THE EFFECT OF TEMPERATURE ON THE PARTITIONING OF NICKEL, COBALT, MANGANESE, CHROMIUM, AND VANADIUM AT 9 GPA AND CONSTRAINTS ON FORMATION OF THE EARTH'S CORE - IMPLICATIONS FOR CORE-MANTLE EQUILIBRIUM

Abstract

The distribution of the elements Ni, Co, Mn, Cr, and V between liquid Fe-Ni-metal and magnesiowustite solid solutions is investigated at 9 GPa in the temperature range 1800-2400°C. The variation of starting material enables the temperature effects to be determined at different redox conditions, although the buffering capacity of the sample plus high pressure assembly appears to be limited at temperatures above 2200°C. The observed metal-magnesiowustite distribution coefficients (normalized to DFe) for the elements Ni and Co show a weak decrease with increasing temperature while those of Mn, Cr, and V progressively increase. The increasingly siderophile behaviour of Mn, Cr, and V with increasing temperature at high pressure is consistent with their depletion in the Earth's mantle being at least partly a consequence of core-mantle fractionation. The presence of silicate melt in some of the experiments also provides metal-silicate distribution coefficients as a function of temperature. The KDMmet/sil values for Mn, Cr, and V also increase with temperature. The metal-silicate distribution coefficients for Cr and V are slightly higher than those for metal-oxide partitioning while the results for Mn show larger differences. Manganese partitions preferentially into silicate phases, while Cr and V partition into magnesiowustite. Comparison of the new metal-oxide partitioning data with previous results suggests that the magnesiowustite composition has a strong influence on Ni partitioning. The distribution coefficients are extrapolated to higher temperatures in order to test the homogenous accretion model of core formation. Constraints on the solubility of Si and O in liquid metal are also obtained at 9 GPa and appear to preclude either of these elements as major light components in the Earth's core.