The Electrical Structure of the Central Pacific Upper Mantle Constrained by the NoMelt Experiment

The NoMelt Experiment imaged the mantle beneath 70Ma Pacific seafloor with the aim of understanding the structure of the lithosphere and the transition from lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analysed using 2-D regularized isotropic and anisotropic inverse modeling. The preferred electrical model for the region contains a 70-80 km thick resistive (>10³ Ωm) lithosphere with a conductive (~50 Ωm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (≤10 Ωm) layer under the 70-80 km thick resistive lithosphere that would be indicative of partial melt. However, the model does contain a more conductive lower lithosphere and asthenosphere than predicted by dry olivine models. We averaged the model into a 1-D profile and compared the result with laboratory data, and models from other locations. Water, in the form of hydrogen dissolved in olivine, is the most likely mechanism responsible for the enhanced conductivity. Predicted bulk mantle water contents are low (20-50ppm) at 100km depth and increase slightly into the asthenosphere (40-500ppm), with the range of values dependent on the conductivity model chosen. The predicted water contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid-ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result.