MINERALOGY, PETROLOGY, AND GENESIS OF FINE-GRAINED REFRACTORY INCLUSIONS IN CARBONACEOUS CHONDRITES

This paper reports data on 38 fine-grained refractory inclusions. The inclusions can be classified on the basis of their mineral composition s into fine-grained refractory inclusions proper (FGI) and amoeboid ol ivine aggregates (AOA). Objects of both types were studied in the Efre movka (CV3), Allende (CV3), PCA-91082 (CR2), and Kaidun (CR) chondrite s. The refractory inclusions (FGI) and olivine aggregates (AOA) are su bdivided into six and two groups, respectively. The bulk compositions are satisfactory approximated by condensation and/or volatilization tr ends. As follows from the fact that most FGI are enriched in light Mg and Si isotopes (MacPherson et al., 1988), volatilization contributed insignificantly to the evolution of their material. The chemistries of AOA are better described by condensation trends for systems depleted in ultrarefractory components, a fact supported by the specific REE fr actionation patterns in most AOA (Mason and Taylor, 1982). The textura l relationships among the minerals in FGI and microbodies in AOA sugge st that they could not form by means of condensation, because the equi librium condensation sequence for 10(-3) - 10(-5) bar (Mel-Sp-Ol-Cpx-A n) does not coincide with the observed sequences: Spl-Mel-An-Cpx (for FGI) and Ol-An-Cpx (for AOA). The natural mineral relationships can be satisfactory described by the crystallization from a melt, because mo st FGI fall into the spinel liquidus field in the CMAS system, and AOA occur in the forsterite liquidus field. The two probable models for t he genesis of the inclusions are: (1) they are products of melting and recrystallization of primary condensates and (2) they consist of aggr egates of particles, which had formed independently in the course of c ondensation, and then melted and consolidated. At present, both models seem to be equally feasible.