Water content and geotherm in the upper mantle above the stagnant slab: Interpretation of electrical conductivity and seismic P-wave velocity models

Geotherm and water content profiles in the upper mantle above the stagnant slab of the Pacific back-arc were estimated from the electrical conductivity and seismic P-wave velocity (V_p) structures. The geothermal profiles were determined by using the electrical conductivity and seismic V_p structures, which, assuming a dry hartzburgite or a dry pyrolite composition, are designated as electrical and seismic geotherms, respectively. In a deeper part of the upper mantle, neither the dry pyrolite nor the dry harzburgite condition provides consistent electrical and seismic geotherms. This discrepancy can be explained by allowing for a small amount of water (500-1000 ppm H/Si) with the seismic geotherm. In a shallower part of the upper mantle, the electrical and seismic geotherms are consistent with each other within 1500-1700°C under the dry harzbutgite condition, whereas they are inconsistent by more than 100°C under the dry pyrolite condition. Alternatively, the wet pyrolite condition applied to the deeper part of the upper mantle also satisfies the electical conductivity and seismic V_p structures in the shallower part.