In the last decade, computational mineral physics has played a fundamental role in our understanding of the Earth. It has complemented experiments by expanding the range of thermodynamics conditions materials properties can be investigated and has provided new insights into the nature of Earth's interior based on atomic scale arguments. This project supports a continuation of studies that use first principles calculations to quantify fundamental chemical and physical properties of some common minerals present deep in the interior of the Earth.
Under this grant it is proposed to expand first principles studies of Earth's materials to i) include anharmonic effects on calculations of thermal properties and phase relations of minerals using methods recently developed by the team; ii) advance studies of phase transformations in solid solutions, particularly for the post-perovskite transition; iii) explore the rheological consequences of the spin-state crossover in ferropericlase, the second most important phase in the lower mantle; iv) couple these studies with geodynamical modeling of the mantle. These studies will make key contributions to our understanding of i) thermodynamic equilibrium in multi-phase aggregates; ii) properties of thermodynamics phase boundaries of major mantle minerals. There are still large uncertainties associated with measurements of Clapeyron slopes, discontinuities, and two-phase loops in divariant systems. These quantities control mantle dynamics and will be better constrained; iii) the rheology of ferropericlase has been implicated in the proposed viscosity minimum around 1,600 km depth and will now be investigated; iv) the geodynamical sensitivity to and consequences of these results. This research, which has intrinsic interdisciplinary value, will shed light on the nature and dynamics in the D" and mid-lower mantle regions.
" are: 1) advance computational methodologies for predictive quantum mechanical calculations of mineral properties, 2) produce results to advance understanding of grand-challenge problems in geophysics; 3) advance the synergy between mineral physics and geodynamics by incorporating predictive results of mineral properties in the simulations and 4) explore phenomenological consequences of these more realistic geodynamics simulations and relate them to geophysical processes. Intellectual merit: During the last funding period several techniques have been developed: a) A semi-analytical method to compute thermal elastic properties that is approximately two orders of magnitude faster than the previous numerical approach b) A technique that enables calculations of anharmonic free energies has been developed. This will allow us to compute thermodynamics properties at very high temperatures approaching melting, and to calculate thermal conductivity. These properties are need for geodynamics simulations. c) A technique to compute electrical conductivity of insulating minerals. This technique will allow us to compute electrical conductivity of mantle minerals. We also have a new formulation to extend these calculations to metals that we hope to implement in the near future. This will allow us to calculate thermal conductivity of liquid metallic iron. Among the most important results we obtained are: a) Thermal elastic propertie of upper mantle and transition zone minerals - olivine, wadsleiyte, and ringwoodite – diamond, and ferropericlase. The latter undergoes a spin crossover and measurements of these elastic properties were still not well understood. b) We have performed for the first time quantum mechanical calculations of anharmonic free energy in the thermodynamics limit (number of particles approaching infinite) for the major phase of Earth’s lower mantle, MgSiO3 perovskite. c) Similar calculations have also been performed for cubic CaSiO3 perovskite to address the controversial stability field of this phase. We have integrated calculated thermodynamics and transport properties in geodynamics simulations a) Parameterized forms of thermal expansivity of minerals at high pressures and temperatures have been included in geodynamics simulations b) Parameterized forms of experimentally determined thermal conductivity have also been incorporated as well c) The influence of spin crossover in ferropericlase on diffusion activation parameters and viscosity has also been parameterized and incorporated in simulations. We have investigated: a) The influence of combined thermal expansivity and conductivity on mantle convection. Whe the combined influence of pressure and temperature on these properties are included, the time scale of the dynamics slow down. b) The influence of the spin crossover through rheology on mantle convection. Spin crossover invigorates convection and mixing in the lower mantle. Other groups have obtained similar results of profound implication for chemical stratification of the lower mantle. Broader impacts: As demonstrated, computational mineral physics is becoming highly integrated with seismology and geodynamics. It provides the basis for interpretation of mantle tomography and is enabling more realistic geodynamics simulations.
