Electrical conductivity of the serpentinised mantle and fluid flow in subduction zones

Abstract

In the mantle wedge of subduction zones, electromagnetic profiles reveal high electrical-conductivity bodies. In hot areas (> 700 °C), water released by dehydration of the slab induces melting of the mantle under volcanic arcs that can explain the observed high conductivities. In the cold (< 700 °C) melt-free fore-arc mantle wedge, fluid water migrates and causes serpentinisation detected as low seismic wave velocities in the mantle wedge. High conductivities in the serpentinized wedge may picture serpentinisation or instantaneous fluid flow depending on serpentine electrical conductivity. We measured the electrical conductivities of three natural serpentine samples from subduction zone context using complex impedance measurements, and find they have low electrical conductivities (< 10 − 4 S.m − 1 ) similar to those of dry mantle minerals below 700 °C. Because of the negligible conductivity of serpentine, electrical conductivity in the hydrated mantle wedge is only sensitive to the fluid content and salinity, not to serpentinisation. A small fraction ( ca. 1% in volume) of connective high-salinity fluids accounts for the highest observed conductivities. The low-salinity fluids (≤ 0.1 m) released by slab dehydration evolve towards high-salinity (≥ 1 m) fluids during progressive serpentinisation of the mantle wedge. These fluids can mix with arc magmas at depths and account for high chlorine/water ratios in arc lavas.