Mineralogy of iron sulfides in CM1 and CI1 lithologies of the Kaidun breccia: Records of extreme to intense hydrothermal alteration

The polymict Kaidun microbreccia contains lithologies of C‐type chondrites with euhedral iron sulfide crystals of hydrothermal origin. Our FIB‐TEM study reveals that acicular sulfides in a CM1 lithology are composed of Fe‐rich pyrrhotite with nonintegral vacancy superstructures (NC‐pyrrhotite), troilite, and pentlandite, all showing distinct exsolution textures. Based on phase relations in the Fe‐Ni‐S system, we constrain the temperature of formation of the originally homogeneous monosulfide solid solution to the range of 100–300 °C. In some crystals the exsolution of pentlandite and the microtextural equilibration was incomplete, probably due to rapid cooling. We use thermodynamic modeling to constrain the physicochemical conditions of the extreme hydrothermal alteration in this lithology. Unless the CM1 lithology was sourced from a large depth in the parent body (internal pressure >85 bar) or the temperatures were in the lower range of the interval determined, the water was likely present as vapor. Previously described light δ³⁴S compositions of sulfides in Kaidun's CM1 lithology are likely due to the loss of ³⁴S‐enriched H₂S during boiling. Platy sulfide crystals in an adjacent, intensely altered CI1 lithology are composed of Fe‐poor, monoclinic 4C‐pyrrhotite and NC‐pyrrhotite and probably formed at lower temperatures and higher fS₂ relative to the CM1 lithology. However, a better understanding of the stability of Fe‐poor pyrrhotites at temperatures below 300 °C is required to better constrain these conditions.