OXYGEN ISOTOPIC ABUNDANCES IN CALCIUM-ALUMINUM-RICH INCLUSIONS FROM ORDINARY CHONDRITES - IMPLICATIONS FOR NEBULAR HETEROGENEITY

The oxygen isotopic compositions of two calcium-aluminum-rich inclusio ns (CAIs) from the unequilibrated ordinary chondrite meteorites Quinya mbie and Semarkona are enriched in O-16 by an amount similar to that i n CAIs from carbonaceous chondrites. This may indicate that most CAIs formed in a restricted region of the solar nebula and were then uneven ly distributed throughout the various chondrite accretion regions. The Semarkona CAI is isotopically homogeneous and contains highly O-16-en riched melilite, supporting the hypothesis that all CAI minerals were originally O-16-rich, but that in most carbonaceous chondrite inclusio ns some minerals exchanged oxygen isotopes with an external reservoir following crystallization.