Mineral physics constraints on the chemical composition of the Earth's lower mantle

Abstract

Mineralogical modeling of the lower mantle critically depends on thermoelastic parameters, such as the expansion coefficient and the Gruneisen parameters. Significant experimental discrepancies exist for these properties of silicate perovskite. We calculate density and seismic velocities of mineral assemblages of (Mg, Fe)SiO_3 perovskite and (Mg, Fe)O magnesiowustite at lower-mantle pressures and temperatures. The results are compared with seismological results (PREM; Preliminary Reference Earth Model) and a statistical test of goodness of fit is applied. We plot the chemical composition of the lower mantle as a function of the thermoelastic parameters of perovskite. The inferred chemical composition depends on the adopted properties. The present uncertainty in the data accommodates most proposed compositional models. A better understanding of lower-mantle mineralogy requires better thermoelastic data, particularly of silicate perovskite. A preferred compositional model is constrained by combining mineral physics with seismological observations of the Earth's deep interior. It appears that the upper mantle and lower mantle are chemically distinct. A chondritic lower mantle cannot be ruled out with present data.