Computational support for a pyrolitic lower mantle containing ferric iron

The dominant minerals in Earthâ €™ s lower mantle are thought to be Fe-and Al-bearing MgSiO 3 bridgmanite and (Mg, Fe)O ferropericlase. However, experimental measurements of the elasticity of these minerals at realistic lower-mantle pressures and temperatures remain impractical. As a result, different compositional models for the Earthâ €™ s lower mantle have been proposed. Theoretical simulations, which depend on empirical evaluations of the effects of Fe incorporation into these minerals, support a pyrolitic lower mantle that contains a significant amount of ferropericlase, much like the Earthâ €™ s upper mantle. Here we present first-principles computations combined with a lattice dynamics approach that include the effects of Fe 2+ and Fe 3+ incorporation. We calculate the densities and elastic-wave velocities of several possible lower-mantle compositions with varying amounts of ferropericlase along a mantle geotherm. On the basis of our calculations of aggregate elasticities, we conclude that neither a perovskitic composition (about 9:1 bridgmanite to ferropericlase by volume) nor an olivine-like composition (about 7:3) reproduces the seismological reference model of average Earth properties. However, an intermediate volume fraction (about 8:2) consistent with a pyrolitic composition can reproduce the reference velocities and densities. Bridgmanite that is rich in ferric iron produces the best fit. Our findings support a uniform chemical composition throughout the present-day mantle, which we suggest is the result of whole-mantle convection. © 2015 Macmillan Publishers Limited. All rights reserved.