Electrical Conductivity of H2O-Rich Silicate Melt: Implications for Subduction Zone Magnetotelluric Anomalies

Abstract

Magnetotelluric surveys reveal high conductivity anomalies in the deep Earth, which can be attributed to the presence of silicate melt, aqueous fluid, or supercritical fluid with intermediate water content. Electrical conductivity data of all of these liquids are needed to interpret magnetotelluric results and place constraints on the composition and volume fraction of the liquid phase. Electrical conductivity experiments to date are limited to silicate melt with H₂O < 12 wt% or aqueous fluid with H₂O > 92 wt%, leaving a vast gap with regard to H₂O content. This study reports the electrical conductivity of two K₂O-Al₂O₃-SiO₂ melts with H₂O content up to 35 wt% at 1.5 GPa and 900–1500°C. We find that the strong increase of electrical conductivity with H₂O content becomes more gradual after H₂O reaches 10 wt%. However, electrical conductivity at 1100°C can approach as high as 40 S/m at 35 wt% H₂O, in contrast with electrical conductivity of <10 S/m for most silicate melts with several wt% H₂O. The variation of electrical conductivity with H₂O correlates with the variation of viscosity and diffusivity, which is rooted in the modification of melt structure and dynamics by H₂O. Application of the electrical conductivity data to interpretation of the magnetotelluric results in the Cascadia subduction zone reveals that the presence of 3 vol% of the extremely water-rich melt could account for the 0.5 S/m electrical anomaly detected at subarc depth on top of the subducted slab.