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 NanoSIMS, we analyzed Fe and Ni in 37 X grains from Type II supernovae 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 X grains and a couple of mainstream grains contain Fe-rich subgrains. Most X grains have large excesses in 57 Fe, 61 Ni, and 62 Ni. 60 Ni excesses are small and the 54 Fe/ 56 Fe ratios of almost all X 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 54 Fe excesses is puzzling in view of the fact that the Si/S zone, whose contribution resulted in the 28 Si excesses in X grains, is very rich in 54 Fe. It has yet to be seen whether elemental fractionation between Si and Fe is an explanation. The 57 Fe deficits observed in a few X grains remain unexplained. In comparison to the X grains, fewer mainstream and AB grains have anomalies. Observed 62 Ni excesses in some mainstream grains are larger than predicted for AGB stars of solar metallicity and are not accompanied by corresponding 61 Ni excesses. A Y grain and a Z grain have excesses in 54 Fe and 62 Ni, but close to normal 57 Fe/ 56 Fe and 60,61 Ni/ 58 Ni ratios. These isotopic compositions are not expected for grains from low-metallicity AGB stars.