Disentangling nebular and asteroidal features of CO3 carbonaceous chondrite meteorites

Analyses of olivines and low-Ca pyroxenes in a suite of porphyritic chondrules of types IA and II in each of ten CO3 chondrites and comparisons with similar chondrules in LL chondrites suggest that the CO3 chondrites may be divided into a metamorphic sequence of subtypes 3.0 to 3.7. Chondrules in Allan Hills A77307 and Colony, which we classify as type 3.0, show only igneous zoning, whereas in the other more metamorphosed type 3.1-3.7 CO chondrites, chondrule silicates are enriched in FeO, most markedly in olivine crystals near the rims of type IA chondrules and along cracks. The FeO enrichments in metamorphosed chondrules result largely from equilibration by solid-state diffusion between chondrules and FeO-rich matrices. Type 3.1-3.7 CO chondrites were formed from chondrites like Colony and ALH A77307 in heated asteroids or planetesimals. We find no evidence for nebular condensation of fayalite rims around these chondrules, as reported for CV3 chondrites, or nebular modification of chondrule silicates, as favored by some workers. Metallic Fe,Ni in ALH A77307 contains up to 0.1-1 wt% Cr, Si, and P, as in the least metamorphosed ordinary chondrites and CM2 chondrites, and grains of cohenite, Fe3C, and haxonite, Fe23C6. These carbides, which are the first reported in carbonaceous chondrites, are associated with pentlandite and magnetite, as in certain LL3 chondrites. We assign the following petrologic subtypes to other CO chondrites; Kainsaz, 3.1; Felix, 3.2; ALH 82101 and Ornans, 3.3; Lance and ALH A77029, 3.4; ALH A77003, 3.5; Warrenton, 3.6; and Isna, 3.7