Homogeneity and constitution of the earth's lower mantle and outer core

Abstract

The lower mantle and outer core of the recent “parametric earth model” (PEM), in which the velocity and density distributions in these regions are both smooth and consistent with the Adams-Williamson equation, are extrapolated to zero pressure using fourth-order finite-strain relations, and the results are compared with recent experimental data on relevant materials. The fourth-order term is found to significantly improve the fits to the model. The lower-mantle velocities are found to be significantly higher than those expected for mixtures of MgO, FeO and SiO 2 of olivine or pyroxene compositions. It is concluded that a slightly denser phase assemblage, with a low FeO content (roughly 5 mole%) is required to explain these results. This Fe content is not significantly different from that in the upper mantle. These results imply that no compositional or thermal constraints on whole-mantle convection can be resolved. The extrapolation of the outer core is well-determined, and the results are consistent with a mixture of Fe and a small proportion of a lighter element.