This project is a laboratory investigation of the electrical conductivity of deformed partially molten rocks to help understand the mechanisms by which plate tectonics operates. The motion of rigid plates that comprise Earth's lithosphere relative to the underlying convecting mantle is thought to promote the formation of sheared rock and/or partially molten zones beneath the lithosphere. This deformation has been suggested to explain geophysical anomalies detected at depths below the lithosphere, such as zones of high electrical conductivity and low seismic velocity. Partial melt is known to redistribute under shear into a texture in which melt is focused into bands with a strong preferred orientation in the direction of the shearing. The proposed experiments are designed to determine the manner in which the electrical conductivity of deformed partially molten mantle rocks varies with shear and orientation. The results will provide information that helps to understand the mechanisms of coupling of lithospheric plates with the underlying mantle and the processes that govern plate tectonics.
This work will involve multidisciplinary investigation of the electrical conductivity signature, in terms of both magnitude and anisotropy relative to the shear direction, of partially molten mantle rocks deformed to high shear strain combined with correlation to field electrical measurements. Melt spatial arrangement in partially molten materials in the upper mantle can result in highly anisotropic geophysical signatures when the materials are deformed to high shear strains. The evolution of melt structure in shear experiments, promoting formation of melt-rich bands, is expected to have significant effects on both bulk conductivity and electrical anisotropy, but such effects have not been fully experimentally investigated. Deformed partially molten samples in the olivine-melt system will be synthesized and electrical conductivity measurements will be performed on prepared sections of these samples, oriented with respect to the maximum applied shear stress, under sub- and super-solidus conditions. Melt compositions will include anhydrous, hydrous and carbonate-bearing melts. The effects of temperature, compaction length and total shear strain will be investigated. Samples will be characterized in 2-D with optical and electron microscopy and in 3-D by synchrotron x-ray tomography. Electrical measurements will be combined with the textural characterization of the samples to develop geometry-based conductivity models as a function of physical and chemical parameters. We will apply our conductivity models to interpret field electrical data. These results will help constrain the nature of and processes in the asthenosphere.
