Water in the mantle: Results from electrical conductivity beneath the French Alps

[1] A deep magnetotelluric sounding in the French Alps provided a vertical electrical conductivity profile between similar to200-1000 km. Two prominent features are observed. First, the conductivity in the depth range 400-800 km is smaller than the conductivity of a pyrolite mantle obtained from laboratory results for a normal geotherm. Second, the data do not require the conductivity to change throughout the transition zone (410-660 km). In this part of the mantle, a temperature of 350-450 C less than normal explains the magnetotelluric conductivity profile. At 200-400 km, our model favors a cold mantle with 1000-1500 ppm of water dissolved in olivine. If correct, this model suggests that the subducted slab is dehydrated before reaching the transition zone.