In this project, we will use geophysically inferred electrical conductivity distribution of the Earth's upper mantle to infer the physical and chemical conditions of the upper mantle. The distribution of water in the mantle reflects the processes by which the upper mantle has evolved. Unlike the oceanic upper mantle, evolution of the continental upper mantle is poorly understood. Consequently, we will focus our attention to the distribution of water in the continental upper mantle from which we infer the evolution of continents. Based on the series of experimental (and theoretical) studies, Karato and his co-workers have shown that water (hydrogen) enhances electrical conductivity of upper mantle minerals dramatically. These experimental studies established the functional relationship between electrical conductivity, water content, temperature, oxygen fugacity etc. These relationships are well established and show that the influence of water is much stronger than other parameters. 'Other parameters' that affect electrical conductivity modestly (major element composition, temperature, oxygen fugacity etc.) can be inferred from other observations (temperature and major element composition from mantle samples, for example). Consequently, we can infer the water distribution from the inferred electrical conductivity distribution through the integration of these observations.
Dr. Kate Selway has extensive experience in analyzing the MT data to construct three-dimensional conductivity distributions from the observations of electromagnetic induction. She has recently obtained new data set from Tanzania, but we will also use the data from other regions including South Africa and North America. Given the knowledge of 'other factors' listed above, we will establish the methodology to map out water distribution by comparison to MT data. Both forward and inverse modeling approaches will be used in this study. For this study, we will have a better understanding of water distribution in the continental upper mantle that will shed important insights into how continents might have grown.
The goal of this project is to explore the magnetotellurics method of remote sensing of Earth’s and planetary interiors. In contrast to a more widely used seismological method, this method provides us with the unique data on the distribution of electrical conductivity. We conducted the following studies: (1) Determined the water distribution in the continental lithosphere in Tanzania. (2) Developed a model to interpret the thermal structure and water content distribution from the combination of seismological and MT observations. (3) Re-evaluated the temperature-water content estimate of the lunar interior (4) Conducted laboratory studies on electrical conductivity in olivine For the Tanzania, we inferred that the water content is heterogeneous, and interpreted these results in terms of the interaction of "original" dry lithosphere with "wet" plume. In addition to MT results (electrical conductivity), seismological observations such as the mid-lithosphere discontinuity (MLD) can be used to infer temperature and water content. We reviewed various geophysical, geochemical and geological observations to evaluate various models for the MLD including compositional layering, layered anisotropy, partial melt and sub-solidus model. We developed forward models of electromagnetic induction of the Moon to help infer the water distribution in the lunar mantle. We conducted a series of laboratory study to determine the influence of hydrogen on the electrical conductivity in olivine including the pressure effects, oxygen fugacity effect and the influence of mechanism change with temperature.
