Comparison of continental and oceanic mantle electrical conductivity: Is the Archean lithosphere dry?

Electrical conductivity profiles derived from magnetotelluric and geomagnetic sounding methods provide a means of constraining upper mantle properties that is complementary to seismic studies. We analyze conductivity profiles for an Archean craton and an oceanic setting, in conjunction with independent constraints on mantle geotherms, to estimate the water content of the upper mantle in these very different geologic environments. Results from this analysis indicate that the Archean lithosphere contains less water than the oceanic mantle in the depth range between ∼150 and ∼250 km. Below ∼250 km these oceanic and continental environments show similar conductivities, suggesting that the water content of the mantle does not vary significantly between ∼250 km and the 410‐km discontinuity. These observations indicate that the Archean lithosphere may be stabilized against convective instabilities partly because it has a high viscosity associated with a dry composition.