The nanoscale mineralogy of Fe,Ni sulfides in pristine and metamorphosed CM and CM/CI-like chondrites: Tapping a petrogenetic record

Abstract

We have sampled sulfide grains from one pristine CM2 chondrite (Yamato [Y-] 791198), one thermally metamorphosed CM2 chondrite (Y-793321), and two anomalous, metamorphosed CM/CI-like chondrites (Y-86720 and Belgica [B-] 7904) by the focused ion beam (FIB) technique and studied them by analytical transmission electron microscopy (TEM). Our study aims at exploring the potential of sulfide assemblages and microstructures to decipher processes and conditions of chondrite petrogenesis. Complex exsolution textures of pyrrhotite (crystallographic NC-type with N≈6), troilite, and pentlandite occur in grains of Y-791198 and Y-793321. Additionally, polycrystalline 4C-pyrrhotite-pentlandite-magnetite aggregates occur in Y-791198, pointing to diverse conditions of gas-solid interactions in the solar nebula. Coarser exsolution textures of Y-793321 grains indicate higher long-term average temperatures in the <100°C range compared to Y-791198 and other CM chondrites. Sulfide mineralogy of Y-86720 and B-7904 is dominated by aggregates of pure troilite and metal, indicating metamorphic equilibration at sulfur fugacities (fS2) of the iron-troilite buffer. Absence of magnetite in equilibrium with sulfide and metal in Y-86720 indicates higher peak temperatures compared with B-7904, in which coexistence of troilite, metal, and magnetite constrains metamorphic temperature to less than 570°C. NC-pyrrhotite occurs in both meteorites as nm-wide rims on troilite grains and, together with frequent anhydrite, indicates a retrograde metamorphic stage at higher fS2 slightly above the fayalite-magnetite-quartz-pyrrhotite buffer. Fine-grained troilite-olivine intergrowths in both meteorites suggest the pre-metamorphic presence of tochilinite-serpentine interlayer phases, pointing to mineralogical CM affinity. Pseudomorphs after euhedral pyrrhotite crystals in Y-86720 in turn suggest CI affinity as do previously published O isotopic data of both meteorites. © The Meteoritical Society, 2013.