Composition of the core, II. Effect of high pressure on solubility of FeO in molten iron

Abstract

An experimental and theoretical investigation of the effect of pressure on the solubility of FeO in molten iron has been carried out. Analyses of shock-wave compression data on iron oxides combined with measurements of the Fe O bond length in “metallic” oxides suggest that the partial molar volume of FeO (V *) dissolved in molten iron is substantially smaller than that of molten wüstite. Hence the effect of high pressure should be to increase the solubility of FeO in molten iron at a given temperature. This inference is confirmed by an experimental investigation of the effect of pressure on the position of the Fe FeO eutectic. Thermodynamic calculations based on these experiments yield an estimate for V * which is in reasonable agreement with the theoretical estimates. The experimental value of V * is used to calculate the effect of high pressure upon the Fe FeO phase diagram. Solubility of FeO in molten iron increases sharply with pressure, the liquid immiscibility region contracts and disappears around 20 GPa and it is predicted that the Fe FeO phase diagram should resemble a simple eutectic system above about 20 GPa. Analogous calculations predict that the solubility of FeO in molten iron in equilibrium with magnesiowüstite (Mg 0.8Fe 0.2)O at 2500°C increase from 14 mol.% (P = 0) to above 25 mol.% at 20 GPa. If the core formed by segregation of metallic iron originally dispersed throughout the earth, it seems inevitable that it would dissolved large amounts of FeO, thereby accounting for the observation that the density of the outer core is substantially smaller than that of pure iron under corresponding P, T conditions.