Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30 degrees N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (->70% olivine), possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poilkilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82-88 in olivine-rich troctolites, to 78-83 in gabbros) and is in Mg-Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depleted in trace elements (e.g., Yb-cpx similar to 5-11 x Chondrite), they are in equilibrium with the same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges.
ISSN: 0009-2541

hal-00420916
https://hal.archives-ouvertes.fr/hal-00420916
DOI : 10.1016/j.chemgeo.2009.02.013

Éditeur(s) : Elsevier
Résumé : The purpose of this study was to determine whether diploid and triploid sea bass differed in terms of main haematological and physiological characteristics. Diploid and triploid fish were produced by sub-optimal pressure treatments and held in communal environments under standard rearing conditions. Total red blood cell count (RBCC), haemoglobin concentration (Hb), hematocrit (Hct), mean cell volume (MCV), mean cellular haemoglobin content (MCH), mean cell haemoglobin concentration (MCHC), plasma metabolites, osmotic pressure, gill Na+/K+-ATPase activity, electrolytes, cortisol, and 3,5,3'-triiodo-L-thyronine (T-3) were measured and compared. Triploidisation in sea bass led to an increase in erythrocyte size (32% in cytoplasm surface area, and 50% in nucleus) and a decrease in erythrocyte number (similar to34%). Haemoglobin and basal plasma cortisol levels were significantly lower in triploid sea bass than in diploids. There were also differences between ploidies in the plasma concentrations of some electrolytes, with triploids showing lower concentrations of K, Fe, Zn, S, and Cu than their diploid counterparts.
Aquaculture (0044-8486) (Elsevier), 2005-02 , Vol. 244 , N. 1-4 , P. 359-367

Droits : 2004 Elsevier B.V. All rights reserved
http://archimer.ifremer.fr/doc/2005/publication-508.pdf
DOI:10.1016/j.aquaculture.2004.11.028
http://archimer.ifremer.fr/doc/00000/508/

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Anisotropy of low-field magnetic susceptibility (AMS) of basaltic lava flows can give some clues about post-emplacement tilting occurring in volcanic sequences. Such a study has been carried out on a sequence of 19 successive lava flows from Kerguelen Archipelago. Surprisingly, two different patterns were observed. The first one - herein called normal fabric - is consistent with the lava-flow direction inferred from geological observations, whereas the second one observed for about 70 per cent of the analysed samples - herein called abnormal fabric - appears to be unrelated to the shear history of the lava flows during their emplacements. These abnormal fabrics are not strictly inverse fabric but seem to have a more complex origin. The unusual nature of these abnormal fabrics has been confirmed by lattice preferred orientation of plagioclases obtained from high-resolution electron backscattered diffraction measurements. The aim of this study is to propose a physical interpretation of these fabrics. A first step in our understanding was to clearly identify the nature and the magnetic properties of the Fe-Ti oxides assemblage. Thus, we present a comprehensive rock magnetic analysis relying on low- and high-temperature thermomagnetic curves (K-T), crossover diagrams, first-order reversal curves, ore-microscopy, Mössbauer measurements and electron backscattered images. We found that in the present case the abnormal fabrics are probably linked to a mixture of prevailing single domain (SD) strongly interacting grains population and a subsidiary multidomain grains population. SD grains are identified as small magnetite crystallized within the interstitial glass in globular aggregates along the silicate framework. Then a question arises: to what extent such SD interacting grains in globular aggregates can affect the magnetic susceptibility and its anisotropy? To answer this question, we developed a physical model in which the total energy density of an SD grain with an uniaxial anisotropy is minimized. In this model, the distribution function of the direction of the magnetic interaction field is comprised between two boundary states. It is either isotropic, which formally corresponds to the thermal demagnetized state, or it is ordered, which formally corresponds to the Alternative Field demagnetized state. We show that in both cases the degree of anisotropy decreases as the interaction field increases. Thus, we conclude that the abnormal fabric encountered in this study can be simply explained by strong magnetostatic interactions that would explain the large scattering of AMS often observed in basaltic lava flows.
ISSN: 0956-540X

hal-00716449
https://hal.archives-ouvertes.fr/hal-00716449
https://hal.archives-ouvertes.fr/hal-00716449/document
https://hal.archives-ouvertes.fr/hal-00716449/file/gji-S-12-HAL.pdf
DOI : 10.1111/J.1365-246X.2012.05421.X

Éditeur(s) : HAL CCSD
Résumé : Eight drill cores cutting through the Mn-carbonate bearing black shales from the Bangombé Plateau were studied. Mn-carbonates (up to 80 m thick) are underlain by laminated pyrite rich shales with organic carbon contents of about 4 wt. % and MnO contents of about 6 wt. %. Mn and Fe, S poor shales are intercalated. Mn-carbonate rich shales (20-29 wt. % MnO) have low contents of phyllosilicates, but are rich in organic carbon (about 7 wt. %) with moderate S contents (about 2 wt. %). All shale facies were deposited in a closed basin through suspension settling and precipitations under anoxic conditions. The Mn-carbonates, towards the top, were formed most likely under suboxic conditions. Pyrite and Mn-carbonates are characterized by bacteriomorphous structures. The latter are composed of micrometric carbonates finely intergrown with clays, quartz and organic matter. They formed around and corroded the dolomite and/or organic matter. Mn-carbonates and pyrite both form also several tenths of micrometer large agglomerates and sub-automorphous grains. The agglomerates can be attributed to evolving diagenesis. Carbonate chemistry varies mainly from rhodochrosite to dolomite. The present results point to primary microbial Mn-carbonate formation. No signs for a Mn-oxide precursor were detected. Slumping and shearing structures, distortion and disruption of thin pyrite beds indicate synsedimentary deformations.
13th Biennial SGA Meeting
Nancy, France

hal-01356155
https://hal.archives-ouvertes.fr/hal-01356155

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We performed forward modelling of seismic anisotropy beneath a migrating strike-slip plate boundary to: (1) test if such a geodynamic context might explain teleseismic shear wave splitting data in the vicinity of the central part of the San Andreas Fault and (2) constrain the power of such data to unravel vertical and lateral variations in deformation patterns in the upper mantle. The modelling involves five steps: (1) thermomechanical modelling, using a finite-element code, of the deformation field, (2) viscoplastic self-consistent modelling of the resulting olivine and pyroxene crystal preferred orientations, (3) calculation of the elasticity tensors for different domains of the finite elements (FEs) model, (4) forward modelling of seismic wave propagation through the model using ray theory, finite-frequency theory and a full-wave approach and (5) performing splitting measurements on the synthetic seismograms. SKS splitting data in central California are best fitted by a model with a hotter geotherm within 60 km of the plate boundary accounting for the opening of an asthenospheric window due to the northward migration of the Mendocino Triple Junction. The westward motion of the plate boundary cannot however explain the rotation of fast polarization directions east of the San Andreas Fault in central California. Comparison between modelled and measured individual shear wave splitting also implies that the homogeneity of the two-layer models accounting for the observations in the vicinity of the San Andreas Fault indicates a sharp transition between lithospheric and asthenospheric deformations beneath this plate boundary. The ability of different synthetic approaches to localize horizontally and vertically the plate boundary-related deformation differs significantly. Splitting in ray theory synthetics closely follow variations in olivine crystallographic preferred orientations in the model. In contrast, splitting analysis on full-wave synthetics, which should be more representative of actual long-period SKS waves, results in smooth apparent lateral variations of anisotropy; the exact location and width of the plate boundary may only be retrieved by comparing fast-polarization profiles obtained using a multichannel analysis on waves with different periods.
ISSN: 0956-540X

hal-01236150
http://hal.univ-reunion.fr/hal-01236150
http://hal.univ-reunion.fr/hal-01236150/document
http://hal.univ-reunion.fr/hal-01236150/file/Bonnin_San_Andreas_modelling_GJI2012.pdf
DOI : 10.1111/j.1365-246X.2012.05650.x

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
The Cerro del Almirez massif (Spain) represents a unique fragment of serpentinized oceanic lithosphere that has been first equilibrated in the antigorite stability field (Atg-serpentinites) and then dehydrated into chlorite–olivine–orthopyroxene (Chl-harzburgites) at eclogite facies conditions during subduction. The massif preserves a dehydration front between Atg-serpentinites and Chl-harzburgites. It constitutes a suitable place to study redox changes in serpentinites and the nature of the released fluids during their dehydration. Relative to abyssal serpentinites, Atg-serpentinites display a low Fe3+/FeTotal(BR) (=0.55) and magnetite modal content (=2.8–4.3 wt%). Micro-X-ray absorption near-edge structure (μ-XANES) spectroscopy measurements of serpentines at the Fe–K edge show that antigorite has a lower Fe3+/FeTotal ratio (=0.48) than oceanic lizardite/chrysotile assemblages. The onset of Atg-serpentinites dehydration is marked by the crystallization of a Fe3+-rich antigorite (Fe3+/FeTotal = 0.6–0.75) in equilibrium with secondary olivine and by a decrease in magnetite amount (=1.6–2.2 wt%). This suggests a preferential partitioning of Fe3+ into serpentine rather than into olivine. The Atg-breakdown is marked by a decrease in Fe3+/FeTotal(BR) (=0.34–0.41), the crystallization of Fe2+-rich phases and the quasi-disappearance of magnetite (=0.6–1.4 wt.%). The observation of Fe3+-rich hematite and ilmenite intergrowths suggests that the O2 released by the crystallization of Fe2+-rich phases could promote hematite crystallization and a subsequent increase in fo2 inside the portion of the subducted mantle. Serpentinite dehydration could thus produce highly oxidized fluids in subduction zones and contribute to the oxidization of the sub-arc mantle wedge.
ISSN: 0010-7999

hal-01172310
https://hal.archives-ouvertes.fr/hal-01172310
DOI : 10.1007/s00410-015-1130-y

Éditeur(s) : HAL CCSD Mineralogical Society
Résumé : International audience
Almost all magmas contain sulfide- or sulfate-bearing phases. In most natural samples the sulfur-bearing phase is a sulfide, which typically is pyrrhotite (Fe1−xS) or pyrite (FeS2) although chalcopyrite (CuFeS2), pentlandite ((Fe,Ni)9S8), sphalerite (ZnS) or molybdenite (MoS2) may be present as well. Sulfate minerals are rare at magmatic conditions, and anhydrite (CaSO4) is the most common. Other magmatic SO4-bearing minerals include the sodalite group minerals (haüyne simplified formula: (Na,Ca)4-8(Al6Si6(O,S)24)(SO4,Cl)1-2), scapolite minerals (silvialite: (Ca,Na)4Al6Si6O24(SO4,CO3)), and S-bearing apatite (Ca5(PO4)3(F, Cl, OH)). Barite (BaSO4) has been mentioned in rare cases (Marchev 1991). Sulfur-bearing minerals usually constitute a negligible fraction of the mineral assemblage in magmatic rocks and thus can be classified as accessory minerals. The crystallization of sulfide, sulfate, and S-bearing minerals strongly depends on melt composition, temperature and pressure, and the S speciation in melt which, in turn, is strongly dependent on the prevailing oxygen fugacity (Baker and Moretti 2011, this volume; Wilke et al. 2011, this volume). Irrespectively of the low abundance of S-bearing minerals, the evolution of sulfur in magmas may be evaluated from the occurrence of sulfides and sulfates. These minerals are critical for estimating the activity of various sulfur-bearing species in magmas and can be used to constrain the oxygen fugacity and the S concentration in the melt (e.g., pre-eruptive sulfur concentration in melts). The presence of either sulfide or sulfate in silicate melt indicates that the predominant dissolved sulfur species in the melt are S2− or S6+, respectively. Typically one of these species is dominant, but there are conditions where S2− ...
ISSN: 1529-6466

hal-00644580
https://hal.archives-ouvertes.fr/hal-00644580
DOI : 10.2138/rmg.2011.73.10

Éditeur(s) : HAL CCSD Mineralogical Society of America
Résumé : International audience
Wadsleyites with various iron contents were synthesized at ~12-14 GPa and 1400 °C under oxidizing and hydrous conditions in coexistence with enstatite. The samples were studied using micro-X-ray absorption near edge structure (XANES) and micro-Mössbauer spectroscopy to determine the ferric iron contents in polyphasic samples and secondary ion mass spectrometry (SIMS) to determine the water concentrations. XANES and Mössbauer analyses show that ferric iron content increases with increasing total iron content, and reaches a maximum of ~30% Fe3+/Fetotal. Two XANES results were cross-checked by Mössbauer analysis and both methods are in reasonable agreement. The use of Fourier transform infrared spectroscopy reveals a new protonation scheme in wadsleyite, with a significant proportion of protons associated with the high-frequency band at 3611 cm−1 and a new band located at 3500 cm−1. The intensity of these two bands is higher for Fe3+-rich wadsleyite. SIMS analyses show that water contents in wadsleyite vary from 4500 to 9400 ppm H2O by weight. Pyroxene water contents range from 790 to 1600 ppm wt H2O. The concentration of water in both phases decreases with increasing iron content. The partition coefficient of water between wadsleyite and pyroxene varies between 5 and 9 and increases with increasing Fe-number of wadsleyite [i.e., XFe/(XFe + XMg) × 100 ratio]. The divalent cation concentrations (i.e., Mg2+ + Fe2+), the Si as well as the H content in wadsleyite decrease with increasing Fe3+ content, indicating an incorporation mechanism via substitution into the metal (Me = Mg2+ and Fe2+) and Si sites with a ratio of 5/3 for (Fe3++H+):Me and of 5/1 for (Fe3++H+):Si, similarly as in the dry system. Thus, coupled substitution of Fe3+ and H+ does not affect the incorporation mechanism of Fe3+, but does affect the location of H+, which is partly incorporated at tetrahedral edges forming [FeSi′-(OH)o*]X neutral defects substituting for Si. In this model 25% of the ferric iron occupies the tetrahedral sites.
ISSN: 0003-004X

hal-00758071
https://hal.archives-ouvertes.fr/hal-00758071
DOI : 10.2138/am.2012.3869

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
Abstract View references (45)Graphene oxide-iron oxide (GO-Fe3O4) nanocomposites were synthesised by co-precipitating iron salts onto GO sheets in basic solution. The results showed that formation of two distinct structures was dependent upon the GO loading. The first structure corresponds to a low GO loading up to 10wt%, associated with the beneficial intercalation of GO within Fe3O4nanoparticles and resulting in higher surface area up to 409m2g-1. High GO loading beyond 10 wt% led to the aggregation of Fe3O4 nanoparticles and the undesirable stacking of GO sheets. The presence of strong interfacial interactions (Fe-O-C bonds) between both components at low GO loading lead to 20% higher degradation of Acid Orange 7 than the Fe3O4 nanoparticles in heterogeneous Fenton-like reaction. This behaviour was attributed to synergistic structural and functional effect of the combined GO and Fe3O4 nanoparticles.
ISSN: 2045-2322

hal-01683208
https://hal.univ-antilles.fr/hal-01683208
DOI : 10.1038/srep04594

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Larsemann Hills area is part of a reworked early Neoproterozoic metamorphic terrain in southwestern Prydz Bay, East Antarctica. Ultramafic and mafic granulites, whose origins remain controversial, occur as lenses, boudins or layered bodies within the para- and ortho-gneiss in the region. The ultramafic and mafic granulites show spinel-olivine-bearing and two-pyroxene-bearing mineral assemblages recrystallized at 860–900 °C. Their bulk rock analyses indicate an origin as igneous cumulates, with high Mg# (molar MgO/(MgO + FeO)) from 0.73 to 0.84 for ultramafic granulite and from 0.46 to 0.78 for mostly mafic granulite as well as high Cr and Ni contents ([Cr] and [Ni] up to 1826 ppm and 1400 ppm respectively for ultramafic granulite and [Cr] of 1460 ppm for mafic granulite). Trace element patterns show pronounced negative Nb anomalies, suggesting a subduction-related tectonic setting for their precursors, consistent with derivation from arc basalts, also suggested by low TiO2 and K-enrichment in mafic granulites. The ultramafic to mafic granulites may have been formed in a subduction-related continental back-arc basin environment simultaneously with peak metamorphism associated with arc-continent collision during the early Neoproterozoic (990–900 Ma) Rayner orogeny.
ISSN: 0743-9547

hal-01622589
https://hal.archives-ouvertes.fr/hal-01622589
DOI : 10.1016/j.jseaes.2017.06.012

Éditeur(s) : HAL CCSD Integrated Ocean Drilling Program
Résumé : International audience
A detailed study of magnetic properties was carried out on ~60 samples from three holes cored during Integrated Ocean Drilling Program Expedition 324, Shatsky Rise. The measurements include scanning electron microscopy (SEM), hysteresis, and first-order reversal curve (FORC) diagrams, as well as measurements of susceptibility at high and low temperatures and variation of saturation isothermal magnetization at low temperature. SEM measurements show the presence of a complex mixture of Fe-Ti oxides with various Ti content x, which is confirmed by the measurements of susceptibility variation with temperature and low-temperature measurements. According to FORC diagrams, the magnetization is carried by single-domain to pseudosingle-domain grains.
ISSN: 1930-1014

hal-00795484
https://hal.archives-ouvertes.fr/hal-00795484
DOI : 10.2204/iodp.proc.324.201.2013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The present study is based on a set of lavas and crosscutting dikes collected by dives along detailed vertical transects on the northern flank of the western part of the Blanco Transform Fault, Northeast Pacific. The studied area consists of a small basin, the Western Blanco Depression (WBD), extending from the southern end of the Juan de Fuca ridge to a pseudofault trace 60 km eastward. The Northern Scarp of the WBD comprises a volcanic unit overlying a sheeted-dike complex. Major and trace element data, coupled with Sr–Nd isotope ratios, reveal a two-component mantle source, composed by an isotopically depleted matrix variably veined by more enriched material. One chemical group (NS2), indistinguishable from the other Northern Scarp samples on the basis of trace element data, has an unusually depleted isotopic composition typical of a nearly pure mantle end-member. Some cogenetic samples of the Northern Scarp have been used to constrain the differentiation modalities. Anorthite and MgO content profiles in plagioclase xenocrysts and phenocrysts reveal (i) the existence of H2O-bearing evolved melts in the mushy zones and (ii) the occurrence of mixing process between these melts and anhydrous mafic liquids. The hydration is supported by other petrographic features such as high magmatic fO2 values, calculated from Fe–Ti oxide pairs, and the presence of pyroxene inclusions in plagioclase phenocrysts. Mixing, consistent with the existence of Ni-rich ferrobasalts, is interpreted to be the consequence of the reservoir refilling by mafic liquids (Mg# = 70). These petrological and geochemical evidences are combined with the evolution of Mg# with depth to suggest a periodic open-system magma chamber evolution beneath the southern end of the Juan de Fuca ridge.
ISSN: 0024-4937

insu-00213426
https://hal-insu.archives-ouvertes.fr/insu-00213426
DOI : 10.1016/j.lithos.2007.06.009

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Geochemical studies of primitive basalts have documented the presence of crustal-derived garnet pyroxenite in their mantle sources. The processes whereby melts with the signature of garnet pyroxenite are produced in the mantle are, however, poorly understood and somewhat controversial. Here we investigate a natural example of the interaction between melts of garnet pyroxenite derived from recycled plagioclase-rich crust and surrounding mantle in the Ronda peridotite massif. Melting of garnet pyroxenite at ∼1.5 GPa generated spinel websterite residues with MREE/HREE fractionation and preserved the positive Eu anomaly of their garnet pyroxenite precursor in whole-rock and clinopyroxene. Reaction of melts from garnet pyroxenite with depleted surrounding peridotite generated secondary fertile spinel lherzolite. These secondary lherzolites differ from common spinel lherzolite from Ronda and elsewhere by their lower-Mg# in clinopyroxene, orthopyroxene and olivine, lower-Cr# in spinel and higher whole-rock Al2O3, CaO, Sm/Yb and FeO⁎ at a given SiO2. Remarkably, secondary spinel lherzolite shows the geochemical signature of ghost plagioclase in the form of positive Eu and Sr anomalies in whole-rock and clinopyroxene, reflecting the transfer of a low-pressure crustal imprint from recycled pyroxenite to hybridized peridotite. Garnet pyroxenite melting and melt-peridotite interaction, as shown in the Ronda massif, may explain how the signature of subducted or delaminated crust is transferred to the mantle and how a garnet pyroxenite component is introduced into the source region of basalts. The efficiency of these processes in conveying the geochemical imprint of crustal-derived garnet pyroxenite to extruded lavas depends on the reactivity of pyroxenite melt with peridotite and the mantle permeability, which may be controlled by prior refertilization reactions similar to those documented in the Ronda massif. Highly fertile heterogeneities produced by pyroxenite-peridotite interaction, such as secondary spinel lherzolite in Ronda, may nucleate magmatic channels that remain chemically isolated from the ambient mantle and act as preferential pathways for melts with the signature of recycled crust.
ISSN: 0012-821X

hal-00814010
https://hal.archives-ouvertes.fr/hal-00814010
DOI : 10.1016/j.epsl.2012.11.047

Éditeur(s) : HAL CCSD Société Géologique de France
Résumé : Xenoliths brought up by kimberlite magmas are rare samples of otherwise inaccessible lithospheric mantle. Eclogite xenoliths are found in most cratons and commonly show a range of mineral and chemical compositions that can be used to better understand craton formation. This study focuses on five new kyanite-bearing eclogites from the Udachnaya kimberlite pipe (367±5 Ma). They are fine-to coarse-grained and consist mainly of “cloudy” clinopyroxene (cpx) and garnet (grt). The clinopyroxene is Al,Na-rich omphacite while the garnet is Ca-rich, by contrast to typical bi-mineral (cpx+grt) eclogites that contain Fe- and Mg-rich garnets. The Udachnaya kyanite eclogites are similar in modal and major element composition to those from other cratons (Dharwar, Kaapvaal, Slave, West African). The kyanite eclogites have lower REE concentrations than bi-mineral eclogites and typically contain omphacites with positive Eu and Sr anomalies, i.e. a “ghost plagioclase signature”. Because such a signature can only be preserved in nonmetasomatised samples, we infer that they were present in the protoliths of the eclogites. It follows that subducted oceanic crust is present at the base of the Siberian craton. Similar compositions and textures are also seen in kyanite eclogites from other cratons, which we view as evidence for an Archean, subduction-like formation mechanism related to craton accretion. Thus, contrary to previous work that classifies all kyanite eclogites as type I (IK), metasomatized by carbonatite/kimberlitic fluids, we argue that some of them, both from this work and those from other cratons, belong to the non-metasomatized type II (IIB). The pristine type IIB is the nearest in composition to protoliths of mantle eclogites because it contains no metasomatic enrichments.
ISSN: 0037-9409

hal-01622617
https://hal.archives-ouvertes.fr/hal-01622617
DOI : 10.1051/bsgf/2017008

Éditeur(s) : HAL CCSD Elsevier
Résumé : Kyanite and zoisite eclogites from the Limousin area in the Variscan French Massif Central (FMC) have been formed by deep subduction to 100 km depth at peak pressure-temperature conditions around 660 degrees C and 2.9 GPa. They belong to a thin tectonic unit comprising ophiolitic bodies devoid of HP-UHP markers and forming a transported suture zone now separating the two main lithotectonic units of the FMC with contrasted tectono-metamorphic features (namely the lower and upper allochthons). Composition of kyanite-eclogites is similar to supra-subduction-zone basalts found in back-arc basins or island arcs (LREE depleted, negative Nb-Ta anomalies, epsilon Nd: +6 to + 9). Zoisite eclogites have the major-element fingerprint of ultrabasic iron-rich plagioclase cumulates common in continental intrusions or forming Fe-Ti oceanic gabbros. The distribution of immobile trace-elements is however more comparable to alkaline Si-undersaturated rocks, but the Sr-Nd isotopic composition (Sr-87/Sr-86: 0.706; epsilon Nd: -3 to -5) suggests the intervention of an old radiogenic crustal component during their petrogenesis. The chemical composition of hydrous zoisite eclogites does not correspond closely to a common magmatic precursor. It is thus suggested that, by analogy with well exposed worldwide HP-UHP terranes, at least a part of the trace-element signature and the isotopic compositions are controlled by channelled fluid-rock interaction at UHP conditions, leading to localised segregation of zoisite that hosts nearly all Sr and LREE budgets of the bulk eclogite. Sr-Nd isotopic composition of these samples is thus controlled by zoisite and probably represents the isotopic composition of the fluid phase present during UHP recrystallisation. Zircons extracted from a zoisite eclogite were dated in-situ by LA-ICP-MS. Crystallisation of the magmatic precursor is bracketed between 489 and 475 Ma and the UHP event is dated at 412 +/- 10 Ma, in agreement with published ages for the Eo-Variscan HP stage in the FMC. A late resetting of the U-Pb isotopic system at 382 +/- 7 Ma is related to an anatectic high pressure-medium pressure event, well known in the Limousin area. The oceanic eclogites dated in this study are tentatively linked to the Galicia-South Brittany Lower Paleozoic Ocean; they show evidence of the existence of an intra-oceanic subduction zone at the Lower Ordovician and of partial closure of this oceanic domain by ocean-continent subduction during Early Devonian.
ISSN: 0024-4937

hal-00512835
https://hal.archives-ouvertes.fr/hal-00512835
DOI : 10.1016/j.lithos.2010.05.013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Almost all Red Sea deeps contain metal-rich sediments covered by brine pools. It is generally agreed that these metal-rich deposits precipitated from overlying metal-rich brines that originated from migrating hydrothermal fluids. No brine pool has ever been reported in Thetis Deep, inciting us to evaluate if such a brine layer ever occurred in the deep during the past. In order to address that questioning, a study combining mineralogical, geochemical (major-, minor-, rare-earth elements) and isotopic (Sr. Nd. Pb) approaches was completed on cored sediments and extracted interstitial waters from inside and outside the deep.;The sediments have an overall hydrothermal origin, as shown by the REE concentrations and patterns, metal contents, and Pb-Nd isotopic data, all pointing to a mantle signature. The intensity of the hydrothermal activity varied with time in the deep; the most intense episode resulting in an almost pure Fe-oxi-hydroxide layer. Varied chemical arguments, especially the Zr and REE data of the sediments, favor the fact that the whole sedimentation in Thetis Deep occurred in the absence of a stable, salt-rich and mineralized brine pool, and that no brine layer ever existed. This conclusion is supported by the constant Sr isotope composition of the sediment and its interstitial waters that are almost identical to that of the Red Sea seawater. The study also suggests that hydrothermal activity monitored fluid supplies that interacted differently with seawater in the different Red Sea deeps, resulting in an overall formation of metal-rich sediments, but along varied local conditions. (C) 2009 Elsevier B.V. All rights reserved.
ISSN: 0375-6742

hal-00475646
https://hal.archives-ouvertes.fr/hal-00475646
DOI : 10.1016/j.gexplo.2009.10.001

Éditeur(s) : Paris : Piget
Résumé : Ou Dénombrement des plantes, des animaux, & des minéraux qui fe trouvent dans l'ifle de Cayenne, les ifles de Remire, fur les côtes de la mer, & dans le continent de la Guyane
Siècle(s) traité(s) : 18
Droits : Domaine public
Provenance : Collectivité territoriale de Guyane. Bibliothèque Alexandre-Franconie
Permalien : http://www.manioc.org/patrimon/FRA11085
FRA11085