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First-principles constraints on diffusion in lower-mantle minerals and a weak D′′ layer

Abstract

Post-perovskite MgSiO3 is believed to be present in the D′′ region of the Earth’s lowermost mantle1,2,3,4. Its existence has been used to explain a number of seismic observations, such as the D′′ reflector and the high degree of seismic anisotropy within the D′′ layer5,6,7,8. Ionic diffusion in post-perovskite controls its viscosity, which in turn controls the thermal and chemical coupling between the core and the mantle, the development of plumes and the stability of deep chemical reservoirs9. Here we report the use of first-principles methods to calculate absolute diffusion rates in post-perovskite under the conditions found in the Earth’s lower mantle. We find that the diffusion of Mg2+ and Si4+ in post-perovskite is extremely anisotropic, with almost eight orders of magnitude difference between the fast and slow directions. If post-perovskite in the D′′ layer shows significant lattice-preferred orientation, the fast diffusion direction will render post-perovskite up to four orders of magnitude weaker than perovskite. The presence of weak post-perovskite strongly increases the heat flux across the core–mantle boundary and alters the geotherm9. It also provides an explanation for laterally varying viscosity in the lowermost mantle, as required by long-period geoid models10. Moreover, the behaviour of very weak post-perovskite can reconcile seismic observation of a D′′ reflector with recent experiments showing that the width of the perovskite-to-post-perovskite transition is too wide to cause sharp reflectors11. We suggest that the observed sharp D′′ reflector is caused by a rapid change in seismic anisotropy. Once sufficient perovskite has transformed into post-perovskite, post-perovskite becomes interconnected and strain is partitioned into this weaker phase. At this point, the weaker post-perovskite will start to deform rapidly, thereby developing a strong crystallographic texture. We show that the expected seismic contrast between the deformed perovskite-plus-post-perovskite assemblage and the overlying isotropic perovskite-plus-post-perovskite assemblage is consistent with seismic observations.

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Figure 1: Comparison of absolute diffusion rates with experiments.
Figure 2: Vacancy diffusion coefficients and viscosity in the lower mantle along a geotherm.
Figure 3: Seismic speed as a function of depth for a gradual transformation to post-perovskite, with an abrupt change in deformation.

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Acknowledgements

The authors thank D. Prior and J. Wheeler for discussions. This work was funded by the European Commission through the Marie Curie Research Training Network c2c (Crust to Core) contract no. MRTN-CT-2006-035957. The authors acknowledge the use of University College London’s Research Computing facility Legion and the use of HECToR, the UK national high-performance computing service, which is provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc. and NAG Ltd, and funded by the Office of Science and Technology through the Engineering and Physical Research Council’s High-End Computing Programme.

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J.P.B. and D.P.D. initiated the project. M.W.A. performed the calculations. J.W. performed the calculations of the seismic profile. M.W.A., J.P.B. and D.P.D. discussed the results and implications and wrote the paper.

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Correspondence to M. W. Ammann or J. P. Brodholt.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Methods and Data, Supplementary Tables S1-S9, Supplementary Figures S1-S4 with legends, Acknowledgements and References. (PDF 659 kb)

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Ammann, M., Brodholt, J., Wookey, J. et al. First-principles constraints on diffusion in lower-mantle minerals and a weak D′′ layer. Nature 465, 462–465 (2010). https://doi.org/10.1038/nature09052

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