Isotopically uniform, 16O-depleted calcium, aluminum-rich inclusions in CH and CB carbonaceous chondrites

We suggest that (i) most of the isotopically uniform and 16O-depleted CAIs resulted from remelting of pre-existing, possibly 16O-rich refractory inclusions. The remelting may have occurred during formation of the magnesian, non-porphyritic (cryptocrystalline and skeletal) chondrules in CHs, CBs, and Isheyevo either by an unspecified, late, single-stage, highly-energetic event or in an impact-generated plume previously hypothesized for their origin; both mechanisms probably occurred in the solar nebula (i.e., in the presence of the nebula gas). The forsterite ± pyroxene rims around 16O-depleted CAIs may have resulted from evaporation-recondensation of silicon and magnesium during this event. Some of the Al-diopside-rich CAIs may have formed by evaporation of the Al-rich chondrule melts. (ii) In addition to these components, the CHs and Isheyevo contain a high abundance of chondrules and refractory inclusions formed by the commonly inferred nebular processes - evaporation, condensation, and incomplete melting of dust aggregates during multiple transient heating events. These include 16O-rich CAIs, amoeboid olivine aggregates, and ferromagnesian and aluminum-rich chondrules with porphyritic textures. Such components are also present in CB chondrites, but they are exceptionally rare. These observations indicate that there are multiple generations of CAIs in metal-rich carbonaceous chondrites. (iii) Because the isotopically uniform, 16O-depleted, igneous CAIs and the magnesian cryptocrystalline and skeletal olivine-pyroxene chondrules are found almost exclusively in metal-rich carbonaceous chondrites, the hypothesized impact-plume mechanism of chondrule formation and recycling of CAIs are not common processes.