PETROGRAPHY AND CHEMISTRY OF FINE-GRAINED DARK RIMS IN THE MOKOIA CV3 CHONDRITE: EVIDENCE FOR AN ACCRETIONARY ORIGIN

Abstract

The matrix and fine-grained dark rims (FGDRs) around a type-A Ca-, Al-rich inclusion (CAI) and a silicate aggregate in the Mokoia CV3 chondrite differ in their major, minor, and trace element abundances. The matrix contains more Nb, Sr, Ba, and V but less Ca than average CI chondrite and has an unfractionated rare earth element (REE) pattern. The two FGDRs are enriched in Na, Al, K, and Ti and depleted in Fe and Mn compared to the matrix. The rim surrounding the CAI is enriched in Hf, Y, and Sc relative to the matrix and has a REE pattern that reflects a complex history. The rim surrounding the silicate aggregate has a REE pattern that suggests a different history and, additionally, is complementary to those of the enclosed enstatite and mesostasis glass. The trace element abundances indicate that Mokoia FGDRs are a mixture of components derived from both high- and low-temperature processes, while the matrix consists of only relatively low-temperature materials. These FGDRs in Mokoia have an accretionary origin and were derived from multiple dust reservoirs.