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 (VpVp) structures. The geothermal profiles were determined by using the electrical conductivity and seismic VpVp 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 VpVp structures in the shallower part.