Incorporating self-consistently calculated mineral physics into thermochemical mantle convection simulations in a 3-D spherical shell and its influence on seismic anomalies in Earth's mantle

Abstract

Phase assemblages of mantle rocks calculated from the ratios of five oxides (CaO-FeO-MgO-Al2O3-SiO2) by free energy minimization were used to calculate the material properties density, thermal expansivity, specific heat capacity, and seismic velocity as a function of temperature, pressure, and composition, which were incorporated into a numerical thermochemical mantle convection model in a 3-D spherical shell. The advantage of using such an approach is that thermodynamic parameters are included implicitly and self-consistently, obviating the need for ad hoc parameterizations of phase transitions which can be complex in regions such as the transition zone particularly if compositional variations are taken into account. Convective planforms for isochemical and thermochemical cases are, however, not much different from those computed using our previous, simple parameterized reference state, which means that our previous results are robust in this respect. The spectrum and amplitude of seismic velocity anomalies obtained using the self-consistently calculated material properties are more "realistic" than those obtained when seismic velocity is linearly dependent on temperature and composition because elastic properties are dependent on phase relationship of mantle minerals, in other words, pressure and temperature. In all cases, the spectra are dominated by long wavelengths (spherical harmonic degree 1 to 2), similar or even longer wavelength than seismic tomographic models of Earth, which is probably due to self-consistent plate tectonics and depth-dependent viscosity. In conclusion, this combined approach of mantle convection and self-consistently calculated mineral physics is a powerful and useful technique for predicting thermalchemical-phase structures in Earth's mantle. However, because of uncertainties in various parameters, there are still some shortcomings in the treatment of the postperovskite phase transition. Additionally, transport properties such as thermal conductivity and viscosity are not calculated by this treatment and are thus subject to the usual uncertainties. © 2009 by the American Geophysical Union.