Determining the temperature of the earth's continental upper mantle from geochemical and seismic data

A method is proposed for determining the temperature of the Earth's upper mantle from geochemical and seismic data. The data are made consistent by physicochemical simulations, which enable one to derive physical characteristics from geochemical compositional models ( direct problem) and to convert seismic velocity profiles into model for the temperature distribution ( inverse problem). The methods were used to simulate temperature distribution profiles in the normal and cold mantle on the basis of profiles for the velocities of P and S waves in the IASP91 model and regional models for the Kaapvaal craton. The constraints assumed for the chemical composition included the depleted material of garnet peridotites and the fertile primitive mantle. The conversion of seismic into thermal profiles was conducted by minimizing the Gibbs free energy with the use of equations of state for the mantle material with regard for anharmonicity and the effects of inelasticity. The sensitivity of the model to the chemical composition and its importance in application to the solution of inverse problems is demonstrated. Temperature profiles derived from the IASP91 and some regional models for depths of 200 - 210 km display an inflection on geotherms toward decreasing temperatures, which is physically senseless. This anomaly cannot be related to either the presence of volatiles or the occurrence of partial melting, because both of them should have resulted in a decrease, but not an increase, in the seismic velocities. Temperature inversion can be ruled out by the gradual fertilization of the mantle with depth. In this situation, the upper mantle material at depths of 200 - 300 km should be enriched in FeO, Al2O3, and CaO relative to garnet peridotites and be simultaneously depleted in these oxides relative to the pyrolite material of the primitive mantle. It can be generally concluded that both the lithosphere and sublithospheric mantle of the Kaapvaal craton, as well as the normal mantle, should be chemically stratified.