Petrogenesis of martian sulfides in the Chassigny meteorite
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Abstract:
The Chassigny meteorite, a Martian dunite, contains trace amounts (0.005 vol.%) of Fe-Ni sulfides, which were studied from two polished mounts in reflected light microscopy, Scanning Electron Microscope (SEM) and Electron Microprobe (EMP).The sulfide phases are, by decreasing order of abundance, nickeliferous (0-3 wt% Ni) pyrrhotite with an average composition M0.88±0.01S(M = Fe+Ni+Co+Cu+Mn), nickeliferous pyrite (0-2.5 wt% Ni), pentlandite, millerite and unidentified Cu sulfides.Pyrrhotite is enclosed inside silicate melt inclusions in olivine and disseminated as polyhedral or near spherical blebs in intergranular spaces between cumulus and postcumulus silicates and oxides.This sulfide is considered to be a solidification product of magmatic sulfide melt.The pyrrhotite Ni/Fe ratios lie within the range expected for equilibration with the coexisting olivine at igneous temperatures.Pyrite occurs only as intergranular grains, heterogeneously distributed between the different pieces of the Chassigny meteorite.Pyrite is interpreted as a by-product of the low-T (200°C) hydrothermal alteration events on Mars that deposited Ca sulfates + carbonates well after complete cooling.The shock that ejected the meteorite from Mars generated post-shock temperatures high (300°C) enough to anneal and rehomogenize Ni inside pyrrhotite while pyrite blebs were fractured and disrupted into subgrains by shock metamorphism.The negligible amount of intergranular sulfides and the lack of solitary sulfide inclusions in cumulus phases (olivine, chromite) indicate that, like other Martian basalts so far studied for sulfur, the parental melt of Chassigny achieved sulfide-saturation at a late stage of its crystallization history.Once segregated, the pyrrhotite experienced a late-magmatic oxidation event that reequilibrated its metal-to-sulfur ratios.Keywords:
Pentlandite
Troilite
Shock metamorphism
Troilite
Pentlandite
Shock metamorphism
Anorthosite
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Abstract— The Kaidun meteorite contains carbonaceous chondrite (CM1) clasts that have been highly altered by reactions with hydrothermal fluids. Pyrrhotite in these clasts occurs as unusual needles wrapped by sheaths of phyllosilicate, and pentlandite forms veins that crosscut aggregates of phyllosilicate and garnet but not pyrrhotite. The isotopic compositions of S (δ 34 S CDT ) in individual sulfide grains, measured by ion micro‐probe, are fractionated compared to troilite in ordinary chondrites. The S in Kaidun sulfides is isotopically light (as much as −4.2% for pyrrhotite and −5.7%0 for pentlandite), unlike sulfides in other carbonaceous chondrites, which are enriched in 34 S. The unusual S‐isotopic composition of these texturally unique sulfides supports the hypothesis that Kaidun CM1 clasts were pervasively altered under extreme thermal conditions, possibly by fluids that had lost isotopically heavy SO 2 .
Troilite
Pentlandite
Parent body
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Abstract– Northwest Africa 4859 (NWA 4859) is a meteorite of LL chondrite parentage that shows unusual igneous features and contains widely distributed pentlandite. The most obvious unusual feature is a high proportion of large (≤3 cm diameter) igneous‐textured enclaves (LITEs), interpreted as shock melts that were intruded into an LL chondrite host. One such LITE appears to have been produced by whole rock melting of LL chondrite, initial rapid partial crystallization, and subsequent slow cooling of the residual melt in the host to produce a differentiated object. Other unusual features include mm‐sized “overgrowth objects,” fine‐grained plagioclase‐rich bands, and coarse troilite (≤7 mm across) grains. All these features are interpreted as having crystallized from melts produced by a single transient shock event, followed by slow cooling. A subsequent shock event of moderate (S3) intensity produced veining and transformed some of the pyroxene into the clinoenstatite polytype. Pentlandite (together with associated troilite) in NWA 4859 probably formed by the breakdown of a monosulfide precursor phase at low temperature (≤230 °C) following the second shock event. NWA 4859 is interpreted to be an unusual impact‐melt breccia that contains shock melt which crystallized in different forms at depth within the parent body.
Pentlandite
Troilite
Breccia
Parent body
Pyroxene
Shock metamorphism
Large igneous province
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Pentlandite
Troilite
Parent body
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Troilite
Parent body
Pentlandite
Chondrule
Ordinary chondrite
Shock metamorphism
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