Disentangling nebular and asteroidal features of CO3 carbonaceous chondrite meteorites

Ten carbonaceous CO3 chondrites (including four chondrites from Antarctica and the Colony, Isna, Kainsaz, Lance , Ornans, and Warrenton chondrites) were analyzed with respect to mean compositions of olivines and low-Ca pyroxenes in order to distinguish the primary nebular features of these chondrites from the secondary (asteroidal or nebular) features. In three of the Antarctic chondrites, matrices and metal grains were analyzed in order to investigate the origin of mineralogical trends in the CO3 sequence. Based on these results, the CO3 chondrites were classified into subtypes 3.0-3.7. In the silicates of chondrites ALH A77307 and Colony, classified as type 3.0, metamorphic effects appear to be absent. Chemical and mineralogical studies suggest that the type 3.1 to 3.7 CO chondrites represent a metamorphic sequence that formed from material closely resembling type 3.0 CO chondrites by metamorphism in one or more planetesimals or asteroids, not by interactions between chondritic ingredients in the solar nebula.