MR32A-10
Experimental Investigation of the Effect of Shear on the Electrical Properties of Polycrystalline Olivine and the Role of Grain Boundaries

Wednesday, 16 December 2015: 12:08
301 (Moscone South)
Anne Pommier, UC San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, Stephen J Mackwell, Lunar & Planetary Institute, Houston, TX, United States, Florian Heidelbach, University of Bayreuth, Bayreuth, Germany, Miki Tasaka, Kanazawa University, Earth Science Course, School of Natural System, Kanazawa, Japan, David L Kohlstedt, University of Minnesota Twin Cities, Minneapolis, MN, United States, Kurt D Leinenweber, Arizona State University, Tempe, AZ, United States and Lars N Hansen, University of Oxford, Oxford, United Kingdom
Abstract:
Sheared rocks are thought to explain electrical anomalies in a deformed uppermost mantle as their high electrical conductivity and anisotropy reproduce electromagnetic data. Although it has been suggested that melt is required to reproduce asthenospheric electrical anisotropy, the electrical properties of the sheared polycrystalline matrix and its contribution to bulk electrical anisotropy at upper mantle conditions are not fully understood and require further investigation. We present a systematic experimental investigation of the effect of shear deformation on the electrical conductivity of polycrystalline olivine (Fo90). Starting samples were sheared at 1200°C and 0.3GPa in a gas-medium apparatus to a strain of up to 7.3. Electrical conductivity and anisotropy of the samples were measured at 3GPa over the temperature range ~700-1300°C in a multi-anvil apparatus using a two-electrode technique.

We observe that shear deformation increases electrical conductivity and that conductivity is highest in the direction of shear, with the sample deformed to the highest strain being the most conductive. Electrical anisotropy is higher at low temperature than at high temperature. For a similar shear strain, comparison with previous work on melt-bearing sheared samples shows that melt increases bulk conductivity but that melt-free samples deformed to high strain are more conductive than melt-bearing samples deformed to low strain. The role of grain boundaries on conduction mechanisms is investigated based on textural analyses and comparison with electrical data on olivine single crystals and undeformed polycrystalline olivine. Application of our results to field data indicates that, even in melt-bearing contexts, electrical anomalies in the asthenosphere can be solely explained by the electrical properties of the sheared solid matrix.

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