Point Defects and Transport Properties of Ferropericlase in the Lower Mantle

The knowledge of transport properties such as atomic diffusion, viscosity and electrical conductivity in the lower mantle is critical for our understanding of the evolution and dynamics of the entire Earth. Those transport properties are largely controlled by point defects, but the relative importance of various species of point defect was poorly constrained in the lower mantle. Laboratory experiments on point defect concentrations in ferropericlase ((Mg,Fe)O) demonstrate that Fe3+ and metal vacancies are the dominant defect pairs in ferropericlase at the top of the lower mantle even at water-saturated conditions. Experiments on Fe-Mg interdiffusion in ferropericlase indicate that the influence of Fe3+ in cation diffusion likely dominates at temperatures expected for the lower mantle, while the influence of both Fe3+ and I1+ is important at lower temperature environments such as near the subduction zone.;Ferropericlase inclusions encapsulated in diamonds derived from the lower mantle are unique proxies for the chemical environment in the lower mantle. Redox conditions inferred from Fe3+ concentration in ferropericlase inclusions indicate that host diamonds were precipitated from carbonatite melts near the top of the lower mantle at significantly oxidized conditions compared with the metal-saturated lower mantle.;A new method was presented to control water fugacity as an independent variable in solid-state high pressure apparatus. Results indicate that water is predominantly incorporated in olivine via two hydrogen atoms substituted into a Mg-site vacancy under the condition we explored.;Iron-rich metallic liquid interacts with silicate/oxides at the core-mantle boundary, but mechanisms of interaction between the core and the mantle have been poorly known. Experiments show that the bulk disequilibrium between solid ferropericlase and the metallic liquid causes migration of metallic liquid in ferropericlase crystals, which provides an efficient mechanism of chemical transport compared with conventional Fe-Mg interdiffusion. Iron entrainment from the core by trans-crystalline melt migration gives a plausible explanation of several geophysical and geochemical observations including the presence of low-velocity and high-density regions near the bottom of the mantle, the observed length of the day variation in the decadal time scale and isotope signatures of the core observed in some of the hot spot volcanisms.