METAL-TROILITE-MAGNETITE ASSEMBLAGE IN SHOCK VEINS OF SIXIANGKOU METEORITE

Abstract

The first natural occurrence of metal-troilite-magnetite assemblage composed of Fe-Ni metal and magnetite dendrites and a groundmass of troilite was identified in the shock veins of the Sixiangkou L6 chondrite, which contain abundant high-pressure minerals. This assemblage suggests a liquid miscibility among metal, FeS, and iron oxide, and subsequently quenching under pressure. Components of magnetite could be, in origin, related to chromite that was embedded and dissolved in an Fe-Ni-S liquid. Cr2O3 dissociated from chromite was mainly incorporated into garnet and magnesiowustite in the fine-grained matrix of shock veins, in which chromium behaves as a lithophile element at the P-T conditions experienced by the shock veins. The occurrence of metal-troilite-magnetite assemblage suggests that the shock veins were still under pressure at temperatures from 900 to 950°C during solidification of Fe-Ni-S or Fe-Ni-S-O liquid, hence indicating a long duration of high pressure in the shock veins.