EXPERIMENTAL-DETERMINATION OF OS METAL SILICATE PARTITIONING/

We report here the first experimental results on the partitioning of O s between metal (Ni-rich OsNi alloys) and silicate melt of anorthite-d iopside eutectic composition at 1400 degrees C and one atmosphere tota l pressure and at oxygen fugacities ranging from 10(-8) to 10(-12) atm . Experiments were done by equilibrating OsNi metal loops with silicat e melt. Metal and glass were separately analyzed by instrumental neutr on activation analysis (INAA) and metal/silicate partition coefficient s for Ni and Os (D-Ni, D-Os) were calculated. The metal/silicate parti tion coefficients of Ni obtained in this study are in agreement with t he results of earlier experiments. In a log D-Ni vs. log fO(2) diagram the results can be fitted to a straight line with a slope of -0.45, c lose to -1/2, implying an effective valence of Ni in the silicate melt of around +2. The Os partition coefficients between OsNi alloy (about 10 and 20 wt.% Os) and silicate melt range from 10(6) to 10(7) (avera ge log D-Os = 6.62 +/- 0.30) and are, if at all, only weakly dependent on oxygen fugacity. The scatter of the partition coefficients exceeds possible variations produced by statistical uncertainties of the anal yses. We suspect formation of Os-nuggets similar to what has been foun d for Ir and Pt. In this sense the results reflect a strict lower limi t for Os metal/silicate partition coefficients. The new data make core -mantle equilibrium for highly siderophile elements (HSEs) more unlike ly and favor the late veneer hypothesis.