Estimating the electrical conductivity of the melt phase of a partially molten asthenosphere from seafloor magnetotelluric sounding data

Seafloor magnetotelluric soundings provide models of electrical conductivity distributions in the Earth’s oceanic mantle. By inverting the sounding results, a high-conductivity (typically 0.01–0.1 S/m) layer is often obtained in the oceanic upper mantle, which is referred to as the electrical asthenosphere. The obtained high conductivity is then interpreted in terms of the amount of additional conduction that accounts for the conductivity value based on a physical model of the asthenosphere. There are two major candidates that are considered as the cause of the asthenosphere: the effect of water and partial melting. In this paper, we consider the partial melting hypothesis. We propose a method to estimate the electrical conductivity of the melt phase that is responsible for high conductivity by assuming the melt fraction distribution in the asthenosphere. We applied this method to one-dimensional conductivity profiles recently obtained from three regions of different seafloor age, and the results indicate that the proposed approach, combined with other approaches such as seismology and mineral physics, will provide useful information in testing a partial melting hypothesis for the cause of the asthenosphere.