A Perovskitic Lower Mantle Inferred from High-pressure, High-temperature Sound Velocity Data

Abstract

Determination of the shear-wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle indicates that perovskite constitutes much more of the lower mantle than predicted by the conventional mantle model and is consistent with the chondritic Earth model. Earth's upper mantle is apparently depleted of silicon when compared with meteorites, which are thought to represent the material from which the Earth formed. This 'missing silicon' problem has provoked intense debate: it suggests that the deficit might be balanced by silicon in the core, or that the upper mantle is not representative of the entire mantle and that the lower mantle is enriched in silicon. Murakami et al. provide evidence to support the latter case. They made laboratory measurements of sound-wave transmission through silicate perovskite and ferropericlase minerals at pressures and temperatures matching those of the lower mantle, and compared the resulting shear-wave values with seismic data that sample the lower mantle. They conclude that the mineralogical model that provides the best fit is one in which perovskite constitutes greater than 93% of the lower mantle. This suggests that the lower mantle is enriched in silicon in comparison with the upper mantle, consistent with the chondritic Earth model, and that there is limited mass transport between the upper and lower mantle. The determination of the chemical composition of Earth’s lower mantle is a long-standing challenge in earth science. Accurate knowledge of sound velocities in the lower-mantle minerals under relevant high-pressure, high-temperature conditions is essential in constraining the mineralogy and chemical composition using seismological observations1, but previous acoustic measurements were limited to a range of low pressures and temperatures. Here we determine the shear-wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle using Brillouin scattering spectroscopy2. The mineralogical model that provides the best fit to a global seismic velocity profile1 indicates that perovskite constitutes more than 93 per cent by volume of the lower mantle, which is a much higher proportion than that predicted by the conventional peridotitic mantle model. It suggests that the lower mantle is enriched in silicon relative to the upper mantle, which is consistent with the chondritic Earth model. Such chemical stratification implies layered-mantle convection with limited mass transport between the upper and the lower mantle.