title: A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data.
abstract: 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 observations, 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 spectroscopy. The mineralogical model that provides the best fit to a global seismic velocity profile 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.
authors.affiliation: Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan. motohiko@m.tohoku.ac.jp
authors.forename: Motohiko
authors.initials: M
authors.lastname: Murakami
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Ohishi
authors.forename: Naohisa
authors.initials: N
authors.lastname: Hirao
authors.forename: Kei
authors.initials: K
authors.lastname: Hirose