Flow and electrical anisotropy in the upper mantle: Finite-element models constraints on the effects of olivine crystal preferred orientation and microstructure

Abstract

Long-period magnetotelluric (MT) data shows that electrical conductivity in the upper mantle is highly anisotropic. Agreement between high electrical conductivity directions and seismic anisotropy fast directions suggests that anisotropic diffusion of hydrogen along oriented olivine crystals controls the anisotropy of electrical conductivity in the upper mantle, since both seismic waves and hydrogen diffusion are faster along the [1 0 0] axis of olivine. Thus, MT electrical anisotropy data, like seismic anisotropy, may be used to map flow patterns in the upper mantle. However, observed electrical anisotropies are significantly higher than seismic ones. To quantify the influence of strain-induced crystal preferred orientations of olivine on upper mantle bulk electrical conductivities, we calculate the macroscopic electrical conductivity anisotropy of a series of naturally and experimentally deformed peridotites using an anisotropic finite-element model. These models, which fully take into account the microstructure: crystal and shape preferred orientations, based on orientation maps obtained by indexation of electron back-scattered diffraction (EBSD) patterns, show macroscopic electrical anisotropy factors ranging from 3 to 16. The intensity of electrical anisotropy depends to first order on the intensity of the olivine crystal preferred orientations, but the relation saturates for strong crystal preferred orientations. In addition, the spatial distribution of the various crystal orientations may significantly enhance influence the anisotropy in strongly textured mantle rocks. The strongest anisotropy factors (>10) occur in mantle rocks in which deformation by dislocation creep has produced not only crystal but also strong shape preferred orientations, even if the latter is masked by recrystallization. Higher anisotropy factors (>100) observed in a few MT experiments imply however an additional, as yet unknown, mechanism controlling electrical conduction at asthenospheric depths in these regions.