Résumé
Summary
Long-period magnetotelluric (MT) data have indicated that electrical conductivity in the upper mantle is highly anisotropic. Rates and anisotropies for self-diffusion of hydrogen in single crystals of mantle minerals are related to electrical resistivity by the Nernst-Einstein relationship. Assuming that the dominant mechanism for electrical conduction in the mantle is hydrogen diffusion, the electrical anisotropy of a peridotite should be controlled by its mineral composition and by the lattice-preferred orientation (LPO) of its constitutive minerals. Macroscopic electrical anisotropies arising from diffusion of hydrogen in upper mantle rocks displaying strain-induced LPO of olivine, enstatite and diopside are calculated using resistor networks in which each resistor has a statistical probability of representing a mineral grain with a particular misorientation relative to the olivine [100] maximum density direction. The orientations of the grains are defined by angular distribution functions describing LPO (1) generated by viscoplastic self-consistent modelling at a range of shear strains and (2) measured in a naturally deformed peridotite. The naturally deformed peridotite displays a strong LPO, but the predicted mean electrical anisotropy factor is less than 3. Geophysical data indicate higher electrical anisotropies for the mantle. This suggests that grain boundary processes that are controlled by shape-preferred orientation of crystals and/or macroscopic heterogeneities further enhance the electrical anisotropy of the mantle. Ambiguities in the conduction mechanism highlight the need for direct laboratory measurements of ionic conductivities in mantle assemblages that can be compared with those calculated from the Nernst-Einstein equation.