Lateral variation in upper mantle viscosity: role of water

Differences in the viscosity of the earth's upper mantle beneath the western US (similar to10(18)-10(19) Pa s) and global average values based on glacial isostatic adjustment and other data ( similar to 10(20) 10(21) Pa s) are generally ascribed to differences in temperature. We compile geochemical data on the water contents of western US lavas and mantle xenoliths, compare these data to water solubility in olivine, and calculate the corresponding effective viscosity of olivine, the major constituent of the upper mantle, using a power law creep theological model. These data and calculations suggest that the low viscosities of the western US upper mantle reflect the combined effect of high water concentration and elevated temperature. The high water content of the western US upper mantle may reflect the long history of Farallon plate subduction, including flat slab subduction, which effectively advected water as far inland as the Colorado Plateau, hydrating and weakening the upper mantle. (C) 2004 Elsevier B.V All rights reserved.