11:00 AM - 11:15 AM
[PPS12-08] Os isotope compositions and fractionation of siderophile elements in metal phases from CB chondrites
Keywords:Chondrite, Metal phase, CB chondrite, Osmium isotopes, Highly siderophile elements, In-situ analysis
We examined multiple metal grains in three CB chondrites: Bencubbin (CBa), Gujba (CBa), and Isheyevo (CBb). The details for Os isotope analysis using a micro milling system and N-TIMS are described in . The concentrations of P, S, Cr, Fe, Co, and Ni in analytical spots adjacent to the sampling pits for Os isotope analysis were determined by EPMA (JEOL-JXA-8530F). The concentrations of HSEs in analytical spots adjacent to the sampling pits were analyzed with fs-LA-ICP-MS (IFRIT, Cyber Laser).
Our Re–Os isotope data are mostly plotted on the 4.567 Ga Re–Os reference line. Nearly homogeneous 187Os/188Os ratios in CB metals indicate that fractionation of Re and Os was minuscule during metal formation at ~4.57Ga. Because Re and Os are ultra-refractory elements with similar 50% condensation temperatures (Re: 1821 K, Os: 1812 K), the limited Re/Os variation may suggest simultaneous condensation of Re and Os from the nebular gas during metal formation. The limited Os isotopic variation suggests that the redistribution of Re and Os during metal formation associated with planetary collision was not significant as are the cases of solidification of liquid metal. A positive correlation of Re/Os ratios calculated from the 187Os/188Os ratios and Os/Ir for CBa metal grains suggests that the condensation of CBa metal grains occurred at an equilibrium condition in a cooling gas until the condensation temperature of Ir (~1600 K). Unlike ultra-refractory HSEs, Pd/Fe and Ni/Fe ratios in CBa and CBb metals exhibit a strong positive correlation. This positive correlation cannot be explained by nebular condensation but condensation in extremely high gas pressure (107 x solar nebula).
References:  Jacquet E. et al. (2013) Meteorit. Planet. Sci. 48, 1981–1999.  Campbell A. et al. (2002) GCA 66, 647–660.  Nakanishi N. et al. (2013) LPSC, abstract #2407.