Iron and nickel isotopic compositions of presolar SiC grains from supernovae

Approximately 1% of presolar SiC grains found in primitive meteorites condensed in the ejecta of Type II supernovae. To explain the isotopic signatures of these so-called X grains, material from different SN zones had to contribute to the mix from which the grains formed. We measured the Fe and Ni isotopic ratios of individual X grains with the NanoSIMS. Most grains have enhanced ⁵⁷Fe/⁵⁶ Fe ratios ranging up to 2×solar. A few grains have depleted ⁵⁷Fe/⁵⁶ Fe ratios down to 0.55×solar. In contrast, ⁵⁴Fe/⁵⁶Fe ratios are close to solar. Anomalies in the Ni isotopes are even more extreme. ⁶¹Ni/⁵⁸Ni ratios range up to 3.5×solar, ⁶²Ni/⁵⁸Ni ratios up to 2×solar, but ⁶⁰Ni/⁵⁸Ni ratios only up to 1.1×solar. One of the grains with ⁵⁷Fe deficits has a ⁶¹Ni deficit, the others have normal ⁶¹Ni/⁵⁸Ni ratios. The grain with the largest ⁵⁷Fe deficit has ⁶²Ni/⁵⁸Ni of 1.8×solar but solar ⁶¹Ni/⁵⁸Ni. The excesses in ⁵⁷Fe and ^{61, 62}Ni observed in most of the grains can be explained by admixture of material from the He/C zone to material from the He/N (or outer) zones. In the He/C zone, neutron capture results in ⁵⁷Fe and ^{61, 62}Ni excesses, and the grain data can be fairly well reproduced by variable admixture from different layers of the He/C zone. The lack of large ⁵⁴Fe excesses in the grains is puzzling in view of the fact that material from the Si/S zone, rich in ⁵⁴Fe, is required to explain the ²⁸Si excesses of X grains. We still do not have a good explanation for the ⁵⁷Fe deficits of several of the grains; they could reflect the initial isotopic compositions of the parent stars.