One of the major challenges in Earth materials research is to develop a firm theoretical basis to understanding the rheological properties of the silicate and oxide materials at high pressure-high temperature conditions. Rheology is a key factor, which controls the complicated mantle dynamics implied by seismological observations and other sources. For instance, the different mechanisms (diffusion or dislocation creep) by which the mantle may deform imply profoundly different pictures of mantle dynamics. Despite recent progress in diffusion and deformation experiments, large extrapolations are still needed to apply the measured data to deep mantle conditions. The PI and collaborators have previously demonstrated that first principles methods (based on density functional theory) being a parameter free approach provide an ideal complement to experiments. In this project, they apply a combination of computational and visualization methods to further promote our understanding of key relevant properties. Specific activities include: 1) Extending the study of point defects to include impurities (including protons) and complexes of defects in mantle minerals. Defect energetics is essential to our understanding of diffusion and deformation in minerals. 2) Investigating grain boundaries and their influence on defect formation and migration. The grain boundary segregation of native defects and impurities is of particular interest. 3) Visualizing simulation data to gain insight into the structures (bonding and coordination) of defect cores, microscopic mechanisms (related to impurity incorporation and diffusion), and electronic properties (defect states and associated localization/trapping of electrons).
The investigators anticipate that the predicted results will have important implications for the nature of mantle deformation, geochemical processes in the Earth's interior and ionic contribution to mantle conductivity. Also, they hope to enrich contact between theory and experiment. The proposal aims to systematically bridge the gap between computational science and Earth materials research and exploit high-end parallel supercomputing and visualization effectively. Its successful completion will have impact on a number of fields including geophysics, materials physics and computational science. The results will be disseminated through publications in geosciences and physics disciplines as well as computational/computer science conference proceedings. Finally, the proposal represents a unique opportunity for training new scientists to develop experience and expertise in more than one area.
Knowledge about the rheological properties of component silicate and oxide materials at high pressure-high temperature conditions is essential to our understanding of complicated mantle dynamics implied by seismological observations and other sources. For instance, the different mechanisms (diffusion or dislocation creep) by which the mantle may deform imply profoundly different pictures of mantle dynamics. Despite major progresses in diffusion and deformation experiments, long extrapolations are still needed to apply the measured data to deep mantle conditions. We have demonstrated that first principles methods based on density functional theory provide the ideal complement to such difficult experiments. In this project, we have applied a combination of computational and visualization methods to further promote our understanding of defects, diffusion, and grain boundaries. Here, we report our results and analysis of grain boundaries and their ability to accommodate point defects and impurities and enhance diffusion rates in mantle materials. Real materials including minerals and ceramics are usually comprised of crystal grains, which are misoriented relative to each other. The boundary regions separating these crystals can dramatically influence various physical properties such as creep and diffusion. Our knowledge about grain boundaries (GBs) and their effects particularly over the mantle pressure regime is limited. In this project, we have simulated different types of grain boundaries in mantle minerals using a first-principles approach. They include {n10}/[001] tilt GBs for MgO and (0l1)/[100], (1l0)/[001] and (012)/[100] GBs for Mg2SiO4 forsterite. Complexity of inter-granular structures with numerous possible configurations makes modeling of GBs that mimics natural samples challenging. Intensive simulations of bi-crystal systems consisting of 500 to over 1000 atoms have revealed the detailed atomic and electronic structures of grain boundaries. They also suggest that several configurations are energetically competitive over the relevant broad range of pressure. At the ambient pressure, the predicted important features of the boundaries include: distorted bonds (Si-O and Mg-O distances changed by 1 and 4 %, respectively), coordination defects (4- and 5-fold Mg-O coordination), and void spaces (0.2 – 0.9 x 10-10 m3/m2). Also, the interface induces splitting of electronic states from the conduction band and kinks at the top of the valence band. The boundary structures vary considerably on compression: Pressure systematically suppresses the excess volume, thereby affecting the type of coordination defects and the degree of bond distortions at the interface, and the relatively open GB core becomes less pronounced in high-pressure boundaries. Our simulations results have shown that native defects and impurities (Ca, Al, and proton modeled so far) favorably segregate to the boundary, with the segregation considerably increasing with pressure. They also imply that grain boundary diffusion is easier, and more anisotropic and complex than bulk (lattice) diffusion: The calculated migration enthalpies for host ions and impurities at the grain boundary are smaller than the bulk values, more so at higher pressures with their values being as low as ~ 1.5 eV at 100 GPa compared to the bulk values of ~ 4 eV. Our results suggest that GBs could be regions of primary storage for incompatible elements in mantle rocks. Fast ionic transport could cover long distances (several kilometers) over the geological time scales. Enhanced ionic diffusion could facilitate creep process. The predicted high defect activity of grain boundaries in key mantle minerals is expected to be relevant to our understanding of mantle rheology and geochemical process. Visualization of first principles molecular dynamics simulation of MgO tilt symmetric grain boundary performed at 0 GPa and 1000 K containing an impurity Ca ion (shown as large white sphere) in the boundary region. Three snapshots (three images provided) show initial, intermediate, and final positions during the migration of Ca ion occurring in the boundary region. As one can see, the Ca ion moves downward by one full lattice constant during a couple of picoseconds. Horizontal and vertical directions represent x ad z directions, respectively, and the y-z plane represents the boundary plane. Green (small) and red (large) spheres denote Mg and O ions, respectively.
