The stability of divalent Ge in silicate melts and its geochemical properties

Abstract

The oxidation state of Ge in silicate glasses, quenched from melts, was determined by X-ray absorption spectroscopy. The melts were equilibrated over the range of relative oxygen fugacities (fO2) from IW-3 to IW +10, where IW is the iron-wüstite oxygen buffer in logarithmic units. X-ray absorption near edge structure (XANES) spectra of the samples show that over the range in fO2 from IW-2.8 to IW +2.4, the Ge 4+ /(Ge 2+ + Ge 4+) ratio increases from 0.05 to 0.95. Modelling of extended X-ray absorption fine structure (EXAFS) gives the Ge 2+-O bond length as 1.89 ± 0.03 Å. Olivine-melt partitioning experiments were also conducted, which show that Ge 2+ is highly incompatible, with D "# $% &'/)#'* < 0.005, whereas D "# +% &'/)#'* is ~ 1, where D is the partition coefficient. The geochemical properties of Ge during the magmatic differentiation of the Moon and other reduced rocky planets and achondrite parent bodies will therefore be entirely different to that familiar from terrestrial examples. In particular, the incompatible nature of Ge 2+ may explain the anomalous enrichment of Ge in KREEP basalts.

Chemical equilibrium calculations for bulk silicate earth material at high temperatures

“…Table 27 and Figure 17a show the results for Ge chemistry in BSE material. Literature reports indicate germanium dissolves in CMAS, CMS-FeO, and CAS-FeO silicate melts as GeO2 and/or GeO (Kegler and Holzheid 2011Mare et al 2020, Yan and Swinbourne 2003, Shuva et al 2016. There are no thermodynamic data for formation of GeO (solid, liquid) from its constituent elements in any of the standard data compilations, so only Ge 4+ (GeO2) was considered.…”