is considered a reliable marker of the presence of seagrass beds. Both sequences terminate with the occurrence of shallow-water, massive coral framestone on the top. The fourth Dağpazarı reef cycle developed on the shelf-edge and started with a coralline red algae bindstone, but in this case, corallines colonizations were followed by a fine-grained, mud-supported biocalcarenite whose stratification dips landward. The top of the succession is characterized by coarser biocalcarenite deposits containing small, coral patches. Due to its stratigraphic setting and composition, this last sequence has been interpreted as a back-reef facies association, related to the progradation of a reef framework (now eroded) as a consequence of a final highstand phase of the sea-level. This work provides new information on some important shallow-water carbonate producers, for example Dağpazarı Halimeda bioherms represent so far the oldest recorded examples of this kind of structures; at the same time it offers a detailed frame for further, specific studies on Middle Miocene reef-building associations.
- Bronze Age pottery from the Aeolian Islands: definition of Temper Compositional Reference Units by an integrated mineralogical
and microchemical approach
[Articolo su rivista]
Brunelli, Daniele; Levi, SARA TIZIANA; P., Fragnoli; A., Renzull; P., Santi; Paganelli, Emanuele; M. C., Martinelli
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.
- Melting of an Hybrid Source Below the Danakil Region
[Abstract in Rivista]
Barbieri, Emiliano; Cipriani, Anna; Brunelli, Daniele; Paganelli, Emanuele
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
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.
- Deep versus shallow melt stagnation in an ultra-slow / ultra-cold ridge segment: the Andrew Bain southern RTI (SWIR)
Paganelli, Emanuele; Brunelli, Daniele; Seyler, M.; Bonatti, E.; Cipriani, Anna; Ligi, M.
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.
- Partial assimilation during crystallization of a (gabbroic) pluton: the gabbronoritic zone of the Atlantis core complex
Barbieri, Emiliano; Brunelli, Daniele; Hellebrand, E.; Johnson, K. T.; Paganelli, Emanuele
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.
- REE pattern rotation in an open system melting: case studies from slow and ultraslow spreading ridges
[Abstract in Rivista]
Cipriani, Anna; Brunelli, Daniele; Seyler, Monique; Paganelli, Emanuele; Barbieri, Emiliano
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.
- Short wavelength vertical fluctuations of the melting regime in the suboceanic melting region
Brunelli, Daniele; Seyler, M.; Paganelli, Emanuele; Barbieri, Emiliano
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.
- Heterogeneous melting and refertilization of a mantle parcel in a cold spot: Andrew Bain fracure zone (SWIR)
Paganelli, Emanuele; Brunelli, Daniele; M., Seyler; E., Bonatti; Cipriani, Anna; M., Ligi
- Multi-stage impregnation of the lithospheric mantle at the Andrew Bain FZ (SWIR)
Paganelli, Emanuele; Brunelli, Daniele; Bonatti, E.; Cipriani, Anna; Ligi, M.
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
- Crust-Poor Lithosphere at Cold Spots in the Mid Atlantic and SW Indian Ridges
[Abstract in Rivista]
Brunelli, Daniele; Bonatti, E; Cipriani, Anna; Grindlay N., R; Ligi, M; Paganelli, Emanuele; Sclater, J.
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.
- Investigation of the Andrew Bain transform fault zone (African-Antartctic Region)
[Articolo su rivista]
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, Daniele; Pertsev, A. N.; Cipriani, Anna; Bortoluzzi, G.; Mercuri, R.; Paganelli, Emanuele; Muccini, F.; Takeuchi, C.; Zaffagnini, F.; Dobrolyubova, K. O.
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.
- ANDREW BAIN TRANSFORM: MULTIPLE CONTINENTAL-TYPE TRANSFORM BOUNDARIES AT MID-OCEAN RIDGE
M., Ligi; E., Bonatti; N. R., Grindlay; J., Sclater; S., Skolotnev; A., Peyve; G., Bortoluzzi; Brunelli, Daniele; Cipriani, Anna; R., Mercuri; F., Muccini; Paganelli, Emanuele; 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; L., Zotov