Metal-silicate partitioning of Co, Ga, and W: Dependence on silicate melt composition

This study investigates the effect of silicate melt composition on metal/silicate partitioning for Co2+, Ga3+, and W4+ at 1300 degreesC, 1 atm, and a log fO(2), of -12. Five glasses in the system MgO-CaO-Al2O3-TiO2-SiO2 with nbo/t (nonbridging oxygens/tetrahedrally coordinated cations) values ranging from 0.25 to 1.52 were used as starting materials. For W and Co experiments the five, glasses were equilibrated with W or Co wire loops, respectively, at the specified run conditions. For Ga experiments the glasses were doped with 2 wt.% Ga2O3 and equilibrated with pure Fe. All phases were analyzed by an electron microprobe. The metal/silicate partition coefficient for W depends strongly on melt basicity, whereas the effect of melt composition on Ga partitioning is less pronounced and for Co it is negligible. D-W decreases rapidly with increasing nbo/t, D-Ga decreases moderately with increasing degree of melt basicity, and D-Co remains relatively constant over our compositional range. The findings of this study indicate that the effect of melt composition on trace element solubility is a function of cation oxidation state such that high valency cations like W4+,, more readily dissolved in depolymerized melts which have a higher ratio of nonbridging oxygens, and lower valency cations like Co2+,, relatively independent of the parameter nbo/t. These results confirm that the composition of the primitive mantle is an important factor in constraining how siderophile trace elements distribute themselves between an Fe-metal core and the bulk silicate Earth during an early magma ocean differentiation event. Copyright (C) 2000 Elsevier Science Ltd.