Atomic-Scale Study of Intercrystalline (Mg,Fe)O in Planetary Mantles: Mechanics and Thermodynamics of Grain Boundaries Under Pressure

Abstract

Polycrystalline (Mg,Fe)O ferropericlase is the second most abundant mantle constituent of the Earth and possibly of super-Earth exoplanets. Its mechanical behavior is expected to accommodate substantial plastic deformation in Earth's lower mantle. While bulk properties of ferropericlase have been extensively studied, the thermodynamics of grain boundaries and their role on mechanical response remain largely unexplored. Here, we use density functional theory calculations to investigate mechanical behavior and thermodynamics of the {310}[001] grain boundary—a representative proxy for high-angle {hk0}[001] tilt grain boundaries—at relevant mantle pressures of the Earth and super-Earth exoplanets. Our results provide evidence that shear-coupled migration and grain boundary sliding are the dominant mechanisms of (Mg,Fe)O grain boundary mobility. We show that pressure-induced structural transformations of grain boundaries can trigger a change in the mechanism and direction of grain boundary motion. Significant mechanical weakening of the grain boundary is observed under multi-megabar pressures, caused by a change in the grain boundary transition state structure during motion. Our results identify grain boundary weakening in periclase as a potential mechanism for viscosity reductions in the mantle of super-Earths. We further demonstrate that structural grain boundary transitions control the spin crossover of Fe2+ in the grain boundaries. We model iron partitioning behavior between bulk and grain boundaries and predict equipartitioning to occur in μm size ferropericlase grains. Our findings suggest that the iron spin crossover pressure in ferropericlase may increase several tens of GPa by pressure-induced structural grain boundary transitions in dynamically active fine-grained lower mantle regions.