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 57 Fe/ 56 Fe ratios ranging up to 2solar. A few grains have depleted 57 Fe/ 56 Fe ratios down to 0.55solar. In contrast, 54 Fe/ 56 Fe ratios are close to solar. Anomalies in the Ni isotopes are even more extreme. 61 Ni/ 58 Ni ratios range up to 3.5solar, 62 Ni/ 58 Ni ratios up to 2solar, but 60 Ni/ 58 Ni ratios only up to 1.1solar. One of the grains with 57 Fe deficits has a 61 Ni deficit, the others have normal 61 Ni/ 58 Ni ratios. The grain with the largest 57 Fe deficit has 62 Ni/ 58 Ni of 1.8solar but solar 61 Ni/ 58 Ni. The excesses in 57 Fe 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 57 Fe 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 54 Fe excesses in the grains is puzzling in view of the fact that material from the Si/S zone, rich in 54 Fe, is required to explain the 28 Si excesses of X grains. We still do not have a good explanation for the 57 Fe deficits of several of the grains; they could reflect the initial isotopic compositions of the parent stars.