1. Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China
2. Institute of Geology, China Earthquake Administration, Beijing 100029, China
3. Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA
4. Department of Mathematics, University of Connecticut, Storr, Connecticut 06269, USA

Correspondence to: Shun-ichiro Karato³ Correspondence and requests for materials should be addressed to S.-i.K. (Email: shun-ichiro.karato@yale.edu).

It is well known that water (as a source of hydrogen) affects the physical and chemical properties of minerals—for example, plastic deformation^{1, 2, 3} and melting temperature⁴—and accordingly plays an important role in the dynamics and geochemical evolution of the Earth. Estimating the water content of the Earth's mantle by direct sampling provides only a limited data set from shallow regions (<200 km depth)⁵. Geophysical observations such as electrical conductivity are considered to be sensitive to water content⁶, but there has been no experimental study to determine the effect of water on the electrical conductivity of olivine, the most abundant mineral in the Earth's mantle. Here we report a laboratory study of the dependence of the electrical conductivity of olivine aggregates on water content at high temperature and pressure. The electrical conductivity of synthetic polycrystalline olivine was determined from a.c. impedance measurements at a pressure of 4 GPa for a temperature range of 873–1,273 K for water contents of 0.01–0.08 wt%. The results show that the electrical conductivity is strongly dependent on water content but depends only modestly on temperature. The water content dependence of conductivity is best explained by a model in which electrical conduction is due to the motion of free protons. A comparison of the laboratory data with geophysical observations^{7, 8, 9, 10} suggests that the typical oceanic asthenosphere contains 10^-2 wt% water, whereas the water content in the continental upper mantle is less than 10^-3 wt%.

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