Experimental determination of partial molar volumes of Ga2O3 and GeO2 in silicate melts: implications for the pressure dependence of metal-silicate partition coefficients

摘要

In this study, densities of gallium and germanium bearing silicate liquids were determined using the double bob Archimedian method. The silicate base components of the liquids were sodium disilicate (NS2) and anorthite-diopside eutectic composition (AD). Densities were converted to molar volumes and plotted against mole fractions of Ga2O3 or GeO2. The mixing behavior of volumes is linear in all systems except in the NS2 + Ga2O3 liquids where a strong non-linearity is observed. In this ease, the partial molar volume at infinite dilution was determined by the common tangent method. At 1600 K, the partial molar volume of Ga2O3 is 50 +/- 8 cm(3) / mol in the NS2 liquid and 36.3 +/- 0.3 cm(3) / mole in the AD liquid. For GeO2 a partial molar volume of 29.7 +/- 0.2 cm(3)/mol is found independent of silicate composition. The knowledge of the partial molar volumes of Ga2O3 and GeO2 in silicate melts allows first order predictions of the pressure dependence of metal / silicate partition coefficients. Such data are important in deciding between homogeneous and inhomogeneous accretion models of the Earth. Using the partial molar volumes of Ga2O3 and GeO2 measured in this study combined with literature data on the metallic components, the volume change of the exchange reaction x/2Fe(met) + MOx/2sil =x/2FEO(sil) + M-met was calculated (M = Ga or Ge). Volume changes for Ga-bearing AD liquids are positive and increase with temperature leading to a decrease in siderophility of Ga relative to Fe with increasing pressure whereas the corresponding change in the Ge-bearing AD liquids is negative implying higher siderophility of Ge with pressure. The decrease of the metal-silicate partition coefficient of Ga with pressure is consistent with models of core formation by global metal-silicate equilibrium at some depth, The increase of the metal/silicate partition coefficient of Ge with pressure is incompatible with global core mantle equilibrium. However, there is some ambiguity regarding the valence state and the coordination of Ge during core formation. Either Ge was present as GeO or as GeO2 while the experimental results obtained in this study are only applicable for GeO,. Our extrapolations are only valid as long as the coordination of Ge does not change because the partial molar volumes depend strongly on the coordination number. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 40]