Thermochemical-thermophysical data on phases in the magnesium-iron-silicon-oxygen system: A synthesis of theory and experimental data and computation of phase equilibrium.

摘要

Thermochemical and thermophysical data for phases in the Mg-Fe-Si-O system are critically evaluated by considering the available experimental phase equilibrium data, the calorimetric measurements, and measured thermophysical properties of solids. An internally consistent data base is thus established. A new equation of heat capacity (C{dollar}sb{lcub}rm p{rcub}{dollar}) for solids is proposed for the extrapolation of C{dollar}sb{lcub}rm p{rcub}{dollar} to high temperatures. The Birch-Murnaghan equation of state with temperature dependence of the bulk modulus is used to calculate the effect of pressure on the Gibbs free energy change of a reaction. Phases included in the data base are polymorphs of Fe (bcc-Fe, fcc-Fe and hcp-Fe), Fe{dollar}sb3{dollar}O{dollar}sb4{dollar} (magnetite and high-pressure magnetite), Fe{dollar}sb2{dollar}O{dollar}sb3{dollar} (hematite and high-pressure hematite), SiO{dollar}sb2{dollar} (quartz, coesite and stishovite), Mg{dollar}sb2{dollar}SiO{dollar}sb4{dollar} (forsterite, {dollar}beta{dollar}-phase, spinel and melt), Fe{dollar}sb2{dollar}SiO{dollar}sb4{dollar}, (fayalite, {dollar}beta{dollar}-phase, spinel and melt), MgSiO{dollar}sb3{dollar} (pyroxene, ilmenite, garnet, perovskite and melt) and FeSiO{dollar}sb3{dollar} (pyroxene and perovskite), and MgO (periclase). Solid solutions considered are MgO-FeO-FeO{dollar}sb{lcub}1.5{rcub}{dollar} (magnesiowustite), (Mg,Fe){dollar}sb2{dollar}SiO{dollar}sb4{dollar} (olivine, {dollar}beta{dollar}-phase and spinel) and (Mg,Fe)SiO{dollar}sb3{dollar} (pyroxene and perovskite). The solid solutions are experimentally studied by determining the Mg and Fe partitioning between coexisting phases, magnesiowustite and olivine, magnesiowustite and {dollar}beta{dollar}-phase, magnesiowustite and spinel, and magnesiowustite and perovskite, under various pressure and temperature conditions. The experiments are performed with the piston-cylinder apparatus, the multi-anvil device and the diamond-anvil cell technique at pressures between 2 and 28 GPa and temperatures between 1200{dollar}spcirc{dollar}C and 1500{dollar}spcirc{dollar}C. The solid solution parameters are obtained by fitting the experimental data simultaneously using the Margules formulation. They are consistent with solution calorimetry and phase equilibrium data.; The internally consistent data base can be used to reproduce the experimentally determined phase relations in the system at pressures up to 30 GPa and to melting temperatures. It is useful in generating phase diagrams for various different compositions for the purpose of planning new experiments and understanding the mineralogy and chemical composition of the Earth's mantle.