Hydrous Silicate Melts in the Earth's Asthenosphere: Evidence from Electrical Conductivity Measurements

The Earth’s asthenosphere is generally associated with low seismic wave velocity and high electrical conductivity. The electrical conductivity anomalies observed from magnetotelluric studies have been attributed to the hydration of mantle minerals, traces of carbonatite melt, or hydrous melts. We report the electrical conductivity of both H2O-bearing (0-6 wt% H2O) and CO2-bearing (0.5 wt% CO2) basaltic melts at 2 GPa and 1473-1923 K, measured using impedance spectroscopy in a piston-cylinder apparatus. CO2 hardly affects conductivity. However, the effect of water on the conductivity of basaltic melt is several times larger than inferred from previous measurements on silicate melts of different composition. The conductivity of basaltic melts with more than 6 wt.% of water approaches the values for carbonatites. Our data are reproduced within a factor of 1.1 by the equation logσ = 2.172 - (860.82 - 204.46 w0.5)/(T - 1146.8), where σ is electrical conductivity in S/m, T is temperature in K, and w is H2O content in wt%. We show that in a mantle with 125 ppm water and for a bulk water partition coefficient of 0.01 between minerals and melt, 2.2% of melt will account for the observed electrical conductivity in the asthenosphere. However, for plausible higher water contents, stronger water partitioning into the melt or melt segregation in tube- or sheet-like structures, even less than 1% of hydrous melt may be sufficient to produce the observed conductivity.