Mantle convection and plate tectonics on Earth-like planets depend on how mantle rocks are deformed. This deformation occurs when defects move in the crystal structures of minerals. Therefore, understanding how these defects behave under pressure is crucial to understanding the dynamics of Earth-like planets.

Dr. was previously a PhD student at the Geodynamics Research Center at the University of Echeim and is currently a research assistant at the Department of Earth Sciences at Utrecht University. A collaborative team of researchers led by Sebastian Ritterex implemented massively parallel, high-performance computer simulations. Quantum mechanical simulations at the atomic scale will shed new light on the mysterious behavior of grain boundaries under the extreme pressure prevailing in the bowels of the planet. This theoretical methodology called “modelling” initially ” allows you to calculate chemical bonding very accurately. It is a powerful tool for determining material properties under extreme conditions in the interiors of planets, where experiments are difficult.

Study of ferropericlase in planetary mantles

Based on the theoretical mineral physics approach above, the team examined the mechanical behavior and thermodynamic properties of high-angle grain boundaries in (Mg,Fe)O ferropericlase, the second most abundant mineral in the Earth’s lower mantle and possibly in the supermantles. Earth outer planets. In this work, the internally consistent LDA+U method was applied in addition to standard density functional theory to more accurately reconstruct the electronic structure of iron.

Results of mechanical behavior show that the very high pressure conditions of terrestrial planets strongly influence the grain boundary motion mechanisms that control intergranular deformation. The research proved for the first time that structural transformations at grain interfaces caused by pressure with increasing depth in planetary mantles cause a change in the mechanism and direction of movement of grain boundaries.

The team also showed that significant mechanical weakening can develop at grain boundaries at pressures of several megabars. This is counterintuitive because it is generally believed that as pressure increases, the arrangement of atoms in materials becomes denser, making them harder to achieve. This phenomenon of grain boundary weakening results from a change in the structure of the transition state of grain boundaries during their movement under extremely high pressure.

Analysis of the data presented Journal of Geophysical Research: Solid EarthPublished in April 2024, it identifies the weakening of grain boundaries in ferropericlase as a potential mechanism that reduces viscosity with increasing depth in the mantle of super-Earth exoplanets.

Casting of iron and its rotational states

The team performed additional thermodynamic modeling of the distribution behavior of iron across bulk and grain boundaries. They determined that grain size is an important factor in controlling the segregation of iron grain boundaries in polycrystalline ferropericlase in the hot, dense lower mantle. It is well known that the incorporation of Fe(II) into bulk MgO has a significant effect on its physical properties, such as density and seismic wave velocity, because Fe(II) undergoes an electron spin transition at high pressure in the Earth’s interior. . There was no previous information about the rotation states of Fe(II) in the particles. Our simulations now show that the electronic spin state of Fe(II) within inclined ferropericlase grain boundaries is controlled by high-pressure grain boundary structural transformations in the Earth’s lower mantle.

This mechanism affects the pressure conditions of the spin transition of iron in polycrystalline (Mg,Fe)O with micrometer or smaller grain sizes. The obtained data show that the cross-pressure of iron during rotation in ferropericlase can increase by up to several tens of GPa due to pressure-induced structural boundary crossings in dynamically active fine-grained regions of the lower mantle compared to thermodynamically more stable regions in the lower mantle. coat.

The group is delighted with these findings, but a better understanding of the collective influence of grain boundaries on the rheological and thermodynamic properties of polycrystalline ferropericlase will require more systematic data from theoretical modeling as well as experimental and electron microscopy observations. corresponding pressures and temperatures in the planets’ mantles.