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Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core−mantle boundary

Dobson, D; Lindsay-Scott, A; Hunt, S; Bailey, E; Wood, I; Brodholt, J; Vocadlo, L; (2019) Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core−mantle boundary. Proceedings of the National Academy of Sciences of USA , 116 (52) pp. 26389-26393. 10.1073/pnas.1914826116. Green open access

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Abstract

The lowermost portion of Earth’s mantle (D″) above the core−mantle boundary shows anomalous seismic features, such as strong seismic anisotropy, related to the properties of the main mineral MgSiO3 postperovskite. But, after over a decade of investigations, the seismic observations still cannot be explained simply by flow models which assume dislocation creep in postperovskite. We have investigated the chemical diffusivity of perovskite and postperovskite phases by experiment and ab initio simulation, and derive equations for the observed anisotropic diffusion creep. There is excellent agreement between experiments and simulations for both phases in all of the chemical systems studied. Single-crystal diffusivity in postperovskite displays at least 3 orders of magnitude of anisotropy by experiment and simulation (Da = 1,000 Db; Db ≈ Dc) in zinc fluoride, and an even more extreme anisotropy is predicted (Da = 10,000 Dc; Dc = 10,000 Db) in the natural MgSiO3 system. Anisotropic chemical diffusivity results in anisotropic diffusion creep, texture generation, and a strain-weakening rheology. The results for MgSiO3 postperovskite strongly imply that regions within the D″ region of Earth dominated by postperovskite will 1) be substantially weaker than regions dominated by perovskite and 2) develop a strain-induced crystallographic-preferred orientation with strain-weakening rheology. This leads to strain localization and the possibility to bring regions with significantly varying textures into close proximity by strain on narrow shear zones. Anisotropic diffusion creep therefore provides an attractive alternative explanation for the complexity in observed seismic anisotropy and the rapid lateral changes in seismic velocities in D″.

Type: Article
Title: Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core−mantle boundary
Open access status: An open access version is available from UCL Discovery
DOI: 10.1073/pnas.1914826116
Publisher version: https://doi.org/10.1073/pnas.1914826116
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10086846
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