Thermodynamically consistent decompression: implications for lower mantle composition

Estimates of the composition of the lower mantle are commonly obtained by a process of adiabatic decompression, in which zero-pressure mantle parameters are determined with a least squares fit of an equation of state to mantle densities. It is shown that this procedure may lead to biased estimates of the zero-pressure density and compressibility of the mantle. The bias can be eliminated or minimized by subjecting the equations of state that are compatible with the lower mantle as well as the thermal properties of selected mineral models. This method was used to evaluate assemblages containing (Mg, Fe)SiO₃ perovskite and (Mg, Fe)O, with Fe/(Fe + Mg) ratios as high as 0.2. Results suggest that unless the mantle is homogeneous in its major oxide components, the lower mantle is likely to be enriched in Fe by at least several percent. -from Authors