The elasticity of the MgSiO3 post-perovskite phase in the Earth's lowermost mantle

Abstract

MgSiO3 perovskite has been assumed to be the dominant component of the Earth's lower mantle, although this phase alone cannot explain the discontinuity in seismic velocities observed 200–300 km above the core–mantle boundary (the D″ discontinuity) or the polarization anisotropy observed in the lowermost mantle1. Experimental and theoretical studies that have attempted to attribute these phenomena to a phase transition in the perovskite phase have tended to simply confirm the stability of the perovskite phase2,3,4,5,6. However, recent in situ X-ray diffraction measurements have revealed7 a transition to a ‘post-perovskite’ phase above 125 GPa and 2,500 K—conditions close to those at the D″ discontinuity. Here we show the results of first-principles calculations of the structure, stability and elasticity of both phases at zero temperature. We find that the post-perovskite phase becomes the stable phase above 98 GPa, and may be responsible for the observed seismic discontinuity and anisotropy in the lowermost mantle. Although our ground-state calculations of the unit cell do not include the effects of temperature and minor elements, they do provide a consistent explanation for a number of properties of the D″ layer.

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Figure 1: The unit cell structures of MgSiO3.
Figure 2: The enthalpy difference between the perovskite phase and post-perovskite phase as a function of pressure.
Figure 3: The variation of compressional (vP) and shear (vS) wave velocities as a function of propagation direction.

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Acknowledgements

We thank S. Kaneshima for discussions, D.M. Bird for providing CASTEP codes, N.M. Harrison for pseudopotentials and the computer centres of RIKEN and NIG for access to the supercomputers. This work was also supported by JASRI/SPring-8 and IFREE/JAMSTEC.

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Correspondence to T. Iitaka.

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Supplementary Figure

The anisotropic compressibility of the PP-phase. (DOC 73 kb)

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Iitaka, T., Hirose, K., Kawamura, K. et al. The elasticity of the MgSiO3 post-perovskite phase in the Earth's lowermost mantle. Nature 430, 442–445 (2004). https://doi.org/10.1038/nature02702

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