Opaque minerals in chondrules and fine-grained chondrule rims in the Bishunpur (LL3.1) chondrite

Abstract

We present a detailed petrographic and electron microprobe study of metal grains and related opaque minerals in the chondrule interiors and rims of the Bishunpur (LL3.1) ordinary chondrite. There are distinct differences between metal grains that are completely encased in chondrule interiors and those that have some portion of their surface exposed outside of the chondrule boundary, even though the two types of metal grains can be separated by only a few microns. Metal grains in chondrule interiors exhibit minor alteration in the form of oxidized P-, Cr-, and Si-bearing minerals. Metal grains at chondrule boundaries and in chondrule rims are extensively altered into troilite and fayalite. The results of this study suggest that many metal grains in Bishunpur reacted with a type-I chondrule melt and incorporated significant amounts of P, Cr, and Si. As the system cooled, some metal oxidation occurred in the chondrule interior, producing metal-associated phosphate, chromite, and silica. Metal that migrated to chondrule boundaries experienced extensive corrosion as a result of exposure to the external atmosphere present during chondrule formation. It appears that chondrule- derived metal and its corrosion products were incorporated into the fine-grained rims that surround many type-I chondrules, contributing to their Fe-rich compositions. We propose that these fine- grained rims formed by a combination of corrosion of metal expelled from the chondrule interior and accretion of fine-grained mineral fragments and microchondrules.