Phase assemblages of mantle rocks calculated from the ratios of five oxides (CaO‐FeO‐MgO‐Al 2 O 3 ‐SiO 2 ) 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 thermal‐chemical‐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.
Abstract We assess the effect of high thermal conductivity of Earth's core, which was recently determined to be 2–3 times higher than previously thought, on Earth's thermochemical‐magnetic evolution using a coupled model of simulated mantle convection and parameterized core heat balance, following the best fit case of Nakagawa and Tackley (2010). The value of core thermal conductivity has no effect on mantle evolution. The core‐mantle boundary heat flow starts high and decreases with time to ~13 TW, which is below the core adiabatic heat flux for the largest thermal conductivity tested (200 W/m/K), meaning that a purely thermal dynamo is not viable. However, gravitational energy release and latent heat associated with inner core growth become important in the last ~0.9 Gyr and allow continuous geodynamo generation despite high core thermal conductivity, although we estimate a subadiabatic region at the top of the core of the order of hundreds of kilometers.
We present preliminary results of an iterative inversion of seismic and density data for the 3-D thermochemical structure of the upper mantle. Our approach relies on a mineral physics model based on current knowledge of material properties at high pressure (P) and temperature (T). The phase equilibria and the elastic properties are computed by using a recent thermodynamical model covering a six oxides (NCFMAS) system. Anelastic properties are implemented with a P, T and frequency dependent law based on available mineral physics knowledge. The model predicts values of physical parameters (e.g., shear velocity, density) as function of pressure (or depth), temperature and composition. Equilibrium compositions or mixtures of different compositions (e.g., MORB and Harzburgite) can be considered. First, we interpret available seismic models for temperature, assuming given compositions. For each model, we predict density and viscosity structure. Second, we compute the geoid kernels considering the average viscosity profiles of each model and perturb the 3-D density model(s). Density variations from the starting models are assumed to be due to lateral variations in composition. Consistently with the origin of such anomalies through melting extraction, we start by assuming only variations along a compositional axis that goes from harzburgite to MORB. We iterate the procedure until convergence. The inversion is implemented using a parametrization in spherical harmonics, a global scale basis which allows a clear analysis of the results in terms of relative contribution of different harmonic degrees (or wavelengths). Although lateral variations in viscosity are not accounted for in the inversion, we evaluate their effects with a forward approach, using the numerical code STAGYY. The synthetics geoids are computed with instantaneous flow calculations and compared with observations. We also use extreme physical laws for the most uncertain material parameters, i.e. viscosity and anelasticity, in order to assess their role on the outcome of the inversion. In general, we found that lateral thermal variations can explain most of the data. Including gravity data helps to determine lateral variations in composition. At a global scale, the dychotomy between continental and oceanic regions clearly emerges. Also, the large temperature variations between continents and oceans down to 300km produce large viscosity variations. In turn, these play a not negligible role on the geoid anomalies at spherical harmonics between 4 and 16 degrees. Including higher frequency seismic data, comparing the results with available petrological information and adding more complexities into the compositional parametrization (e.g., water effects) can help to better resolve the thermochemical anomalies at a regional scale.
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