Distribution of the pre-solar component in Allende and other carbonaceous chondrites

摘要: Excess 16 O, relative to terrestrial abundances, has been found in all samples of C2, C3 and C4 carbonaceous chondrites which have analyzed, amounting nine meteorites thus far. Whole-rocks mineral separates from the fall on a single mixing line consistent with admixture 1–5% excess O. The anhydrous silicates C2 same line, but hydrous silicate matrix C2's define mass-fractionation trend parallel trend, displaced towards higher All analyzed are isotopically heterogeneous sub-millimeter scale. Detailed analyses separated phases several Allende Ca Al-rich inclusions reveal pattern large O enrichments spinel, pyroxene sometimes olivine, small melilite, feldspathoids grossular. distribution excesses, together their enhancement minerals believed be early solar nebular condensates, implies existence pre-solar “carriers” isotopic anomaly, probably grains or molecules oxygen was nearly pure These carriers not unequivocally indentified, corundum SiO possibilities. anomalies may also disturbed by diffusion-controlled processes exchange between condensates surroundings, either nebula, later parent body. No direct correlation yet observed isotope recently neon, magnesium xenon.