Silicate melt under pressure: coordination changes and trace element partitioning

Abstract

Many geochemical models of magmatic processes, such as the formation of the Earth’s metallic core, are based on trace element partition coefficients. Fundamental to these models is an understanding of how partition coefficients vary with pressure. The main objective of this thesis is to explore one factor that controls the pressure-dependence of partitioning: the coordination environment (i.e. the number of bonded oxygens) of cations in silicate melt. Changes in the coordination number of major elements, Si and Al, are well known to occur in natural melts with pressure, but similar changes have been demonstrated for only a few trace elements: Ni, Co and Lu. In this work, coordination environments of Ge were Ga were studied. X-ray absorption spectroscopy of aluminosilicate glasses was used to show that both Ge and Ga begin to change their coordination from about 1 GPa, and this change is not yet complete at 10 GPa. In glasses quenched from high-pressure melts, Ge and Ga average coordination increased rapidly between 4 and 5 GPa, suggesting that a change in major element coordination might influence the coordination of both trace elements. To assess this possibility, nuclear magnetic resonance spectroscopy was used to determine changes in the local environments of major elements in the same or similar glasses to which Ge and Ga coordination was studied. Al coordination changed rapidly between 4 and 5 GPa, as had been observed for Ge and Ga. In particular, the Ga and Al average coordination numbers correlate well. These observations indicate that changes in the coordination of major elements may indeed influence the coordination of trace elements. The effect of a Ge coordination change on partitioning was determined by conducting olivine-melt partitioning experiments up to 4.5 GPa. The results show that Ge becomes more incompatible with increasing pressure, whereas if no coordination change took place, the opposite behaviour would be expected. However, existing models are insufficient to describe the effect of coordination changes on partitioning behaviour. The observed coordination changes of Ge4+ will be relevant in models of the Earth’s core formation only if Ge4+ is the stable species at the reducing conditions of the magma ocean at that time (below the iron-wüstite oxygen buffer, IW). However, previous work has indicated the possibility of Ge2+ stability in silicate melts around these conditions. This was tested using X-ray absorption spectroscopy of glasses quenched from melts prepared at varying oxygen fugacity. The spectra show that the Ge4+–Ge2+ transition occurs over the range IW +2 to IW -2. Olivine-melt partitioning experiments indicate that Ge2+ is highly incompatible, in contrast to Ge4+, which has a partition coefficient close to one. Show more