Determination of the formal Ge-oxide species in silicate melts at oxygen fugacities applicable to terrestrial core formation scenarios

Abstract

High-temperature, high-pressure experiments were performed at 1350 °C and 0.5 GPa to determine the dependence of germanium partitioning behavior between coexisting solid metal and liquid silicate phases on oxygen fugacity and melt composition. A piston cylinder apparatus was used to perform experiments under effectively closed-system conditions to avoid possible loss of volatile Ge species. The oxygen fugacity of the experiments was adjusted by using silicate melts with different amounts of FeO. This causes a change of the melt composition. Additional experiments with different melt compositions but identical FeO contents were performed to determine the effect of melt composition on the metal–silicate partitioning behavior. The partitioning behavior of Ge depends on both oxygen fugacity ( f O2) and silicate melt composition (NBO/T). The dependences can be described by the equation: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[logD^{met{\mbox{--}}sil}Ge= 3.32({\pm}0.07) {-}0.50({\pm}0.03) {\cdot} log\mathit{f}O_{2} {-} 0.29 ({\pm}0.04) {\cdot} \frac{NBO}{T} (\mathit{R}^{2} = 0.99).\] \end{document} The slope of the correlation between Dmet–silGe and oxygen fugacity allows one to determine the valence state of Ge in the silicate melt. Within the f O2 range of the experiments (−0.9 to −2.7 log units relative to the Fe-FeO (IW) buffer), the valence state of Ge is 2.00 (±0.12), i.e . GeO is the stable species in the silicate melt. This is in good agreement with an earlier study performed in 1 atm furnaces at 1300 °C and a comparable f O2 range (−0.5 to −2.7 log units relative to the IW buffer), but at variance with another study that determined tetravalent Ge cations dissolved as oxides in silicate melts in 1 atm experiments performed at more oxidizing conditions (−1.3 to +3.8 log units relative to IW buffer). The data of the present study in combination with these earlier data suggest a change in the dominant formal Ge oxide species in silicate melts at oxygen fugacities slightly below the IW buffer.