Van Driel, Jack; (2022) Understanding the Role of Grain Boundaries in the Lower Mantle: From the Atom to the Continuum. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis explores the properties of grain boundaries within Earth’s Lower Mantle for both MgO periclase and MgSiO3 bridgmanite with atomistic methods. Following introductory chapters, including a geophysical outline, grain boundary theory and computational methods, the thesis includes three results chapters. The first results chapter explores simple twin grain boundary structures within MgO periclase. Following ab-initio structure searching routines, the strength of twin boundaries was investigated from 0 120 GPa. Results found that the energy barrier for shear-coupled migration of the 310/[001] interface varies enormously with pressure and exhibits sudden strong discontinuities in strength. The second results chapter displays the approach taken to develop an accurate and transferable machine learning potential (MLP) for MgSiO3 bridgmanite. Through a structural database that in- cludes liquid crystalline and amorphous environments, the developed ML potential was shown to match a wide range of physical and chemical properties achieved with density functional theory (DFT). The MLP presented in this thesis has been shown to match phonon dispersion curves generated through high-quality DFT calculations. Additionally, the versatility of the MLP is illustrated in accurately modelling atomic environments such as grain boundaries. Finally, the MLP was used to obtain a melting curve via the two-phase coexistence approach, in which results match previous calculations and experiments. The final results chapter utilises the developed MLP to establish low energy grain boundary structures for MgSiO3 bridgmanite at six different orientations. Following the analysis of grain boundary properties such as width, energy and structure, the diffusivity of the grain boundary region is obtained through large scale molecular dynamics calculations and the mean squared displacement analysis. The properties of grain boundary width and diffusivity are then used to place constraints on the grain-size dependent viscosity of the lower mantle via the Coble Creep formula. Finally, Chapter 7, summarises the findings of each section as well as exploring the directions of future research.

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