Core formation and the oxidation state of the Earth: additional constraints from Nb, V and Cr partitioning

We have combined metal-silicate partitioning data from the literature with new experimental results at 1.5-8 GPa and 1480-2000 °C to parameterize the effects of pressure, temperature and composition on the partitioning of V, Cr and Nb between liquid Fe metal (with low S and C content) and silicate melt. Using information from the steelmaking literature to correct for interactions in the metal phase, we find that, for peridotitic silicate melts, metal-silicate partition coefficients are given by:(log fenced(D_Nb ^met/sil) = 2.5 log fenced(D_Fe ^met/sil) + 1.57 - frac(11, 930, T) - frac(114 (± 43) P, T) ± 0.41; log fenced(D_V ^met/sil) = 1.5 log fenced(D_Fe ^met/sil) + 0.582 - frac(6493, T) - frac(62 (± 19) P, T) ± 0.08; log fenced(D_Cr ^met/sil) = log fenced(D_Fe ^met/sil) + 0.643 - frac(4232 (± 538), T) - frac(22 (± 13) P, T))where D_i ^met/sil is the partition coefficient for element i. Partitioning of V and Cr is insensitive to silicate melt composition, but Nb shows a considerable compositional effect. The new data enable us to examine different models of terrestrial accretion and core formation. If we fix the Fe content of the mantle at the current value and use the Ni content of the mantle to estimate pressure of equilibration then temperatures about 1200 °C above the silicate liquidus are required to match vanadium partitioning to the current concentration of V in the mantle. Under these conditions the core concentrations of Si (∼15%) and Nb (>60% of Earth's budget) are implausibly high. A more realistic approach is to assume that the metal of accreting planetesimals equilibrated at the base of a deep magma ocean whose temperature was close to the silicate liquidus. As the magma ocean deepened in proportion to the size of the Earth the metal was continuously extracted to the core without further re-equilibration in the lowermost mantle. In this case the V, Cr and Nb contents of core and mantle can easily reproduce the expected values provided the Earth became more oxidized as it accreted (O'Neill H. S. (1991) The origin of the Moon and the early history of the Earth-a chemical model. 2. The Earth Geochim. Cosmochim. Acta 55(4), 1159-1172; Wade J. and Wood B. J. (2005) Core formation and the oxidation state of the Earth. Earth Planet. Sci. Lett. 236, 78-95; Wänke H. and Dreibus G. (1988) Chemical-composition and accretion history of terrestrial planets. Philos. Trans. Roy. Soc. Lond. A-Math. Phys. Eng. Sci. 325(1587), 545-557). Increasing oxidation requires the oxidized Fe content of the mantle to increase from 0.5% to 1.0% to the current value of 6.26% as the Earth grew. This model, with magma ocean thickness corresponding to 35% of mantle depth, reproduces the calculated core-mantle partitioning of Ni, and Co and yields a Si content of the core of approximately 6%, in good agreement with cosmochemical estimates.