Seismic velocities of major silicate and oxide phases of the lower mantle

We have recently investigated the elastic properties of five compounds gene rally considered as potential consituents of the Earth's lower mantle. Full elastic constant tensors of MgSiO3 and CaSiO3 perovskites, MgO, CaO, and S iO2 have been determined as a function of pressure up to 140 GPa and at zer o temperature using the first principles plane wave pseudopotential method within the local density approximation. We report comprehensive comparisons of the calculated high-pressure elastic wave velocities of these phases wi th seismically derived lower mantle properties. The effects of iron and tem perature are estimated in the case of Mg-silicate perovskite based on avail able experimental data. Comparison of the P and S wave velocity profiles of perovskite along a series of high-temperature isotherms to the lower mantl e supports the view that this region is primarily composed of Mg-rich silic ate perovskite. Our study also suggests that among the candidate phases usu ally considered, none are invisible. Minor phases which may be present, inc luding magnesiowustite, CaSiO3 perovskite, and the high-pressure polymorphs of SiO2 and CaO, have elastic wave velocities that differ significantly fr om those of Mg-rich silicate perovskite and may be detectable seismically.