A Computational Study of the Earth's Lower Mantle Thermal Structure and Composition

Abstract

An enhanced determination of the internal dynamic processes that take place in our planet's interior could be achieved if its composition and thermal structure are well resolved. However, its extreme pressure and temperature conditions make this task a grand challenge in the geophysical sciences. Alternate methods of study are required to overcome such physical constraints. In this work, we use the thermoelastic properties of various minerals, computed from ab initio calculations, to study the composition and thermal structure of the Earth's lower mantle. The mineral phases of this region of the Earth are: (Fe, Al)-bearing MgSiO3 bridgmanite, (Fe, Al)-bearing MgSiO3 post-perovskite, (Mg, Fe)O ferropericlase, and CaSiO3 perovskite. The thermoelastic properties of different lower mantle aggregates are computed by varying the molar concentration of these mineral phases, whose seismic velocities and densities are compared to one-dimensional seismic models, for validation, along their self-consistent temperature gradient. We particularly focus on the effect of a pressure-induced reordering of the electronic structure of Fe in ferropericlase, known as spin-crossover, on the temperature profile of the lower mantle. The anomalous behavior caused by spin-crossover has not been observed in one-dimensional seismic studies. Thus, we present a novel way to achieve this by using a common seismic observable known as the Bullen's parameter. Finally, we study the seismic and thermodynamic signatures of a major phase transition, bridgmanite to post-perovskite, which are of vital importance to shed light on the enigmatic behavior of the deep lower mantle, otherwise known as the D" region.