Petrology of types 4–6 carbonaceous chondrites

Abstract

We have studied the petrology of three type 4 carbonaceous chondrites, Coolidge, Karoonda, and Pecora Escarpment 82500, which was previously classified as an LL6 chondrite, and compared them to published descriptions of the other known metamorphosed C chondrites, Yamato 6903 and Mulga (west). All except Coolidge have abundant matrix, typically 80 vol %, composed largely of homogeneous olivine, Fa 29–33, and surprisingly heterogeneous plagioclase, mostly An 20–75. Their opaque mineralogy indicates high oxygen fugacities during metamorphism: metallic Fe,Ni and troilite are rare, and magnetite and pentlandite are abundant with accessory pyrrhotite, chalcopyrite, and pyrite. Possibly because of a lower matrix abundance, Coolidge has more reduced minerals: metallic Fe,Ni and olivine with Fa 14. The properties of all C4–6 chondrites appear to be consistent with an origin by metamorphism of C3‐like precursors by processes analogous to those operating in ordinary chondrite parent bodies or planetesimals. However, type 4–6 material was not well mixed with type 3 material in the CV3 parent body, as it was in the ordinary chondrite parent bodies. Conceivably, C4–6 chondrites come from one or more separate parent bodies.