Experimental determination of the metal/silicate partition coefficient of Germanium: Implications for core and mantle differentiation

Abstract

Germanium is a moderately siderophile, and volatile element that exhibits a depletion in Earth's mantle and other planetary and asteroidal basalts (relative to chondritic values). Attempts to explain these depletions have not been satisfactory, but there have also been too few experimental studies to constrain the chemical behavior of Ge. Experimental data have been challenging due to difficulties in analyzing the low (1 to 100 ppm) concentrations of Ge in silicate melts. In this study we examine the role of temperature and silicate melt composition in controlling the value of D(Ge) metal/silicate, by carrying out new experiments at high temperature and pressure. The experimental glasses were analyzed using laser-ablation ICP-MS, which can detect levels of Ge much lower than possible with the electron microprobe. Our new results show that D(Ge) metal/silicate decreases with increasing temperature over the range from 1500 to 1900 °C. Additionally, changing silicate melt composition from basalt to peridotite causes a small increase in D(Ge) metal/silicate, compared to the decrease observed in previous studies across a different range of melt composition. These results, combined with previous studies at higher pressures and with S-bearing metallic liquids, show that there are high PT conditions at which the Ge concentration Earth's primitive upper mantle can be explained through metal-silicate equilibrium in a magma ocean stage, where equilibration took place at 40 (± 4) GPa and 3200 (± 200) °C. Additionally, estimated Ge contents of the lunar, Martian and HED parent body mantles can be explained by magma ocean conditions in all of these bodies early in the solar system. These calculations all take into account both the volatile and siderophile nature of Ge.