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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 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.

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Figure 1: Brillouin spectra of lower-mantle phases.
Figure 2: Shear velocity for lower-mantle phases.
Figure 3: Effect of aluminium and iron on shear modulus and its pressure derivative.
Figure 4: Lower-mantle geotherms and calculated shear- and longitudinal-wave velocity profiles for whole-mantle and layered-mantle convection models.

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Acknowledgements

We greatly appreciate the comments by I. Jackson. Suggestions from E. Ohtani, C. Bina, S. Karato and S.-H. Shim improved the manuscript. We also thank N. Sata and Y. Asahara for their experimental assistance at SPring-8. This study was performed under the approval of SPring-8 (proposals no. 2008B0099 and 2009A0087).

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M.M. planned the research and did experimental and analytical work. M.M. and K.H. wrote the paper. All authors were involved in the experiments and discussed the results.

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Correspondence to Motohiko Murakami.

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The authors declare no competing financial interests.

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Murakami, M., Ohishi, Y., Hirao, N. et al. A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data. Nature 485, 90–94 (2012). https://doi.org/10.1038/nature11004

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