Presolar Silicate And Oxide Grains Found In Lithic Clasts From Isheyevo And The Fine-Grained Matrix Of Northwest Africa 801

We report on the discovery of 33 oxygen-anomalous grains from the CH3/CB(b)3 chondrite Isheyevo and the CR2 chondrite Northwest Africa (NWA) 801. Oxygen isotopic compositions indicate the origin of the majority grains in stellar outflows of low-mass (similar to 1.2 to similar to 2.2 M), solar-metallicity red giant or asymptotic giant branch stars, while highly O-17-enriched grains probably have nova origins. Isotopic compositions of the eight O-18-rich grains, including an extremely O-18-rich grain (similar to 16 times solar O-18/O-16 ratio), are reproduced by zone mixing of SNe II ejecta. Close-to-normal silicon, magnesium, and calcium isotopic compositions of grains are consistent with the isotope exchange in the interstellar medium or the meteorite parent body, while two grains with Si isotopic anomalies and one grain with Mg isotopic anomalies reflect the Galactic chemical evolution. An Isheyevo clast showed several hot spots with moderate to high N-15 enrichments, including a hot spot with an extreme N-15 excess of (7225 316)parts per thousand. However, no correlation between N-15 enrichment and presolar oxygen-rich grain abundance is found. Grains with elliptical shapes probably indicate primary condensation feature. Two complex grains possibly display decoupling of the isotopic and elemental compositions in the grain formation environments. The low silicate-to-oxide abundance ratio for the fine-grained chondrule rims in NWA 801 likely reflects the preferential destruction of silicates due to terrestrial weathering. In NWA 801, the presolar O-rich grain abundance in fine-grained chondrule rims is higher than in the interchondrule matrix, similar to the trend observed for some aqueously altered chondrites of petrologic type 2.