Distribution of the pre-solar component in Allende and other carbonaceous chondrites
Abstract
Excess 16O, relative to terrestrial abundances, has been found in all samples of C2, C3 and C4 carbonaceous chondrites which have been analyzed, amounting to nine meteorites thus far. Whole-rocks and mineral separates from all the C3 and C4 meteorites fall on a single mixing line consistent with admixture of 1-5% of excess 16O. The anhydrous silicates of C2 meteorites fall on the same line, but the hydrous silicate matrix of C2's define a mass-fractionation trend parallel to the terrestrial trend, but displaced towards higher 16O. All meteorites analyzed are isotopically heterogeneous on a sub-millimeter scale. Detailed analyses of separated phases of several Allende Ca sbnd Al-rich inclusions reveal a consistent pattern of large 16O enrichments in spinel, pyroxene and sometimes olivine, and small 16O enrichments in melilite, feldspathoids and grossular. The heterogeneous distribution of the 16O excesses, together with their enhancement in minerals believed to be early solar nebular condensates, implies the existence of pre-solar "carriers" of the isotopic anomaly, probably grains or molecules with oxygen which was nearly pure 16O. These carriers have not been unequivocally indentified, but pre-solar grains of corundum or spinel, and pre-solar molecules of SiO are possibilities. The isotopic anomalies may also have been disturbed by diffusion-controlled processes of exchange between early condensates and their surroundings, either in the nebula, or later in the parent body. No direct correlation has yet been observed between the oxygen isotope anomalies and recently observed isotope anomalies in neon, magnesium and xenon.
- Publication:
-
Earth and Planetary Science Letters
- Pub Date:
- March 1977
- DOI:
- 10.1016/0012-821X(77)90005-X
- Bibcode:
- 1977E&PSL..34..209C
- Keywords:
-
- Abundance;
- Carbonaceous Chondrites;
- Meteoritic Composition;
- Oxygen Isotopes;
- Abnormalities;
- Microstructure;
- Minerals;
- Olivine;
- Pyroxenes;
- Spinel;
- Tables (Data)