Iron isotopic measurements in presolar silicate and oxide grains from the Acfer 094 ungrouped carbonaceous chondrite

We carried out Fe isotopic analyses on 21 O‐rich presolar grains from the Acfer 094 ungrouped carbonaceous chondrite. Presolar grains were identified on the basis of oxygen isotopic ratios, and elemental compositions were measured by Auger spectroscopy. The Fe isotopic measurements were carried out by analyzing the Fe isotopes as negative secondary oxides with the NanoSIMS to take advantage of the higher spatial resolution of the Cs⁺ primary ion beam. Our results demonstrate the effectiveness of this approach for measuring both ⁵⁴Fe/⁵⁶Fe and ⁵⁷Fe/⁵⁶Fe. The ion yield for FeO^– is significantly lower than for Fe⁺, but this is not a serious limitation for presolar silicate grains with Fe as a major element. Most of the grains analyzed are ferromagnesian silicates, but we also measured four oxide grains. Iron contents are high in all of the grains, ranging from 10 to 40 atom%. Three of the grains belong to oxygen isotope Group 4. All of them have ⁵⁴Fe/⁵⁶Fe and ⁵⁷Fe/⁵⁶Fe ratios that are solar within errors, consistent with an origin in the outer zones of a Type II supernova, as indicated by their oxygen isotopic compositions. The remaining grains belong to oxygen isotope Group 1, with origins in low‐mass AGB stars. The majority of these also have solar ⁵⁴Fe/⁵⁶Fe and ⁵⁷Fe/⁵⁶Fe ratios. However, four grains are depleted in ⁵⁷Fe; one is also slightly depleted in ⁵⁴Fe. Current AGB models predict excesses in ⁵⁷Fe with ⁵⁴Fe/⁵⁶Fe ratios that largely reflect the metallicity of the parent star. While the solar ⁵⁷Fe/⁵⁶Fe ratios are consistent with formation of the grains in early third dredge‐up episodes, these models cannot account for the grains with ⁵⁷Fe depletions. Comparison with galactic evolution models suggests formation of these grains from stars with significantly subsolar metallicity; however, these models also predict large depletions in ⁵⁴Fe, which are not observed in the grains. Thus, the isotopic compositions of these grains remain unexplained.