Electrical Conductivity of Hydrous Ultramafic Melts With Implications for Origin of Low Velocity Layer Atop of the 410 km Seismic Discontinuity

Low-velocity layers (LVLs) above the 410-km discontinuity are commonly attributed to partial melt generated by dehydration melting when hydrous mantle transition-zone material rises into the upper mantle, where water solubility in nominally anhydrous minerals decreases. Interpreting coexisting high-conductivity anomalies requires constraints on the intrinsic conductivity of the melt phase at pressures just above the transition zone. We measured electrical conductivity of hydrous ultramafic melts representative of incipient melts, ranging from 6.3 to 18.6 wt% H2O at 13 GPa using impedance spectroscopy in a Kawai-type multi-anvil apparatus. Hydrous ultramafic melts are extremely conductive, and conductivity increases systematically with H2O content to values comparable to alkali-carbonate melts. The high conductivity implies that even small fractions of interconnected hydrous melt can dominate bulk mantle conductivity. Combining our measurements with geophysical conductance estimates indicates that <1 vol% melt can produce conductive layers thicker than 10 km, consistent with seismological constraints on LVL structure.