Electrical Transport in Mantle and Crustal Materials

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NSF EAR 0073987 PI: James A. Tyburczy, Arizona State University Magnetotellurics and geomagnetic depth sounding have revealed zones of anomalously high electrical conductivity in a variety of crustal and mantle environments. Crustal and shallow mantle high conductivity zones are observed beneath continental collision zones, mid ocean ridges and the ocean floor, and continental shields, among other areas, and mantle transition zone high-conductivity zones are generally observed at depths of 300-400 km. A variety of explanations for these high conductivity features have been invoked including the presence of partial melt, hydrogen in minerals, aqueous fluids, and the presence of interconnected or oriented metals or other highly conducting minerals (with different explanations for different areas). In order to properly invert and interpret magnetotelluric response functions, the electrical response of Earth materials in the low frequency range must be determined. This proposal outlines an experimental effort to determine the electrical conductivity and the complex electrical impedance of relevant geological materials at elevated temperatures and pressures over the frequency range 0.0001 to 100,000 Hz. In particular, 1) the influence of melt composition and texture at low melt fraction on the bulk electrical properties of partially molten systems and 2) the influence of hydrogen in hydrous minerals (amphibole, serpentine) and nominally anhydrous minerals (olivine and pyroxene) on electrical properties will be examined. The electrical properties of texturally equilibrated partially molten olivine-basalt systems will be studied at one bar total pressure and at elevated temperatures and pressures (up to 2000 degrees C and 20 GPa) in a multiple anvil device. The electrical properties of hydrous minerals and hydrogen-containing nominally anhydrous minerals will be examined at temperatures up to 1200 degrees C and .8 - 1 GPa in an internally heated device under appropriate water fugacity conditions. The results will be interpreted in terms of equivalent electrical circuits that can be related to models of ionic processes. The results of this study will be important for the interpretation and modeling of magnetotelluric data and will also shed new light on physical transport processes in a variety of geological environments. The ultimate benefits will be more detailed understanding of the temperature profile and physical state of matter at depth in the Earth's interior.