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

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-diopsi de eutectic composition (AD). Densities were converted to molar volumes and plotted against mole fractions of Ga2O3 or GeO2. The mixing behavior of vo lumes is linear in all systems except in the NS2 + Ga2O3 liquids where a st rong non-linearity is observed. In this ease, the partial molar volume at i nfinite dilution was determined by the common tangent method. At 1600 K, th e partial molar volume of Ga2O3 is 50 +/- 8 cm(3) / mol in the NS2 liquid a nd 36.3 +/- 0.3 cm(3) / mole in the AD liquid. For GeO2 a partial molar vol ume 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 m elts allows first order predictions of the pressure dependence of metal / s ilicate partition coefficients. Such data are important in deciding between homogeneous and inhomogeneous accretion models of the Earth. Using the par tial molar volumes of Ga2O3 and GeO2 measured in this study combined with l iterature data on the metallic components, the volume change of the exchang e 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 siderophil ity of Ga relative to Fe with increasing pressure whereas the corresponding change in the Ge-bearing AD liquids is negative implying higher siderophil ity of Ge with pressure. The decrease of the metal-silicate partition coeff icient of Ga with pressure is consistent with models of core formation by g lobal metal-silicate equilibrium at some depth, The increase of the metal/s ilicate partition coefficient of Ge with pressure is incompatible with glob al core mantle equilibrium. However, there is some ambiguity regarding the valence state and the coordination of Ge during core formation. Either Ge w as present as GeO or as GeO2 while the experimental results obtained in thi s 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 vo lumes depend strongly on the coordination number. (C) 2001 Elsevier Science B.V. All rights reserved.