Éditeur(s) : HAL CCSD E Schweizerbart Science Publishers
Résumé : The occurrence and preservation of aragonite in eclogite-facies rocks of north-eastern Corsica is linked to an uncommon microtexture. Aragonite exclusively occurs as oriented fibres in garnet crystals of a graphitic, more or less siliceous marble that immediately overlies a serpentinite body of the meta-ophiolitic unit. The arrangement of the fibres is grossly radial, but more clearly sectoral in subhedral garnet, the fibres growing perpendicular to the garnet/matrix interface. Raman mapping reveals that the carbonate is calcite in the matrix and in poikiloblastic garnet cores, and that the fibres in the garnet mantle are aragonite alone in the case of a carbonate matrix, and both aragonite and quartz (in distinct fibres) in a quartz–carbonate matrix. These features are interpreted as prograde intergrowths, the result of garnet nucleation and growth in a calcite and then aragonite matrix (±quartz). Upon further heating and/or decompression, the aragonite matrix transformed back to calcite while the carbonates included in garnet retained their original structure, in spite of the relatively high temperature attained (ca. 500 °C). These aragonite relics are one more example of the preservation of a high-pressure polymorph through mechanical shielding of inclusions in a rigid host. The aragonite–garnet intergrowths are similar to quartz–garnet intergrowths described in amphibolite-facies graphitic schists. They are evidence that oriented inclusions in garnet are not necessarily precipitates (‘exsolutions'). Unlike precipitates, their orientation is controlled more by the shape of the garnet growth front than by symmetry constraints.
ISSN: 0935-1221

hal-00411136
https://hal.archives-ouvertes.fr/hal-00411136
DOI : 10.1127/0935-1221/2008/0020-1892

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
Separated olivine grains from a deeply subducted serpentinized wehrlite (Changawuzi in the western Tianshan ultrahigh-pressure belt, China) were analysed with unpolarized transmission FTIR and the Ti contents were determined using LA-ICP-MS. The major bands in the olivine spectra display striking similarities to Ti-clinohumite and are interpreted as OH in lamellae. The quantification of the water content in lamellae requires calibration of the IR-absorption for such bands. We have obtained a new absorption coefficient for Ti-clinohumite of 0.125+/−0.017 ppm cm2 based on polarized FTIR measurements on three orthogonal sections through a large single crystal of Ti-clinohumite from Val Malenco, which has a known water content of 1.53 wt%. The resulting water content in olivine using this calibration factor ranges from 440 to 2,600 ppm and correlates positively with the Ti content that ranges from 130 to 1,400 ppm. For the quantification of the water content in Ti-chondrodite and Ti-clinohumite that are associated with olivine, we developed a new method using attenuated total reflectance FTIR spectroscopy. Ti-chondrodite and Ti-clinohumite display similar IR bands at ~3,562, 3,525 and ~3,583–3,586 cm−1. As in olivine, the water content and Ti content correlate in both Ti-clinohumite and Ti-chondrodite, indicating an intergrowth of these minerals, which has been confirmed by TEM analyses. Our results confirm previous suggestions that there is a strong correlation between the Ti content of ultramafic rocks and their capacity to transport water to the deeper mantle in subduction zones beyond conditions where hydrous phases are stable.
ISSN: 0010-7999

hal-01172237
https://hal.archives-ouvertes.fr/hal-01172237
DOI : 10.1007/s00410-014-0992-8

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 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 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