Microstructural evidence for multiple generations of sulfide precipitation and impact‐induced thermal alteration under acidic conditions on Asteroid Ryugu

Abstract Here we report on the microstructure and chemistry of sulfide minerals returned from asteroid (162173) Ryugu. We identify a unique sulfide population in particle A0016 composed of violarite (FeNi 2 S 4 ), pyrite (FeS 2 ), chalcopyrite (CuFeS 2 ), pyrrhotite (Fe 1‐x S), and pentlandite ([Fe,Ni] 9 S 8 ). Violarite, pyrrhotite, and pyrite reveal evidence of formation from a fluid including porosity, phyllosilicate inclusions, and gaps along grain boundaries due to volume change. Violarite grains are polycrystalline with spatially correlated Ni and Co enrichments and contain pyrrhotite lamellae measuring 100s nanometers across. Pyrite assemblages are polycrystalline with spatially anticorrelated Ni and Co enrichments. In comparison, the chalcopyrite grain is a compositionally homogeneous single crystal spatially associated with rutile (TiO 2 ). The data suggest that particle A0016 experienced multiple generations of fluid flow. The first generation of fluid flow occurred at low temperatures (25 to 100 °C) under alkaline (pH > 8) conditions, leading to the precipitation of pyrrhotite and pentlandite. In comparison, the second generation of fluid flow was acidic (pH < 6.5) and reducing (−0.14 ≤ Eh ≤ −0.36) at elevated temperatures (230 to 300 °C), leading to formation of pyrite, violarite, and chalcopyrite. The elevated temperatures required to produce this sulfide population are consistent with heating from an impact on Ryugu's parent body.