Isotopic analysis of presolar graphite grains from Orgueil

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.