The solubility and oxidation state of tungsten in silicate melts: Implications for the comparative chemistry of W and Mo in planetary differentiation processes

Abstract

The solubility of W in 18 melt compositions in the system CaO–MgO–Al 2O 3–SiO 2 in equilibrium with W metal was determined as a function of oxygen fugacity ( fO 2) at 1400 °C and atmospheric pressure, using CO–CO 2 and H 2–CO 2 gas mixtures to control fO 2. Samples were analysed by both laser-ablation ICP-MS and electron microprobe. The variation of W solubility with fO 2 establishes that W dissolves predominantly as W 6+, with a possible contribution from W 4+ only at the very lowest fO 2s accessible to the experimental method, in which regime experimental difficulties make the reliability of the results uncertain. X-ray absorption near edge structure (XANES) spectroscopy at the L3-edge of representative samples confirms the oxidation state of W as 6+, and suggests that W 6+ occurs in tetrahedral coordination in silicate melts. Activity coefficients of WO 3 derived from the solubility measurements correlate exactly with those of MoO 3 obtained previously by similar experiments using the same melt compositions and temperature (O'Neill and Eggins, 2002). The effect of TiO 2 on W solubility is shown to be mainly one of dilution, from an investigation at one fO 2 in the pseudobinary between the anorthite–diopside eutectic composition (ADeu) and TiO 2. The solubilities of W and also Mo may be combined with thermodynamic data from the literature for Fe–W and Fe–Mo alloys to calculate partition coefficients for W and Mo between silicate melt and Fe-rich metal. The calculated partition coefficients for W and Mo differ by ∼ 10 3 over the range of fO 2 appropriate for equilibrium between liquid metal and silicate melt during planetary core formation at low pressures and moderate temperatures (∼ 1400 °C). Because the ratio of D W sil-melt/met/ D Mo sil-melt/met is predicted to decrease only moderately with temperature (e.g., to ∼ 10 2 at 2200 °C), and is independent of fO 2, melt composition and degree of partial melting, the large fractionation of Mo/W expected for equilibrium conditions could provide a useful means of discriminating between models of heterogenous and homogenous accretion and core formation, once the effect of pressure is better understood. However, comparison of our calculated metal/silicate partition coefficients with direct experimental determinations reveals an apparent lack of internal consistency among the latter, which may partly reflect a strong influence of minor components in the metal phase (e.g., carbon) on partitioning, which will also need to be understood before Mo/W systematics can be applied with confidence.