^1,2Wei Jia Ong, ¹Christine Floss
¹Laboratory for Space Sciences and Physics Department, Washington University, St. Louis, Missouri, USA
²National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan, USA

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.

Reference
Ong WJ, Floss C (2015) Iron isotopic measurements in presolar silicate and oxide grains from the Acfer 094 ungrouped carbonaceous chondrite. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12475]

Publsihed by arrangement with John Wiley&Sons

⁵D. Schwander, ^2,3,4L. Kööp, ¹T. Berg, ⁵G. Schönhense, ^2,3,4P.R. Heck, ^2,3,4,5A.M. Davis, ^1,6,7U. Ott
¹Institut für Physik, Johannes Gutenberg-Universität, Staudingerweg 7, D-55128 Mainz, Germany
²Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, United States
³Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL, United States
⁴Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, United States
⁵Enrico Fermi Institute, The University of Chicago, Chicago, IL, United States
⁶University of West Hungary, H-9700 Szombathely, Hungary
⁷Max-Planck-Institut für Chemie, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany

Ca, Al-rich inclusions (CAIs) often contain numerous refractory metal nuggets (RMNs), consisting of elements like Os, Ir, Mo, Pt and Ru. The nuggets are usually thought to have formed by equilibrium condensation from a gas of solar composition, simultaneously with or prior to oxide and silicate minerals. However, the exact mechanisms responsible for their extremely variable compositions, small sizes and associations with CAI minerals remain puzzling. Expanding on previous work on chemically separated RMNs, we have studied a large number of RMNs within their host CAIs from three different meteorite types, i.e., the highly primitive chondrite Acfer 094, Allende (CV3ox) and Murchison (CM2). Our results show several inconsistencies between the observed features and a direct condensation origin, including a lack of correlated abundance variations in the refractory metals that is expected from variations in condensation temperature. Instead, we show that most RMN features are consistent with RMN formation by precipitation from a CAI liquid enriched in refractory metals. This scenario is additionally supported by the common occurrence of RMNs in CAIs with clear melt crystallization textures as well as the occurrence of synthetic RMNs with highly variable compositions in run products from Schwander et al. (2015). In some cases, the sizes of meteoritic RMNs correlate with the sizes of their host minerals in CAIs, which indicates common cooling rates.

Reference
Schwander D, Kööp L, Berg T, Schönhense G, Heck PR, Davis AM, Ott U (2015) Formation of refractory metal nuggets and their link to the history of CAIs. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.014]

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