Electrical Conductivity of Carbonated Hydrous Basaltic Melt: Implications for the Conductivity Anomaly Beneath the Ocean Floors

We measured the electrical conductivity of CO2 and H2O-bearing basaltic melts up to 1750 K at 2 GPa, corresponding to pressure around the lithosphere-asthenosphere boundary. The electrical conductivity of the dry and hydrous samples is comparable to those reported by previous studies on the Fe-free basaltic melt. The substantial CO2 can limit the water solubility in basaltic melt at 2 GPa. Both CO2 and H2O, which cannot completely dissolve in the melt, coexist as fluid phases, resulting in reduced electrical conductivity of the basaltic melt, which has a lower water content relative to the amount of volatile components in the bulk starting system. The activation enthalpy of basaltic melt is markedly higher than those of more evolved silicate melts, especially on the H2O-poor condition, due to the more enriched alkaline earth elements. The present results suggest that an overall melt fraction of 0.1–5.3 vol% is needed to account for the high electrical conductivity anomalies (10−1.3 to 10−0.3 S/m) beneath the oceanic plate near the East Pacific Rise and Cocos plate. However, for those regions where the electrical conductivity is extremely high (≥10−0.3 S/m), more than 6 wt% H2O is expected to incorporate to maintain a melt fraction that will not trigger mechanical instability. In turn, it requires a low CO2 budget or degree of carbonation within these regions.