Dehydration of chlorite explains anomalously high electrical conductivity in the mantle wedges

International audienceMantle wedge regions in subduction zone settings show anomalously high electrical conductivity (~1 S/m) that hasoften been attributed to the presence of aqueous fluids released by slab dehydration. Laboratory-based measurementsof the electrical conductivity of hydrous phases and aqueous fluids are significantly lower and cannot readilyexplain the geophysically observed anomalously high electrical conductivity. The released aqueous fluid also rehydratesthe mantle wedge and stabilizes a suite of hydrous phases, including serpentine and chlorite. In this presentstudy, we have measured the electrical conductivity of a natural chlorite at pressures and temperatures relevant forthe subduction zone setting. In our experiment, we observe two distinct conductivity enhancements when chloriteis heated to temperatures beyond its thermodynamic stability field. The initial increase in electrical conductivity to~3 × 10−3 S/m can be attributed to chlorite dehydration and the release of aqueous fluids. This is followed by aunique, subsequent enhancement of electrical conductivity of up to 7 × 10−1 S/m. This is related to the growth of aninterconnected network of a highly conductive and chemically impure magnetite mineral phase. Thus, the dehydrationof chlorite and associated processes are likely to be crucial in explaining the anomalously high electricalconductivity observed in mantle wedges. Chlorite dehydration in the mantle wedge provides an additional sourceof aqueous fluid above the slab and could also be responsible for the fixed depth (120 ± 40 km) of melting at thetop of the subducting slab beneath the subduction-related volcanic arc front