The Influence of Silicate Melt Structure on the Partitioning of W During Core Formation

The partitioning of W between silicate and metallic melts has important implications for the core formation. W is siderophile, but an increase affinity for silicate melts is recorded at low degrees of polymerization. We observe the same trend in our multi-anvil experiments. This phenomena has been explained intuitively based on its atomic charge and size (Siebert et al. 2011). The influence of NBO/T on the partitioning of elements has been extensively discussed in literature (Palme et al. 2011; Righter 2011a; Righter 2011b) but it does not address the problem in its entirety since structural changes imposed by compression to high pressure are not included in the NBO/T parameter. In our experimental data we looked at the partitioning of W as a function of NBO/T, at 8GPa and 2000oC, and we observe a correlation with the amount of Ca present in the melt. This observation lead to the idea that the atomic volume of W and the available space in the silicate melts is responsible for the partitioning variation with NBO/T. We decided to test this hypothesis by using first principles molecular dynamics (FPMD) simulations to look at the structural changes in the silicate melts as a function of Si/O ratio and the behaviour of W in such different environments. Here we present new data regarding the coordination and charge of W in different types of silicate melts and we offer a structural interpretation for the partitioning behaviour of W as a function of the degree of polymerization of silicate melts. References:Palme, H. et al., 2011. Comment on "Prediction of metal - silicate partition coefficients for siderophile elements : An update and assessment of PT conditions for metal - silicate equilibrium during accretion of the Earth" by K . Righter, EPSL, 312(3-4), pp.516-518Righter, K., 2011a. Prediction of metal-silicate partition coefficients for siderophile elements: An update and assessment of PT conditions for metal-silicate equilibrium during accretion of the Earth. EPSL, 304(1-2), pp.158-167Righter, K., 2011b. Reply to the Comment by Palme et al. EPSL, 304(1-2), pp.158-167Siebert, J. et al., 2011. Systematics of metal-silicate partitioning for many siderophile elements applied to Earth's core formation. GCA, 75(6), pp.1451-1489