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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