Éditeur(s) : HAL CCSD Elsevier
Résumé : High-precision Ni isotopic variations are reported for the metal phase of equilibrated and unequilibrated ordinary chondrites, carbonaceous chondrites, iron meteorites, mesosiderites, and pallasites. We also report new Zn and Cu isotopic data for some of these samples and combine them with literature Fe, Cu, and Zn isotope data to constrain the fractionation history of metals during nebular (vapor/solid) and planetary (metal/sulfide/silicate) phase changes. The observed variations of the Ni-62/Ni-58, Ni-61/Ni-58, and Ni-60/Ni-58 ratios vary linearly with mass difference and define isotope fractionation lines in common with terrestrial samples. This implies that Ni was derived from a single homogeneous reservoir. While no Ni-60 anomaly is detected within the analytical uncertainties, Ni isotopic fractionation up to 0.45 parts per thousand per mass-difference unit is observed. The isotope compositions of Ni and Zn in chondrites are positively correlated. We suggest that, in ordinary chondrites, exchange between solid phases, in particular metal and silicates, and vapor followed by mineral sorting during accretion are the main processes controlling these isotopic variations. The positive correlation between Ni and Zn isotope compositions contrasts with a negative correlation between Ni (and Zn) and Cu isotope compositions, which, when taken together, do not favor a simple kinetic interpretation. The observed transition element similarities between different groups of chondrites and iron meteorites are consistent with the genetic relationships inferred from oxygen isotopes (IIIA/pallasites and IVA/L chondrites). Copper is an exception, which we suggest may be related to separate processing of sulfides either in the vapor or during core formation. (C) 2007 Elsevier Ltd. All rights reserved.
ISSN: 0016-7037

hal-00408304
https://hal.archives-ouvertes.fr/hal-00408304
DOI : 10.1016/j.gca.2007.06.049

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
CK chondrites are highly oxidized meteorites containing abundant magnetite and trace amounts of Fe,Ni metal. Although the group is predominately composed of equilibrated meteorites (types 4-6), in recent years a significant number of new samples have been classified as being either CK3 or CK3-anomalous. These unequilibrated CKs often display a close affinity with members of the CV oxidized subgroup. CKs and CVs (oxidized subgroup) may therefore form a continuum and by implication could be derived from a single common parent body. To investigate the relationship between these two groups a detailed study of the oxygen isotope composition, opaque mineralogy and major and trace element geochemistry of a suite of CV and CK chondrites has been undertaken. The results of oxygen isotope analysis confirm the close affinity between CV and CK chondrites, while excluding the possibility of a linkage between the CO and CK groups. Magnetites in both CV and CK chondrites show significant compositional similarities, but high Ti contents are a diagnostic feature of the latter group. The results of major and trace element analysis demonstrate that both CV and CK chondrites show overlapping variation. Supporting evidence for a single common source for both groups comes from their similar cosmic-ray exposure age distributions. Recent reflectance spectral analysis is consistent with both the CVs and CKs being derived from Eos family asteroids, which are believed to have formed by the catastrophic disruption of a single large asteroid. Thus, a range of evidence appears to be consistent with CV and CK chondrites representing samples from a single thermally stratified parent body. In view of the close similarity between CV and CK chondrites some modification of the present classification scheme may be warranted, possibly involving integration of the two groups. One means of achieving this would be to reassigned CK chondrites to a subgroup of the oxidized CVs. It is recognized that a full evaluation of this proposal may require further study of the still poorly understood CK3 chondrites.
ISSN: 0016-7037

hal-00474633
https://hal.archives-ouvertes.fr/hal-00474633
DOI : 10.1016/j.gca.2009.11.029

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report trace element concentrations of silicate phases in chondrules from LL3 ordinary chondrites Bishunpur and Semarkona. Results are similar to previously reported data for carbonaceous chondrites, with rare earth element (REE) concentrations increasing in the sequence olivine < pyroxene < mesostasis, and heavy REE (HREE) being enriched by 1–2 orders of magnitude (CI-normalized) relative to light REE (LREE) in ferromagnesian silicates, although no single olivine with very large LREE/HREE fractionation has been found. On average, olivine in type II chondrules is poorer in refractory lithophile incompatible elements (such as REE) than its type I counterpart by a factor of ∼2. This suggests that olivine in type I and II chondrules formed by batch and fractional crystallization, respectively, implying that type II chondrules formed under faster cooling rates (>∼10 K/h) than type I chondrules. Appreciable Na concentrations (3–221 ppm) are measured in olivine from both chondrule types; type II chondrules seem to have behaved as closed systems, which may require chondrule formation in the vicinity of protoplanets or planetesimals. At any rate, higher solid concentrations in type II chondrule forming regions may explain the higher oxygen fugacities they record compared to type I chondrules. Type I and type II chondrules formed in different environments and the correlation between high solid concentrations and/or oxygen fugacities with rapid cooling rates is a key constraint that chondrule formation models must account for.
ISSN: 0016-7037

hal-01174084
https://hal.archives-ouvertes.fr/hal-01174084
DOI : 10.1016/j.gca.2015.02.005

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
We report in situ LA-ICP-MS trace element analyses of silicate phases in olivine-bearing chondrules in the Sahara 97096 (EH3) enstatite chondrite. Most olivine and enstatite present rare earth element (REE) patterns comparable to their counterparts in type I chondrules in ordinary chondrites. They thus likely share a similar igneous origin, likely under similar redox conditions. The mesostasis however frequently shows negative Eu and/or Yb (and more rarely Sm) anomalies, evidently out of equilibrium with olivine and enstatite. We suggest that this reflects crystallization of oldhamite during a sulfidation event, already inferred by others, during which the mesostasis was molten, where the complementary positive Eu and Yb anomalies exhibited by oldhamite would have possibly arisen due to a divalent state of these elements. Much of this igneous oldhamite would have been expelled from the chondrules, presumably by inertial acceleration or surface tension effects, and would have contributed to the high abundance of opaque nodules found outside them in EH chondrites. In two chondrules, olivine and enstatite exhibit negatively sloped REE patterns, which may be an extreme manifestation of a general phenomenon (possibly linked to near-liquidus partitioning) underlying the overabundance of light REE observed in most chondrule silicates relative to equilibrium predictions. The silicate phases in one of these two chondrules show complementary Eu, Yb, and Sm anomalies providing direct evidence for the postulated occurrence of the divalent state for these elements at some stage in the formation reservoir of enstatite chondrites. Our work supports the idea that the peculiarities of enstatite chondrites may not require a condensation sequence at high C/O ratios as has long been believed.
ISSN: 1086-9379

hal-01217095
https://hal.archives-ouvertes.fr/hal-01217095
DOI : 10.1111/maps.12481

Éditeur(s) : HAL CCSD Elsevier
Résumé : The uppermost mantle in back arc regions is the site of complex interactions between partial melting, melt percolation, and fluid migration. To constrain these interactions and evaluate their consequences on geochemical cycles, we carried out an in situ trace element and water study of a suite of spinel peridotite xenoliths from two regions of the Japan back arc system, Ichinomegata (NE Japan) and Oki-Dogo (SW Japan), using LA-ICPMS and FTIR spectrometry, respectively. This study provides the first full dataset of trace element and hydrogen compositions in peridotites including analyses of all their main constitutive silicate minerals: olivine, orthopyroxene and clinopyroxene. The Ichinomegata peridotites sample a LREE-depleted refractory mantle (Mg# olivine = 0.90; Cr# spinel = 0.07–0.23; Yb clinopyroxene = 7.8–13.3 × C1-chondrite, and La/Yb clinopyroxene = 0.003–0.086 × C1-chondrite), characterized by Th-U positive anomalies and constant values of Nb/Ta. The composition of the studied Ichinomegata samples is consistent with that of an oceanic mantle lithosphere affected by cryptic metasomatic interactions with hydrous/aqueous fluids (crypto-hydrous metasomatism). In contrast, the Oki-Dogo peridotites have low Mg# olivine (0.86–0.93) and a broad range of compositions with clinopyroxene showing “spoon-shaped” to flat, and LREE-enriched patterns. They are also characterized by their homogeneous compositions in the most incompatible LILE (e.g., Rb clinopyroxene = 0.01–0.05 × primitive mantle) and HFSE (e.g., Nb clinopyroxene = 0.01–2.16 × primitive mantle). This characteristic is interpreted as resulting from various degrees of melting and extensive melt-rock interactions. FTIR spectroscopy shows that olivine in both Ichinomegata and Oki-Dogo samples has low water contents ranging from 2 to 7 ppm wt. H2O. In contrast, the water contents of pyroxenes from Ichinomegata peridotites (113–271 ppm wt. H2O for orthopyroxene, and 292–347 ppm wt. H2O for clinopyroxene) are significantly higher than in Oki-Dogo peridotites (9–35 ppm wt. H2O for orthopyroxene, and 15–98 ppm wt. H2O for clinopyroxene). This indicates a relationship between melt-rock interaction and water concentrations in pyroxenes. Our study suggests that the water content of the Japan mantle wedge is controlled by the late melt/fluid/rock interactions evidenced by trace element geochemistry: a mechanism triggered by magma-rock interactions may have acted as an efficient dehydrating process in the Oki-Dogo region while the Ichinomegata mantle water content is controlled by slab-derived crypto-hydrous metasomatism.
ISSN: 0016-7037

hal-01553105
https://hal.archives-ouvertes.fr/hal-01553105
DOI : 10.1016/j.gca.2017.03.042

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Orogenic lherzolites exhibit a specific (although reproducible) platinum-group element signature characterized by slight positive deviations of Pd/Ir, Rh/Ir and Ru/Ir ratios from the canonical chondritic model. Such a signature was alternatively considered to be a compositional feature of the primitive upper mantle or resulting from sulfide melt addition by refertilisation reactions in the continental lithosphere. To shed additional light on this conundrum, the distribution of PGE in FON B 93, an unserpentinized, fertile (3.24% Al2O3) orogenic lherzolite (French Pyrenees) used as in-house standard by our group, has been studied to different scales, from the bulk rock to trace minerals. In addition to new ICPMS analyses after separation into Nis and Te coprecipitation, its whole-rock PGE concentrations were redetermined by three different laboratories using very precise ID-ICP-MS methods after digestion of the sample in Carius tube at T=250 degrees C and 320 degrees C or in high-pressure asher (HPA-S). The four methods produce reproducible Ru, Rh and Pd contents (7.1 +/- 0.18 ppb; 1.43 +/- 0.05 ppb; 7.1 +/- 0.30 ppb) whereas the non-ID NiS-fire assay method underestimates the Os and Ir concentrations by c.a. 10 and 15% compared to ID-ICP-MS analyses (4.40 +/- 0.07 and 4.00 +/- 0.17 ppb, respectively). Platinum was the most difficult to analyse. If performed on powder aliquots smaller than 3 g, the ID-ICP-MS analyses generate strong nugget effects while the non-ID NiS-fire assay method yields statistically lower (but highly reproducible) Pt concentrations (6.92 +/- 0.26 ppb). These features reflect the strong partitioning of Pt into trace phases that SEM and laser ablation-ICP-MS analyses on thin sections identify as both high-temperature Pt-Ir-Os alloys and Pt-(Pd) tellurides of likely subsolidus origin; ten to fifteen grains ranging in maximum dimension from a few micrometres to a few hundreds of nanometers, were identified by standard polished thin section. LA-ICP-MS data on base metal sulfides (25 grains analysed), coupled with the whole-rock S concentration (277 +/- 10 ppm) and modal composition of the BMS (90% pentlandite+accessory pyrite and secondary pyrrhotite + 10% chalcopyrite) allowed the contribution of the BMS phase to the PGE budget of FON B 93 to be estimated. Except Pt that exhibits a 95% deficit in the BMS phase, the PGE concentrations measured by ID-ICP-MS can be balanced by BMS while Al-spinel is a negligible contributor, accounting for less than 0.5% of the Ru budget. The occurrence of Pt in trace phases may bias the whole-rock Pt concentrations because 1) mechanical collection of Pt-rich trace phases remains problematic with the Nis button, and 2) Pt-Ir-Os alloys may be prone to digestion problems in conventional Carius tube procedures. Since they could be stable at mantle depth, such refractory alloys that contain Os and Ir may also have enhanced the heavy PGEs/light PGEs fractionation. Our observations likely pertain to orogenic lherzolites as a whole because BMS assemblages in these mantle rocks record evolution at low sulfur fugacity, which prevents Pt-Ir-Os alloy from entering the Mss in the mantle; moreover, at subsolidus temperature, pentlandite, the major BMS, cannot accommodate 0 valence state Pt.
ISSN: 0009-2541

hal-00411486
https://hal.archives-ouvertes.fr/hal-00411486
DOI : 10.1016/j.chemgeo.2007.06.030

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The platinum-group element (PGE) systematics of continental mantle peridotites show large variability, reflecting petrogenetic processing of the upper mantle during partial melting and melt/fluid percolation inside the lithosphere. By removing Pd-Cu-Ni rich sulfides, partial melting events that have stabilized the sub-continental mantle lithosphere fractionated PPGEs (Palladium-group PGE; Pt, Pd) relative to IPGEs (Iridium-group PGE; Os, Ir, Ru, Rh). Residual base-metal sulfides (BMS) survive as enclosed IPGE-enriched Monosulfide Solid Solutions (Mss), which otherwise decompose into Ru-Os-Ir-rich refractory platinum-group minerals (PGMs) once the partial melts become S-undersaturated. The small-scale heterogeneous distribution of these microphases may cause extreme nugget effects, as seen in the huge variations in absolute PGE concentrations documented in cratonic peridotites. Magmas fluxing through the lithospheric mantle may change the initial PGE budgets inherited from the melting events, resulting in the great diversity of PGE systematics seen in peridotites from the sub-continental lithosphere. For instance, melt-rock reactions at increasing melt/rock ratios operate as open-system melting processes removing residual BMS/PGMs. Highly percolated peridotites are characterized by extreme PGE depletion, coupled with PGE patterns and Os-isotope compositions that gradually evolve toward that of the percolating melt. Reactions at decreasing melt-rock ratios (usually referred to as "mantle metasomatism") precipitate PPGE-enriched BMS that yield suprachondritic Pd/Ir and occasionally affect Pt/Ir and Rh/Ir ratios as well. Moreover, volatile-rich, small volume melts fractionate Os relative to Ir and S relative to Se, thereby producing rocks with supra-chondritic Os/Ir and S/Se coupled with supra-chondritic Pd/Ir and Pt/Ir. Major magmatic inputs at the lithosphere-asthenosphere boundary may rejuvenate the PGE systematics of the depleted mantle. Integrated studies of "refertilized" peridotites with worldwide provenance provide evidence for mixing between old PGM-rich harzburgitic protoliths and newly-precipitated BMS. Long-lived PGMs carry the Os-isotope compositions of ancient melt‐depletion events into seemingly undepleted fertile lherzolites. Another diagnostic feature of major refertilization processes is the increasing modal abundance of Pt-Pd-Te-Bi or Pt-As-S microphases. Due to regional-scale refertilization processes, sizeable (> 100 km) domains of the upper lithospheric mantle are now significantly enriched in Pd, Au, Cu, Se, and other incompatible chalcophile elements that are of considerable importance in PGE-ore forming events.
ISSN: 0024-4937

hal-00828626
https://hal.archives-ouvertes.fr/hal-00828626
DOI : 10.1016/j.lithos.2012.08.017

Éditeur(s) : HAL CCSD GeoScienceWorld
Résumé : International audience
Due to their "iron-loving" properties, platinum-group elements (PGE) are expected to be stored in the Earth's core. Although very low, at a few parts per billion, PGE concentrations measured in mantle-derived rocks are too high to be in chemical equilibrium with the core. The "late veneer" model offers the best explanation for this paradox-it postulates that a flux of primitive meteorites hit the early Earth after core formation had ceased. However, the inferred PGE composition of the hypothetical primitive mantle exhibits slight positive excesses of Ru, Rh, and Pd compared to the canonical chondritic signature. Such deviations have trigger debate about the composition of the late veneer and the extent of reworking of PGE signatures by igneous processes within the Earth's mantle.
ISSN: 1811-5209

hal-00411483
https://hal.archives-ouvertes.fr/hal-00411483
DOI : 10.2113/GSELEMENTS.4.4.247

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Whole-rock concentrations of Se and Te were determined in 53 peridotite samples collected in orogenic bodies from the Eastern Pyrenees, including the Lherz massif (33 samples), the type-locality of Iherzolite. Orogenic Iherzolites tectonically emplaced into the continental crust escaped devolatilisation that may strongly alter concentrations of those volatile trace elements in basalt-hosted peridotite xenoliths. The Se and Te concentration ranges (<3.5-100.7 ppb; <1-11 ppb) are consistent with the few analyses of mantle samples yet available, yet extending to higher Se concentrations. In-situ analyses performed by LAM-ICPMS on base metal sulfides indicate that the Se budget is controlled by sulfides whereas a large fraction of Te (50 +/- 30%) does not reside in sulfides. The missing fraction is accounted for by fractional crystallization of Pt-Pd tellurides from Cu-Ni sulfide melts. A maximum of one third of the S budget of Eastern Pyrenean peridotites was added by assimilation of crustal sulfides, which crystallized as pyrite. Disregarding those contaminated samples, some features of the Se and Te systematics (e.g. bivariate plots vs. fertility index) cannot be explained by partial melting processes. The best fit for 33 peridotites (mostly from Lherz) yields higher than chondritic S/Se (3.1213 +/- 500; r(2) = 0.96) for near chondritic Se/Te (8.6 +/- 2; r(2) = 0.9). Our study suggest that such superchondritic S/Se ratios are better explained by petrogenetic processes involving refertilization of harzburgitic lithosphere by metal (Cu,Ni)-rich sulfide melts precipitated along with clinopyroxene and Al-spinel from silicate melts en-route to the surface.
ISSN: 0009-2541

hal-00544577
https://hal.archives-ouvertes.fr/hal-00544577
DOI : 10.1016/j.chemgeo.2010.09.007

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
We report trace element analyses by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of metal grains from nine different CR chondrites, distinguishing grains from chondrule interior ("interior grains"), chondrule surficial shells ("margin grains"), and the matrix ("isolated grains"). Save for a few anomalous grains, Ninormalized trace element patterns are similar for all three petrographic settings, with largely unfractionated refractory siderophile elements and depleted volatile Au, Cu, Ag, S. All three types of grains are interpreted to derive from a common precursor approximated by the least-melted, fine-grained objects in CR chondrites. This also excludes recondensation of metal vapor as the origin of the bulk of margin grains. The metal precursors were presumably formed by incomplete condensation, with evidence for high-temperature isolation of refractory platinum-group-element (PGE)-rich condensates before mixing with lower temperature PGE-depleted condensates. The rounded shape of the Ni-rich, interior grains shows that they were molten and that they equilibrated with silicates upon slow cooling (1-100 K h 1), largely by oxidation/evaporation of Fe, hence their high Pd content, for example. We propose that Ni-poorer, amoeboid margin grains, often included in the pyroxene-rich periphery common to type I chondrules, result from less intense processing of a rim accreted onto the chondrule subsequent to the melting event recorded by the interior grains. This means either that there were two separate heating events, which formed olivine/ interior grains and pyroxene/margin grains, respectively, between which dust was accreted around the chondrule, or that there was a single high-temperature event, of which the chondrule margin records a late "quenching phase," in which case dust accreted onto chondrules while they were molten. In the latter case, high dust concentrations in the chondrule-forming region (at least three orders of magnitude above minimum mass solar nebula models) are indicated.
ISSN: 1086-9379

hal-01003975
https://hal.archives-ouvertes.fr/hal-01003975
DOI : 10.1111/maps.12212

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Chromitite pods in the Mayarí-Cristal ophiolitic massif (eastern Cuba) were formed in the Late Cretaceous when island arc tholeiites and MORB-like back-arc basin basalts reacted with residual mantle peridotites and generated chromite-rich bodies enclosed in dunite envelopes. Platinum-group minerals (PGM) in the podiform chromitites exhibit important Os-isotope heterogeneities at the kilometric, hand sample and thin section scales. 187Os/188Os calculated at the time of chromitite crystallization (~90 Ma) ranges between 0.1185 and 0.1295 (γOs = −7.1 to +1.6, relative to enstatite chondrite), and all but one PGM have subchondritic 187Os/188Os. Grains in a single hand sample have initial 187Os/188Os that spans from 0.1185 to 0.1274, and in one thin section it varies between 0.1185 and 0.1232 in two PGM included in chromite which are only several millimeters apart. As the Os budget of a single micrometric grain derives from a mantle region that was at least several m3 in size, the variable Os isotopic composition of PGM in the Mayarí-Cristal chromitites probably reflects the heterogeneity of their mantle sources on the 10-100 m scale. Our results show that this heterogeneity was not erased by pooling and mingling of individual melt batches during chromitite crystallization but was transferred to the ore deposits on mineral scale. The distribution of the Os model ages calculated for PGM shows four main peaks, at ~100, 500, 750 and 1,000 Ma. These variable Os model ages reflect the presence of different depleted domains in the oceanic (Pacific-related) upper mantle of the Greater Antilles paleo-subduction zone. The concordance between the age of crystallization of the Mayarí-Cristal chromitites and the most recent peak of the Os model age distribution in PGM supports that Os in several grains was derived from fertile domains of the upper mantle, whose bulk Os isotopic composition is best approximated by that of enstatite chondrites; on the other hand, most PGM are crystallized by melts that tapped highly refractory mantle sources.
ISSN: 0010-7999

hal-00822742
https://hal.archives-ouvertes.fr/hal-00822742
DOI : 10.1007/s00410-010-0575-2

Éditeur(s) : HAL CCSD The Geochemical Society of Japan
Résumé : International audience
The Western Cordillera of Ecuador consists of Cretaceous crustal fragments of oceanic plateaux and superimposed insular arcs, which were accreted to the northwestern South American margin during the Late Cretaceous and Paleocene. Slices of high-grade metabasites, ultramafic rocks, gabbros and basalts, unmetamorphosed radiolarian cherts and scarce garnet-bearing metasediments were randomly exhumed along Miocene to Recent transcurrent faults crosscutting the Western Cordillera. The basalts show geochemical characteristics of oceanic plateau basalts (flat REE patterns, La/Nb = 0.85). The gabbros differ from the basalts in having lower REE levels, positive Eu anomalies, and negative Nb and Ta anomalies; they are interpreted as resulting from arc magmatism. The amphibolites and banded amphibolites have major and trace element chemistry similar to that of oceanic plateau basalts (flat REE patterns, La/Nb = 0.86) or to cumulate gabbros. The granulite shares with oceanic plateaus similar trace element chemistry (flat REE patterns, La/Nb < 1) and εNdi values (+7.6). Continent-derived metasediments are depleted in heavy REE (La/Y = 4.8) and have a negative Eu anomaly. Foliated lherzolites, melagabbronorites and pyroxenites consist of serpentinized olivine + cpx + opx ± Ca-plagioclase. Lherzolites, melagabbronorites and pyroxenites are LREE depleted with positive Eu anomalies, while the harzburgite displays a U-shaped REE pattern. The trace element abundances of the ultramafic rocks are very low (0.1 to 1 times the chondritic and primitive mantle values). The ultramafic rocks represent fragments of depleted mantle, deformed cpx-rich cumulate, and continental lithospheric mantle or mantle contaminated by subduction-fluid. Except the scarce quartz-rich metasediments, all these rocks likely represent remnants of accreted oceanic crustal fragments and associated depleted mantle. Since these samples were randomly sampled at depth by the fault, we propose that the Western Cordillera and its crustal root are mainly of oceanic nature.
ISSN: 0016-7002

insu-00679228
https://hal-insu.archives-ouvertes.fr/insu-00679228

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Mantle-derived xenoliths hosted by melilitite lavas from In Teria (Ahaggar, SE Algeria) include garnet and spinel peridotites, pyroxenite and phlogopite megacrysts. The spinel and garnet peridotites record an early deformation event, which formed porphyroclastic microstructures and olivine crystal preferred orientations, followed by static infiltration of hydrous alkaline melts. This metasomatic stage (stage 1) is characterized by the crystallization of phlogopite in the garnet and spinel peridotites, amphibole in the spinel peridotites and clinopyroxene in the garnet peridotite, which record chemical equilibration with an alkaline silicate melt. These early events were largely overprinted by carbonatitic metasomatism (stage 2), which is observed only in the spinel peridotites. Spinel peridotite major and trace element compositions, as well as the compositions of newly formed minerals, are characteristic of interaction with carbonate melt, associated with strong enrichment in incompatible trace elements in clinopyroxene. This second stage was followed by crystallization of pyroxenites (stage 3) in vein conduits, probably segregated from alkaline melts. We propose a scenario in which the different metasomatic imprints record successive stages of interaction between lithospheric mantle and sublithospheric melts throughout the Cenozoic. In Sr–Nd isotope space, the host melilitites and several xenoliths are clustered and plot close to the HIMU mantle end-member. However, some peridotite xenoliths are shifted towards more radiogenic 87Sr/86Sr values. In 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–06Pb/204Pb space the In Teria samples define a relatively large domain characterized by high 206Pb/204Pb and 208Pb/204Pb, consistent with a contribution of an HIMU component, considered to represent a sublithospheric signature. The highest 87Sr/86Sr values are comparable with those ascribed to the EM1 mantle end-member, representing the signature of the lower continental lithosphere, and are probably inherited from the pre-metasomatic lithospheric mantle beneath In Teria. Numerical modelling of porous percolation of melt of sublithospheric origin through an EM1-like lithospheric mantle protolith reproduces the In Teria peridotite compositions, using moderately sub-chondritic Sr/Nd values for the peridotite (e.g. In Teria garnet peridotite) and moderately super-chondritic Sr/Nd values in the melt (approximately ocean island basalt values). A few spinel peridotites require a component characterized by a 143Nd/144Nd signature higher than both the EM1 end-member and the local Ahaggar basalts; the 208Pb/204Pb compositions of several samples point to a component with a depleted mid-ocean ridge basalt (MORB) mantle (DMM) signature. Thus the lithospheric mantle beneath In Teria probably did not have a uniform EM1 signature before the onset of metasomatism; it included a DMM peridotite component as well as some peridotites with elevated 143Nd/144Nd values recording long-term LREE depletion.
ISSN: 0022-3530

hal-01356781
https://hal.archives-ouvertes.fr/hal-01356781
DOI : 10.1093/petrology/egw009

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The Ronda orogenic peridotite (southern Spain) contains a variety of pyroxene-rich rocks ranging from high-pressure garnet granulites and pyroxenites to low-pressure plagioclasespinel websterites. The 'asthenospherized' part of the Ronda peridotite contains abundant layered websterites ('group C' pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitions towards their host peridotites. Previous studies have suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modelling study of a layered outcrop of group C pyroxenite near the locality of Tolox aimed at constraining the origin of these pyroxenites after host peridotites by pervasive pyroxene-producing, refertilization meltrock reactions. Mg-number [=Mg/(Mg + Fe) cationic ratio] numerical modelling shows that decreasing Mg-number with increasing pyroxene proportion, characteristic of Ronda group C pyroxenites, can be accounted for by a melt-consuming reaction resulting in the formation of mildly evolved, relatively low Mg-number melts (similar to 0.65) provided that the melt fraction during reaction and the time-integrated melt/rock ratio are high enough (>0.1 and >1, respectively) to balance Mg-Fe buffering by peridotite minerals. This implies strong melt focusing caused by melt channelling in high-porosity domains resulting from compaction processes in a partial melted lithospheric domain below a solidus isotherm represented by the Ronda peridotite recrystallization front. The chondrite-normalized rare earth element (REE) patterns of group C whole-rocks and clinopyroxenes are convex-upward. Numerical modeling of REE variations in clinopyroxene produced by a pyroxene-forming, melt-consuming reaction results in curved trajectories in the (Ce/Nd)(N) vs (Sm/Yb)(N) diagram (where N indicates chondrite normalized). Based on (Ce/Nd)(N) values, two transient, enriched domains between the light REE (LREE)-depleted composition of the initial peridotite and that of the infiltrated melt may be distinguished in the reaction column: (1) a lower domain characterized by convex-upward REE patterns similar to those observed in Ronda group C pyroxeniteperidotite; (2) an upper domain characterized by melts with strongly LREE-enriched compositions. The latter are probably volatile-rich, small-volume melt fractions residual after the refertilization reactions that generated group C pyroxenites, which migrated throughout the massifincluding the unmelted lithospheric spinel-tectonite domain. The Ronda mantle domains affected by pyroxenite- and dunite- or harzburgite-forming reactions (the 'layered granular' subdomain and 'plagioclase-tectonite' domain) are on average more fertile than the residual, 'coarse granular' subdomain at the recrystallization front. This indicates that refertilization traces the moving boundaries of receding cooling of a thinned and partially melted subcontinental lithosphere. This refertilization process may be widespread during transient thinning and melting of depleted subcontinental lithospheric mantle above upwelling asthenospheric mantle.
ISSN: 0022-3530

hal-00412200
https://hal.archives-ouvertes.fr/hal-00412200
DOI : 10.1093/petrology/egn014

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Highly siderophile elements (Platinum-group elements, Au and Re) are currently assumed to reside inside base metal sulfides (BMS) in the convecting upper mantle. However, fertile lherzolites sampled by Pyrenean orogenic peridotite massifs are unexpectedly rich in 0.5-3 mu m large micronuggets of platinum-group minerals (PGM). Among those, sulfides from the laurite-erlichmanite series (Ru, Os(Ir)S(As)2), Pt-Ir-Os alloys and Pt-Pd-Te-Bi phases (moncheite-merenskyite) are predominant. Not only the BMS phases but also the PGM micronuggets must be taken into account in calculation of the PGE budget of orogenic fertile lherzolites. Laurite is a good candidate for equilibrating the whole-rock budget of Os, it and Ru while accounting for supra-chondritic Ru/Ir-N. Textural relationships between PGMs and BMS highlight heterogeneous mixing between refractory PGMs (laurite/Pt-Ir-Os alloys) inherited from ancient refractory lithospheric mantle and late-magmatic metasomatic sulfides precipitated from tholeiitic melts. "Low-temperature" PGMs, especially Pt-Pd bismuthotellurides should be added to the list of mineral indicators of lithosphere refertilization process. Now disseminated within fertile lherzolites, "lithospheric" PGMs likely account for local preservation of ancient Os model ages (up to 2 Ga) detected in BMS by in-situ isotopic analyses. These PGMs also question the reliability of orogenic lherzolites for estimating the PGE signature of the Primitive Silicate Earth.
ISSN: 0012-821X

hal-00486109
https://hal.archives-ouvertes.fr/hal-00486109
DOI : 10.1016/j.epsl.2009.11.017

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Despite a relatively 'uniform' fertile composition (Al2O3 = 2 center dot 19-4 center dot 47 wt %; Fo% = 89 center dot 2 +/- 0 center dot 3%; Cr#(Spl) = 8 center dot 9 +/- 1 center dot 5%), the Montferrier peridotite xenoliths show a wide range of S contents (22-590 ppm). Most sulphides are interstitial and show peculiar pyrrhotite-pentlandite intergrowths and low abundances of Cu-rich phases. Sulphide-rich samples are characterized by strong enrichment in the light rare earth elements and large ion lithophile elements without concomitant enrichment of the high field strength elements. Such trace-element fractionation is commonly ascribed to metasomatism by volatile-rich melts and/or carbonatitic melts. S and Se (11-67 ppb), as well as S/Se (up to approximate to 12 000), are correlated with La/Sm. Cu, however, remains broadly constant (30 +/- 5 ppm). These features strongly suggest that the percolation-reaction of such volatile-rich fluids has led to sulphide enrichment with an atypical signature marked by strong fractionation of the chalcophile elements (i.e. S vs Se and Cu). S-rich xenoliths are also characterized by high (Pd/Ir)(N) (1 center dot 2-1 center dot 9; where subscript N indicates normalized to chondrite), (Pd/Pt)(N) between 1 center dot 5 and 2 center dot 2, and (Os/Ir)(N) up to 1 center dot 85. Despite the relative uniform fertile composition of the xenoliths, Re/Os ranges between 0 center dot 02 and 0 center dot 18. Os-187/Os-188 is extremely variable even within a single sample and can be as high as 0 center dot 1756 for the most S-rich samples. Sulphides show highly fractionated and variable abundances of the highly siderophile elements (HSE) [0 center dot 03 (Pd/Ir)(N) < 1283] and Re-Os isotopic composition (0 center dot 115 < Os-187/Os-188 < 0 center dot 172). Such variation can be observed at the thin-section scale. Whole-rock and in situ sulphide data demonstrate that chalcophile and HSE systematics and the Os isotopic composition of the upper mantle could be significantly modified through metasomatism, even with volatile-rich fluids. These features highlight the complex behaviour of the HSE in fluid-rock percolation-reaction models and suggest a complex interplay between sulphide addition (crystallization or sulphidation) and partial equilibration of pre-existing sulphide. The specific fractionations observed in chemical proxies such as S and Se, Os and Ir, and Pd and Pt, as well as the low abundance of Cu-rich sulphides, suggest that sulphide addition may not have occurred via sulphide melts. Rather, we infer that S was present as a dissolved species in a (supercritical) oxidizing, volatile-rich fluid (C-O-H-S +/- Cl) along with other chalcophile and siderophile elements such as Os, Pd, Re and Au. The highly radiogenic Os composition of this fluid ( Os-187/ Os-188 > 0 center dot 17) would imply that such fluids are derived from an uncommon type of mantle source possibly related to carbonatite melts.
ISSN: 0022-3530

hal-00644524
https://hal.archives-ouvertes.fr/hal-00644524
DOI : 10.1093/petrology/egr038

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) : HAL CCSD Springer Verlag
Résumé : International audience
Cobalt-bearing pyrite (0.5-2.0 wt.% Co) is abnormally abundant (up to 35 vol.% of the total volume of the sulfide phase) in some eastern Pyrenean peridotite massifs, compared to pieces of subcontinental mantle studied so far for sulfides. Pyrite occurs as vermicular intergrowths inside pentlandite and/or chalcopyrite or as coarser, blocky grains in the intergranular pores of host peridotites. Those different pyrites are characterized by different platinum-group element systematics (measured by laser ablation microprobe and ICP-MS). Vermicular pyrite intergrown with pentlandite displays Os-, Ir-, Ru- and Rh-enriched chondrite normalized PGE patterns of Monosulfide solid solution (Mss). In contrast, coarse-grained intergranular ("blocky") pyrites, are PGE-poor. Chalcophile trace elements (i.e. Zn, Pb, Ag, Au) that are not usually concentrated in mantle-derived sulfides were commonly detected. By contrast, selenium contents are generally low, yielding thus pyrite with high S/Se ratio (> 10(5)), consistent with a sedimentary sulfur source. Pyrite microtextures and chalcophile trace element contents support a process of assimilation of crustal sulfur from the metamorphosed sedimentary country rocks. These latter generated highly reactive CO2-S fluids, which were injected into structural discontinuities of the lherzolitic bodies. Sulfur has reacted at T = 300-550A degrees C with pre-existing, mantle-derived, metal-rich sulfide assemblages (pentlandite-chalcopyrite). Addition of crustal sulfur did produce Mss which, on cooling, exsolved the Os-rich pyrite in addition to pentlandite. The coarse-grained pyrite types have crystallized directly from S-rich fluids.
ISSN: 0010-7999

hal-00617425
https://hal.archives-ouvertes.fr/hal-00617425
DOI : 10.1007/s00710-010-0138-2

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Highly siderophile element concentrations (HSE: Re and platinum-group elements (PGE)) are presented for gabbros, gabbroic eclogites and basaltic eclogites from the high-pressure Zermatt-Saas ophiolite terrain, Switzerland. Rhenium and PGE (Os, Ir, Ru, Rh, Pt, Pd) abundances in gabbro- and eclogite-hosted sulphides, and Re-Os isotopes and elemental concentrations in silicate phases are also reported. This work, therefore, provides whole rock and mineral-scale insights into the PGE budget of gabbroic oceanic crust and the effects of subduction metamorphism on gabbroic and basaltic crust. Chondrite-normalised PGE patterns for the gabbros are similar to published mid-ocean ridge basalts (MORB), but show less inter-element fractionation. Mean Pt and Pd contents of 360 and 530 pg/g, respectively, are broadly comparable to MORB, but gabbros have somewhat higher abundances of Os, Ir and Ru (mean: 64, 57 and 108 pg/g). Transformation to eclogite has not significantly changed the concentrations of the PGE, except Pd which is severely depleted in gabbroic eclogites relative to gabbros (similar to 75% loss). In contrast, basaltic eclogites display significant depletion of Pt (>= 60%), Pd (>85%) and Re (50-60%) compared with published MORB, while Os, Ir and Ru abundances are broadly comparable. Thus, these data suggest that only Pt, Pd and Re, and not Os, Ir and Ru, may be significantly fluxed into the mantle wedge from mafic oceanic crust. Re-Os model ages for gabbroic and gabbroic eclogite minerals are close to age estimates for igneous crystallisation and high-pressure metamorphism, respectively, hence the HSE budgets can be related to both igneous and metamorphic behaviour. The gabbroic budget of Os, Ir, Ru and Pd (but not Pt) is dominated by sulphide, which typically hosts >90% of the Os, whereas silicates account for most of the Re (with up to 75% in plagioclase alone). Sulphides in gabbroic eclogites tend to host a smaller proportion of the total Os (10-90%) while silicates are important hosts, probably reflecting Os inheritance from precursor phases. Garnet contains very high Re concentrations and may account for >50% of Re in some samples. The depletion of Pd in gabbroic eclogites appears linked, at least in part, to the loss of Ni-rich sulphide. Both basaltic and gabbroic oceanic crust have elevated Pt/Os ratios, but Pt/Re ratios are not sufficiently high to generate the coupled Os-186-Os-187 enrichments observed in some mantle melts, even without Pt loss from basaltic crust. However, the apparent mobility of Pt and Re in slab fluids provides an alternative mechanism for the generation of Pt- and Re-rich mantle material, recently proposed as a potential source of Os-187-Os-186 enrichment.
ISSN: 0016-7037

hal-00420072
https://hal.archives-ouvertes.fr/hal-00420072
DOI : 10.1016/j.gca.2008.11.036

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The decay of 187Re to 187Os provides a unique tracer of recycled lithosphere in the Earth's upper mantle, and the radiogenic Os isotope compositions of many mid-ocean ridge basalts (MORB) have been taken to indicate the presence of recycled oceanic crust in the mantle source. However, others have attributed the data to contamination by seawater or an age effect (due to 187Re decay following igneous crystallisation). This study presents new Re Os, U Th Ra and B isotope data for normal-MORB (N-MORB) glass (including 18 samples analysed previously). The scale of the variation in 187Os/188Os observed here in N-MORB glass ranges from 0.126 to 0.148, with a mean value of 0.133 ± 0.009 (2?). Previously published Os isotope data for MORB is shown to have been affected by an analytical artefact, and in particular the very radiogenic Os isotope compositions reported earlier are now shown not to exist. Nevertheless, some radiogenic Os isotope compositions are still seen in the data obtained here. Coupled Th and Ra excesses appear to rule out an age effect, at least, for those samples studied, however, ?11B covariations with 187Os/188Os are consistent with the assimilation of older oceanic crust (affected by seawater contamination). Individual sulphides (less likely to be affected by such contamination) are always less radiogenic than their glass host, with 187Os/188Os values ranging from 0.123 to 0.131. This indicates that sulphide precipitation occurred prior to contamination of the surrounding melt, but the data also suggest that low-Os sulphides have been affected by contamination through partial equilibration with that melt. Taken together, these indicate that dominant process responsible for the radiogenic Os isotope compositions of N-MORB glass is contamination by older oceanic crust. Although the influence of age effects and the presence of recycled material cannot be completely ruled out, Os-rich sulphides yield sub-chondritic Os isotope compositions which point rather to a long-term depletion in Re in the MORB source likely to have resulted from formation of the continental crust over much of Earth's history.
ISSN: 0012-821X

hal-00320075
https://hal.archives-ouvertes.fr/hal-00320075
DOI : 10.1016/J.EPSL.2007.05.014