Magnetotellurics show anomalously high electrical conductivity in the young oceanic asthenosphere, which is higher than dry olivine conductivity by 1~2 orders of magnitude. Previous workers attempted to attribute this anomaly free proton conduction of olivine, (e.g. Karato, 1990; Wang et al., 2006), however, such hypothesis is denied because it requires unrealistically high volatile contents (Katsura et al., 2017).
The Mg-diffusion controlled ionic conduction mechanism in olivine, in contrast, has received minimal attention. Incorporation of water produces additional Mg vacancies and thus may enhance Mg diffusion and ionic conductivity. Therefore, we hypothesis that although the contribution of ionic conduction to dry olivine is relatively small at upper mantle temperature conditions, it may become important by considering the small amount of water in the asthenosphere.
To evaluate this hypothesis, we performed Mg self-diffusion experiments and direct conductivity measurements in olivine as functions of pressure, temperature, and water content. The experimental results show that both Mg diffusivity and ionic conductivity are dramatically enhanced by water incorporation. They have water content exponents of 1.2±0.2 and 1.3±0.2, respectively. The conductivity anomaly in the asthenosphere is well explained by Mg diffusion controlled ionic conductivity in olivine enhanced by ~60 wt. ppm water.
Electrical conductivity of olivine with various mechanisms. The pink rectangular with the black frame: conditions of the high conductivity layer in the oceanic asthenosphere. Cyan: small polaron; brown: ionic conductivity under dry conditions; blue: ionic conductivity with 60 wt. ppm H2O; gray: free proton conduction with 60 wt. ppm H2O, red: the sum of small polaron and ionic conductivity with 60 wt .ppm H2O, which interpret the high conductivity layer.
Fei, H., Druzhbin, D., Katsura, T., The effect of water on ionic conductivity in olivine. Journal of Geophysical Research: Solid Earth 125, e2019JB019313, 2020.
Fei, H., Koizumi, S., Sakamoto, N., Hashiguchi, M., Yurimoto, H., Marquardt, K., Miyajima, N., Katsura, T., Mg lattice diffusion in iron-free olivine and implications to conductivity anomaly in the oceanic asthenosphere. Earth and Planetary Science Letters 484, 204-212, 2018.