Electrical conductivity of magnetite-bearing antigorite upon dehydration and implications for high conductivity anomalies in forearc mantle wedges

The electrical conductivities of antigorite with magnetite fractions of 0, 5, 10, 20, 25 and 30 vol% were measured before and after dehydration at 2.0 GPa and temperatures of 573–1073 K, and the oxygen fugacity of the aggregates after dehydration was estimated. Our results indicate that before dehydration, the electrical conductivity was significantly enhanced when the connectivity threshold for magnetite exceeded 21 vol%. The interaction between the magnetite and the fluid liberated the magnetite from the antigorite, resulting in a decrease in the magnetite fraction. The iron in the magnetite was dissolved in the liquid, increasing the oxygen fugacity. The oxygen fugacity of the aggregates increased with increasing magnetite content, and the oxygen fugacity of the oxidizing fluid released by the decomposition of the serpentine was between QFM + 1.5 and QFM + 2.8 (QFM = Quartz-Fayalite-Magnetite oxygen buffer), which caused an increase in the electrical conductivity of the aggregates of up to 0.1 S/m. Based on geophysical observations, we conclude that ∼2 vol% of oxidizing fluid with QFM + 2.8 can generate a high conductivity anomaly of ∼10–0.5 S/m in a warm subduction zone, such as the Southwest Japan and Cascadia subduction zones.