HR: 0800h
AN: V31D-0686    [Abstracts]
TI: New experimental approach toward the determination of Os partitioning between metal and silicate
AU: * Yokoyama, T
EM: yokoyama@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742, United States
AU: Walker, R J
EM: rjwalker@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742, United States
AU: Walker, D
EM: dwalker@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, United States
AB: Highly siderophile elements (HSE; including Re, Os, Ir, Ru, Rh, Pt, Pd, Au), generally defined by their extremely high partition coefficients between Fe-rich metal and silicate (D^met/sil >10⁴), are of great interests for understanding core formation processes. Of these, Os is a key element due to applications of the ¹⁸⁷Re- ¹⁸⁷Os isotope system. Precise experimental determination of the metal-silicate partition coefficients for HSE have been hindered by analytical difficulties such as: 1) extremely low HSE concentrations in quenched glass that are close to or less than the detection limits of applicable measurement methods, and 2) the common development of HSE micro-nuggets scattered in the silicate glass. The presence of such nuggets can potentially lead to incorrect interpretations regarding D^met/sil values. Here we employed a new approach to determine the D_Os^met/sil at natural Os abundances by applying graphite capsule experiments in a piston cylinder apparatus. A paired sample of well-characterized iron meteorite and a komatiitic basalt, which have very different ¹⁸⁷Os/¹⁸⁸Os ratios, was put in a graphite capsule and processed at 10 kbar, 1450^oC for 24-92 hours. The ¹⁸⁷Os/¹⁸⁸Os and Os concentration in the quenched glass containing a dispersion of metal nuggets were simultaneously measured by isotope dilution (NTIMS) by processing very small (~1 mg) samples liberated from the capsule. When the results are plotted on a ¹⁸⁷Os/¹⁸⁸Os-1/Os diagram, we observed a linear correlation which converges at the composition of the starting metal. A projection of the trend to the Os isotopic composition of the starting silicate indicates a pure silicate concentration that is several-times lower Os than the starting basalt. This method allows estimation of a D value for silicate free of nuggets without the need to separate nuggets from the silicate. Our new result implies extremely low solubility of Os in the silicate melt, evidently resulting from Os removal from the silicate. The Os concentration in the endmember silicate glass free of nuggets gives D_Os^met/sil = 6×10⁵.
DE: 1015 Composition of the core
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting