Isotopic analysis of presolar graphite grains from Orgueil

Astronomy and Astrophysics – Astronomy

Scientific paper

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98.38.Cp, 98.80.Ft, 81.05.Uw, 97.60.Bw, 97.20.Jg, 97.10.Cv

Scientific paper

We report the successful isolation and isotopic analysis of presolar graphite grains from the Orgueil CI chondrite. Isotopic measurements were made on seven density fractions, with grain sizes >1 μm: ORG1b (1.59 1.67 g cm-3), 1c (1.67 1.75 g cm-3), 1d (1.75 1.92 g cm-3), 1f (2.02 2.04 g cm-3), 1g (2.04 2.12 g cm-3), 1h (2.12 2.16 g cm-3) and 1i (2.16 2.30 g cm-3). We measured C, N, O, and Si isotopes with the NanoSIMS. All the fractions, except ORG1b and ORG1h, contain presolar graphite as demonstrated by the large range of 12C/13C ratios (4 1746) measured in individual grains. The abundance of grains with isotopically light carbon increases with increasing density. Some of the low-density grains are enriched in 18O, 15N and 28Si. As the density increases, the grains mostly exhibit solar oxygen and nitrogen isotopic ratios. However, the high-density grains are enriched in 29Si and 30Si. The 18O and 28Si excesses indicate that the low-density grains originated from supernovae, while the high-density grains (with 30Si and 12C excesses) probably originated from AGB stars with low metallicities.

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