SMYER H-CHONDRITE IMPACT-MELT BRECCIA AND EVIDENCE FOR SULFUR VAPORIZATION

Abstract

Smyer is an H-chondrite impact-melt breccia containing ~20 vol% 0.5- to 13-mm-thick silicate-rich melt veins surrounding unmelted subrounded chondritic clasts up to 7 cm in maximum dimension. At the interface between some of the melt veins and chondritic clasts, there are troilite-rich regions consisting of unmelted, crushed 0.2- to 140-μm-size angular silicate grains and chondrule fragments surrounded by troilite and transected by thin troilite veins. Troilite fills every available fracture in the silicates, including some as thin as 0.1 μm. Little metallic Fe-Ni is present in these regions: the FeS/Fe modal ratio ranges from ~25:1 to ~500:1, far higher than the eutectic weight ratio of 7.5:1. The texture of these regions indicates that the sulfide formed from a fluid of very low viscosity. The moderately high viscosity (0.2 poise) and large surface tension of liquid FeS, its inability to wet silicate grain surfaces at low oxygen fugacities, and the supereutectic FeS/Fe ratios in the troilite-rich regions indicate that the fluid was a vapor. It seems likely that during the shock event that melted Smyer, many silicates adjacent to the melt veins were crushed. Upon release of shock pressure, some of the troilite evaporated and dissociated. Molecules of S2 were transported and condensed into fractures and around tiny silicate grains; there, they combined with Fe from small adjacent metallic Fe-Ni grains to form troilite. The Ni content at the edges of some of these metal grains increased significantly; Co from these Ni-rich grains diffused into nearby kamacite. Impact-induced S volatilization may have played a major role in depleting the surface of 433 Eros (and other chondritic asteroids) in S.