Earth mineralogical model: Gibbs free energy minimization computation in the system MgO-FeO-SiO2

A thermodynamic database which is consistent with most available phase equilibrium experiments and calorimetric and physical measurements on the solids in the system MgO-FeO-SiO2 is established for the phases with the compositions (Mg, Fe)SiO3 (garnet, perovskite, pyroxene, and ilmenite), (Mg, Fe)(2)SiO4 (olivine, beta, and gamma phases), SiO2 (stishovite and coesite), and (Mg, Fe)O (periclase and wustite). The data are systematized by using the high temperature Birch-Murnaghan equation of state which includes the pressure and temperature dependent bulk modulus (K) and temperature dependent thermal expansion (alpha) of the solids. The systematized thermodynamic data contains heat capacity (C-p) data, which is internally consistent with the data on alpha, K, volume, and temperature. Such a systematized database is used to calculate, by the method of minimization of Gibbs free energy, the mineralogical composition of the peridotitic/pyrolitic and chondritic MgO-FeO-SiO2 mantles. The model corresponds closely to the seismological PREM (Preliminary Earth Reference Model) in predicting the major seismic discontinuities. However, such discontinuities resulting from reactions or phase transformation are not as sharp as the seismic ones. Calculated adiabatic geothermal gradient starting at 6 GPa and 1500 K reaches a temperature of 2046 K at the core/mantle pressure (135 GPa) in a pyrolite mantle. The model Earth parameters in the lower mantle are (PREM parameters in bracket): K-s = 308 (306) to 687 (656) GPa; phi = 70 (69) to 121 (118)km(2) s(-2).