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Electrical conductivity anisotropy of dry and hydrous olivine at 8GPa

Abstract : The effects of dissolved HO on the electrical conductivity and its anisotropy in olivine (Fo) at 8GPa were investigated by complex impedance spectroscopy. At nominally anhydrous conditions, conduction along [100] and [001] is slightly higher than along [010] in contrast to observations made at lower pressures in earlier studies. Increasing HO content increases conductivities but activation energies are lower and HO concentration dependent. The use of polarized FTIR spectroscopy to determine HO concentrations reveals a weaker than expected effect that water has on olivine conductivity and distinguishes our results from earlier studies based on analyses using non-polarized infrared spectroscopy. We show that at HO concentrations of a few hundred wt ppm or less, that the dominant conduction mechanism at mantle temperatures continues via small polarons, such as that observed for anhydrous olivine. Our results also suggest that at depths greater than 200km, the presence of HO may not be necessary to explain regions in the upper mantle where both electrical and seismic anisotropy are observed. This can be explained by differences in the pressure dependence of the activation energy for conduction along each of the three crystallographic axes. However, while electrical anisotropy of anhydrous olivine remains weak at 8GPa, it is nevertheless enhanced by elevated concentrations (> several hundred wt ppm) of dissolved HO. At these conditions dominated by proton hopping, conductivity along [010] is highest, approximately an order of magnitude greater than along [100]. Additionally, at 1000wt ppm and 1500°C, an isotropic conductivity derived from the data is about 1 order of magnitude higher than that for nominally anhydrous olivine. Thus, in regions of the mantle characterized by anomalously high conductivities and both electrical and seismic anisotropy, significant amounts of dissolved hydrogen can be expected.
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Brent T. Poe, Claudia Romano, Fabrizio Nestola, Joseph R. Smyth. Electrical conductivity anisotropy of dry and hydrous olivine at 8GPa. Physics of the Earth and Planetary Interiors, Elsevier, 2010, 181 (3-4), pp.103. ⟨10.1016/j.pepi.2010.05.003⟩. ⟨hal-00660106⟩

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