Velocity structure and composition of the lower mantle with spin crossover in ferropericlase

Abstract

The composition of the Earth's lower mantle (LM) is critical in understanding the Earth's interior and dynamics. Previous reports on the composition of the LM are controversial. The composition of LM, constrained here using high-temperature and high-pressure data of the velocities and density of minerals from first-principles calculations, spans a large range: from pyrolite with ~ 15% ferropericlase (Fp) in weight (wt %) to perovskitic-rich composition with ~ 10 wt % Fp. Any composition well constrained by preliminary reference Earth model (PREM) has a sufficient amount of Fp to exhibit the positive temperature dependence of VΦ (= Kρ) at the middle LM, leading to the insensitivity of VP to temperature. The depth at which VP is insensitive to temperature variation deepens significantly with increasing temperature because the spin transition in Fp has the positive Clapeyron slope (~17 MPa/K). The Large Low Shear Velocity Provinces below Africa is estimated to be ~ 750 K higher than the ambient mantle.