Re-Os isotope systematics and fractionation of siderophile elements in metal phases from CBa chondrites

1Nao Nakanishi,1Tetsuya Yokoyama,1Satoki Okabayashi,2Tomohiro Usui,1,3Hikaru Iwamori
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13050]
1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo, Japan
2Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo, Japan
3Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa, Japan
Published by arrangement with John Wiley & Sons

We report Os isotope compositions of metal grains in two CBa chondrites (Bencubbin and Gujba) determined using a micromilling sampling coupled with thermal ionization mass spectrometry, together with the abundances of major and trace siderophile elements obtained by electron probe microanalysis and femtosecond laser ablation inductively coupled plasma–mass spectrometry. The CBa metal grains presented 187Os/188Os ratios akin to carbonaceous chondrites with limited variations (0.1257–0.1270). Most of the CBa metal grains were scattered along a 187Re-187Os reference isochron of IIIAB iron meteorites, indicating that the CBa metals experienced limited Re-Os fractionation at the time of their formation. The Re/Os ratios of sampling spots for the CBa metals, recast from the observed 187Os/188Os ratios, had a positive correlation with their Os/Ir ratios. In addition, the metal grains showed a positive correlation in a Pd/Fe versus Ni/Fe diagram. These correlations suggest that the CBa metal grains have formed via equilibrium condensation or evaporation from a gaseous reservoir at ~10−4 bar with enhanced metal abundances. Compared to the Bencubbin metals, the Gujba metals are characterized by having systematically lower Pd/Fe and Ni/Fe ratios that span subchondritic values. Such a difference was most likely induced by the compositionally heterogeneous impact plume from which the metals were condensed.

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