DI24A-05
Experimental Investigation of the Electrical Anisotropy of the Lithosphere–Asthenosphere System

Tuesday, 15 December 2015: 17:00
303 (Moscone South)
Anne Pommier1, Kurt D Leinenweber2, David L Kohlstedt3, Chao Qi4, Edward Garnero5, Stephen J Mackwell6 and James A Tyburczy2, (1)UC San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, (2)Arizona State University, Tempe, AZ, United States, (3)University of Minnesota Twin Cities, Minneapolis, MN, United States, (4)University of Pennsylvania, Philadelphia, PA, United States, (5)Arizona State University, EarthScope National Office, School of Earth and Space Exploration, Tempe, AZ, United States, (6)Lunar & Planetary Institute, Houston, TX, United States
Abstract:
Electromagnetic profiles of the lithosphere–asthenosphere system show zones of high electrical conductivity and electrical anisotropy, which vary with depth. These anomalies can be explained by localized rock deformation near the lithosphere–asthenosphere boundary due to the relative motion of tectonic plates and underlying mantle. Such localization may result from a small amount of melt or water in the asthenosphere, reducing viscosity. In particular, the effect of melt on the physical properties of deformed materials at upper-mantle conditions remains poorly constrained.

Here we present electrical anisotropy measurements at high temperatures and quasi-hydrostatic pressures of about 3 GPa and temperatures up to 1300°C on previously deformed olivine aggregates and sheared partially molten rocks. For all samples, electrical conductivity is highest when parallel to the direction of shear. At temperatures >900°C, a deformed solid matrix with nearly isotropic melt distribution has an electrical anisotropy factor <5. To explain the higher electrical anisotropies observed in our experiments, we propose an experimentally based model in which layers of sheared olivine are alternated with layers of sheared olivine plus MORB or of pure melt. Conductivities are up to 100 times greater in the shear direction than perpendicular to the shear direction and reproduce stress-driven alignment of melt.

Our results reproduce both the magnitude and the anisotropy of electrical conductivity in various melt-bearing geological contexts. Field data are best fit by an electrically anisotropic asthenosphere overlain by an isotropic, high-conductivity lowermost lithosphere. The high conductivity could arise from partial melting associated with localized deformation resulting from plate motion relative to the mantle, with subsequent upward melt percolation from the asthenosphere. This interpretation appears to be consistent with seismic observations.

<< Previous Abstract | Next Abstract >>