Oxygen Fugacity Constraints During Core Formation - Metal-Silicate Partitioning of Nominally Lithophile Elements

Based on the relative depletion in Si of the Earth's mantle compared to chondritic material it has frequently been proposed that some Si was extracted into the Earth's core during accretion. However, in order to generate a core containing ~ 7 wt% Si relatively reducing conditions would have been required during core formation. Important constraints can be placed on the lowest redox state attained during core formation, however, by studying the partitioning of nominally lithophile elements. Ta for example is not depleted in the Earth's mantle, although simple thermodynamic calculations suggest that Ta should have metal-silicate partition coefficients similar to Si. Other elements that could be used as such indicators are In, Ga and Zn which are only depleted in the Earth's mantle compared to CI due to volatility. Liquid metal-liquid silicate partitioning experiments on Ta, Ga, In and Zn have been performed between 6 and 20 GPa at 2100oC. In each experiment the staring material was doped with 4 wt% of these elements added to 56 wt% synthetic primitive mantle composition and 40 wt% Fe-alloy. The metallic component was mixed from varying proportions of Fe powder and Fe84Si16 or Fe71Si29 and doped with 2 wt% Ni and 1 wt% Co. Various redox conditions were achieved by adjusting the amount of Si metal. Graphite and MgO single crystal capsules were employed. Recovered liquid metal and quenched silicate melt phases were analyzed with EMP and SIMS was employed for some silicate melt measurements. In contrast to previous studies, our results show that between 6 and 20 GPa metal-silicate partition coefficients for Ta are 1 to 2 orders of magnitude higher than those of Si. There seems to be no pressure dependency of the partition coefficient for the 4 elements studied. These results imply that if metal silicate equilibration occurred at pressures below 25 GPa Si is unlikely to be the light element in the core as this would have caused a strong depletion of Ta from the mantle.