PREDICTION OF SIDEROPHILE ELEMENT METAL-SILICATE PARTITION-COEFFICIENTS TO 20-GPA AND 2800-DEGREES-C - THE EFFECTS OF PRESSURE, TEMPERATURE, OXYGEN FUGACITY, AND SILICATE AND METALLIC MELT COMPOSITIONS

We report new metal-silicate partition coefficients for Ni, Co and P a t 7.0 GPa (1650-1750 degrees C), and Ni, Co, Mo, W and P at 0.8, 1.0 a nd 1.5 GPa (1300-1400 degrees C). Guided by thermodynamics, all availa ble metal-silicate partition coefficients, D(i), where i is Ni, Co, P, Mo and W, are regressed against 1/T, P/T, ln f(O-2), ln(1-X-S) (X-S i s mole fraction of S in metallic liquid) and nbo/t (non-bridging oxyge n/tetrahedral cation ratio, a silicate melt compositional-structural p arameter) to derive equations of the following form: ln D(i) = aln f(O -2) + (b/T) + (cP/T) + d(nbo/t) + eln(1-X-S) + f. Expressions for soli d metal-liquid silicate and liquid metal-liquid silicate partition coe fficients are derived for S-free and S-bearing systems. We investigate whether Earth's upper-mantle siderophile element abundances can be re conciled with simple metal-silicate equilibrium. Sulfur-free metallic compositions do not allow a good fit. However, Ni, Co, Mo, W and P abu ndances in the upper mantle are consistent with simple metal-silicate equilibrium at mantle pressures and temperatures (27 GPa, 2200 K, Delt a IW(iron-wustite) = -0.15, nbo/t = 2.7; X-S = 0.15). Although these c onditions are near the anhydrous peridotite solidus, they are well abo ve the hydrous solidus and probably closer to the liquidus. A hydrous magma ocean and early mantle are consistent with predicted planetary a ccretion models. These results suggest that siderophile element abunda nces in Earth's upper mantle were established by liquid metal-liquid s ilicate equilibrium near the upper-mantle-lower-mantle boundary. (C) 1 997 Elsevier Science B.V.