Grain Boundaries' Weakening Impact on Planetary Mantles

Grain Boundaries' Weakening Impact on Planetary Mantles
by Riko Seibo
Tokyo, Japan (SPX) Jul 18, 2024

The deformation of mantle rocks, which drives mantle convection and plate tectonics on planets like Earth, is influenced by the movement of defects within the crystal lattices of minerals. These defects' physical properties under pressure are crucial for understanding the dynamics of Earth-like planets.

A team of researchers, led by Dr. Sebastian Ritterex, formerly of the Geodynamics Research Center at Ehime University and now with the Department of Earth Sciences at Utrecht University, employed extensive high-performance computer simulations. These simulations, rooted in quantum mechanical atomic-scale modeling, explored the behavior of grain boundaries under the intense pressures found in planetary interiors. Utilizing "ab initio simulations," the team accurately computed chemical bonding, providing a valuable tool for assessing material properties in conditions where direct experimentation is challenging.

Focusing on high-angle tilt grain boundaries in (Mg,Fe)O ferropericlase, the Earth's second most abundant lower mantle mineral, the study applied both standard density functional theory and the LDA+U method to accurately reproduce the electronic structure of iron.

The study's findings revealed that high-pressure conditions significantly impact grain boundary motion mechanisms, which dictate intercrystalline deformation. "Our research proved for the first time that structural transformations of grain interfaces, induced by pressure with increasing depth in planetary mantles, trigger a change in the mechanism and direction of grain boundary motion," explained Dr. Ritterex. Additionally, the research demonstrated that grain boundaries could experience significant mechanical weakening under multi-megabar pressures.

This discovery challenges the conventional belief that materials become harder as atomic arrangements become more closely packed under pressure. The weakening is attributed to changes in the transition state structure of grain boundaries during motion at extremely high pressures. The Journal of Geophysical Research: Solid Earth, published in April 2024, highlights this weakening in ferropericlase as a potential mechanism for viscosity reduction in the mantles of super-Earth exoplanets.

Further thermodynamic modeling examined iron partitioning between bulk and grain boundaries. The research indicated that grain size significantly influences the segregation of iron in polycrystalline ferropericlase in the lower mantle's hot and dense conditions. Incorporating substitutional Fe(II) in bulk MgO notably affects properties like density and seismic wave velocities due to an electronic spin transition at high pressure within the Earth's interior.

Prior to this study, there was no information on the spin states of Fe(II) within grain boundaries. The current modeling shows that the electronic spin state of Fe(II) in ferropericlase tilt grain boundaries is governed by structural transformations at high pressures in the Earth's lower mantle. This mechanism affects the iron spin crossover pressure in polycrystalline (Mg,Fe)O with small grain sizes, potentially increasing it by several tens of GPa due to pressure-induced structural transitions in dynamically active fine-grained lower mantle regions.

Dr. Ritterex emphasized the need for more systematic data from theoretical modeling, experiments, and electron microscopy observations to better understand grain boundaries' collective effects on polycrystalline ferropericlase's rheological and thermodynamic properties at relevant mantle pressures and temperatures.