Iron and nickel isotopic ratios in presolar sic grains

We report the first Fe isotopic anomalies and the first Ni isotopic ratio measurements in presolar SiC grains of separate KJG from the Murchison meteorite. With Nano SIMS, we analyzed Fe and Ni in 37 × grains from Type II supemovae and 53 SiC grains of other types. The Ni/Fe and Co/Fe ratios in grains of all types are much higher than in the gas from which the grains are believed to have condensed. A majority of the × grains and a couple of mainstream grains contain Fe-rich sub grains. Most × grains have large excesses in ⁵⁷Fe, ⁶¹ Ni, and ⁶²Ni. ⁶⁰Ni excesses are small and the ⁵⁴Fe/⁵⁶Fe ratios of almost all × grains are normal. These isotopic compositions are best explained by mixing of material from the He/N zone of Type II supernovae with material from the He/C zone. The lack of any ⁵⁴Fe excesses is puzzling in view of the fact that the Si/S zone, whose contribution resulted in the ²⁸Si excesses in × grains, is very rich in ⁵⁴Fe. It has yet to be seen whether elemental fractionation between Si and Fe is an explanation. The ⁵⁷Fe deficits observed in a few X grains remain unexplained. In comparison to the × grains, fewer mainstream and AB grains have anomalies. Observed ⁶²Ni excesses in some mainstream grains are larger than predicted for AGB stars of solar metallicity and are not accompanied by corresponding ⁶¹Ni excesses. AY grain and a Z grain have excesses in ⁵⁴Fe and ⁶²Ni, but close to normal ⁵⁷Fe/⁵⁶Fe and ^60,61'Ni/⁵⁸Ni ratios. These isotopic compositions are not expected for grains from low-metallicity AGB stars.