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Computational support for a pyrolitic lower mantle containing ferric iron

Nature Geoscience volume 8, pages 556–559 (2015)Cite this article

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Abstract

The dominant minerals in Earth’s lower mantle are thought to be Fe- and Al-bearing MgSiO3 bridgmanite and (Mg, Fe)O ferropericlase1. However, experimental measurements of the elasticity of these minerals at realistic lower-mantle pressures and temperatures remain impractical. As a result, different compositional models for the Earth’s lower mantle have been proposed2,3,4. Theoretical simulations, which depend on empirical evaluations of the effects of Fe incorporation into these minerals, support a pyrolitic lower mantle that contains a significant amount of ferropericlase5,6, much like the Earth’s upper mantle. Here we present first-principles computations combined with a lattice dynamics approach that include the effects of Fe2+ and Fe3+ incorporation. We calculate the densities and elastic-wave velocities of several possible lower-mantle compositions with varying amounts of ferropericlase along a mantle geotherm. On the basis of our calculations of aggregate elasticities, we conclude that neither a perovskitic composition (about 9:1 bridgmanite to ferropericlase by volume) nor an olivine-like composition (about 7:3) reproduces the seismological reference model of average Earth properties. However, an intermediate volume fraction (about 8:2) consistent with a pyrolitic composition can reproduce the reference velocities and densities. Bridgmanite that is rich in ferric iron produces the best fit. Our findings support a uniform chemical composition throughout the present-day mantle, which we suggest is the result of whole-mantle convection.

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Figure 1: Volume compression curves of MgBr.
Figure 2: Longitudinal-wave velocities, bulk-sound velocities, shear-wave velocities, and densities of possible LM compositions along the geotherm.
Figure 3: Optimized Fe3+ ratio (Fe3+/ΣFe) in MgBr of pyrolitic composition.

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Acknowledgements

We thank Z. Zeng and S. Whitaker for helpful comments. This research was supported by JSPS Grants-in-Aid for Scientific Research under Grant No. 25 03023, KAKENHI 20001005 and 21740379, and by the X-ray Free Electron Laser Priority Strategy Program (MEXT). X.W. was supported by research fellowships of the Global CoE program of Earth and Planetary Science, Ehime University, Japan, and of JSPS for Overseas Researchers.

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Authors and Affiliations

  1. Geodynamics Research Center, Ehime University, Ehime 790-8577, Japan

    Xianlong Wang, Taku Tsuchiya & Atsushi Hase

  2. Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan

    Xianlong Wang & Taku Tsuchiya

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  1. Xianlong Wang
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Contributions

X.W. and T.T. designed the study and wrote the manuscript. X.W., T.T. and A.H. performed the calculations. X.W. analysed the data.

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Correspondence to Xianlong Wang or Taku Tsuchiya.

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Wang, X., Tsuchiya, T. & Hase, A. Computational support for a pyrolitic lower mantle containing ferric iron. Nature Geosci 8, 556–559 (2015). https://doi.org/10.1038/ngeo2458

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