Distribution functions for anisotropic electrical resistivities due to hydrogen diffusivity in aligned peridotite and their application to the lithosphere–asthenosphere boundary

Abstract

The scaling properties of electrical anisotropy due to hydrogen diffusivity are investigated by representing the lattice-preferred-orientation (LPO) of a naturally-deformed peridotite xenolith using resistor network models. Whereas hydrogen diffusivities are approximately 40 (or 20) times faster along the a-axis of olivine single crystals than along their c-axes (or b-axes), a highly strained aggregate produces bulk electrical anisotropy factors of less than 4 (i.e. the more conductive direction can be expected to be less than 4 times more conductive than the less conductive direction) with 95% confidence. The bulk resistivities in both directions lie between those of the a-axes and the c-axes of olivine single crystals. Resistivity-depth profiles derived by assuming self-diffusion of hydrogen in a highly strained mantle indicate that for a realistic geotherm, electrical anisotropy arising due to hydrogen diffusivity within the thermal lithosphere is unlikely to be resolvable using magnetotelluric (MT) sounding, because the increase in resistivity with depth in the more resistive direction obscures the anisotropy. This is a consequence of the inherent lack of vertical resolution – i.e. inability to resolve vertical layering – of the MT method. Therefore, electrical anisotropy due to hydrogen self-diffusion in mantle minerals aligned within the lithosphere by palaeoflow is unresolvable using MT methods. However, a resolvable signature of such electrical anisotropy should be detectable, if present, in the adiabatic region below the thermal lithosphere or thermal boundary layer (TBL), where the geotherm is shallower. Since the mechanical lid or plate is generally assumed to be thinner than the TBL, this raises the question whether any anisotropic signature detected by MT can be attributed to LPO of olivine as a consequence of plate motion. If so, what does this tell us about heat transfer and strain dissipation in the enigmatic region between the mechanical lid and the TBL?