Variations in the electrical conductivity of the upper mantle beneath North America and the Pacific Ocean

Variations in the electrical conductivity of the mantle beneath Carty Lake in the Canadian Shield, Tucson in the southwest United States, and Honolulu and Midway in the north central Pacific were determined through the invers ion of long-period magnetotelluric and geomagnetic depth sounding data. Inv ersion of computed response functions is carried out using a minimum struct ure, regularized approach. The upper mantle beneath Carty Lake is approxima tely an order of magnitude more resistive than the upper mantle beneath Tuc son and nearly 1.5 orders of magnitude more resistive than Honolulu and Mid way Island. Inversions were also carried out where the minimum structure co nstraint was removed at known upper mantle discontinuities. These models sh ow a jump in conductivity of similar to 1.5 orders of magnitude across the 660 km discontinuity, a result that is consistent with laboratory experimen ts on realistic mantle assemblages. Mantle conductivity profiles at Carty L ake are significantly more resistive than those at Tucson, Honolulu, and Mi dway to depths of similar to 300-400 km. These observations likely reflect differing thermal states, the presence (or absence) of partial melt and vol atiles, and may also be related to chemical differences between depleted an d undepleted upper mantle. The observed conductivity variations may be inte rpreted as lateral variations in temperature, partial melt, and/or dissolve d hydrogen in olivine.