Thermoelasticity, Water Partitioning, and Equations of State of Lower Mantle Minerals

How is water distributed in the Earth, and how might it be detected from seismic obser- vations? Recent geochemical, geodynamic, and astrophysical results suggest that the Earth may contain significantly more water than exists on its surface. Yet, how water might be distributed between lower mantle minerals is poorly understood. This dissertation encompasses three projects that explore the properties of materials under lower mantle pressures and temperatures. In the first study I used density functional theory (DFT) calculations to investigate the effects of water on the elastic properties of bridgmanite (brg), and postperovskite (ppv), at pressures of the lowermost mantle (called D''). The results indicate that the reduction in elastic moduli in D'' is expected to be approximately 1%, and 3%, per weight % water, for the bulk and shear moduli, respectively. In the second study I used DFT and ab-initio lattice dynamics to study the distribution of water between brg and ppv with aluminum-bearing and aluminum-free hydrogen defects. The results indicate that aluminous ppv may be a host for primordial water in D'' as suggested by recent geochemical measurements, and the presence of Al-bearing hydrous ppv reduces the adiabatic bulk sound velocity contrast across the brg-ppv phase transition, and increases the radial velocity gradient. Together, these two projects indicate that ppv may be a host for primordial water in the D'' region and suggest that the state of hydration of the lowermost mantle may potentially be inferred through correlated regions of low impedance contrast and high velocity gradients. In the third study I describe a computer program, GEOST, designed to help researchers re- fine accurate equations of state (EOS) from experimental measurements of pressure, volume, and temperature.