Discovery of Abundant In Situ Silicate and Spinel Grains from Red Giant Stars in a Primtive Meteorite

Abstract

We report the discovery of 12 in situ presolar silicate and spinel grains, 140-590 nm in size, in the Acfer 094 meteorite. These grains represent a matrix-normalized abundance of presolar O-rich dust of 170 parts per million. Among the 10 silicate grains are three olivines, four pyroxenes, and three grains with glasslike composition. Eleven grains have large excesses in 17 O with 17 O/ 16 O ratios of up to 2.9 times the solar ratio and slightly lower than or close-to-solar 18 O/ 16 O ratios. These grains most likely formed in 1.5-1.65 M ⊙ red giant branch (RGB) or asymptotic giant branch (AGB) stars with close-to-solar metallicity. One pyroxene grain has close-to-solar 17 O/ 16 O and 18 O/ 16 O of 3.8 times solar. Silicon- and Fe-isotopic ratios of this grain are suggestive of the formation in an RGB or AGB star, probably with higher-than-solar metallicity. 29 Si/ 28 Si and 30 Si/ 28 Si ratios of the silicate grains vary by ∼16%, are positively correlated with one another, and fall to the 30 Si-poor side of the Si mainstream line characteristic for presolar SiC from AGB stars. This gives independent confirmation for the view that the Si mainstream line reflects the Galactic chemical evolution of the Si isotopes, except for a small shift due to dredge-up of matter from the He shell in AGB stars.