It is well known that water (as a source of hydrogen) affects the physical and chemical properties of minerals--for example, plastic deformation and melting temperature--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). Geophysical observations such as electrical conductivity are considered to be sensitive to water content, 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 suggests that the typical oceanic asthenosphere contains approximately 10(-2) wt% water, whereas the water content in the continental upper mantle is less than approximately 10(-3) wt%.

Electrical conductivity of most minerals is sensitive to hydrogen (water) content, temperature, major element chemistry and oxygen fugacity. The influence of these parameters on electrical conductivity of major minerals has been characterized for most of the lower crust, upper mantle and transition zone minerals. When the results of properly executed experimental studies are selected, the main features of electrical conductivity in minerals can be interpreted by the physical models of impurity-assisted conduction involving ferric iron and hydrogen-related defects. Systematic trends in hydrogen-related conductivity are found among different types of hydrogen-bearing minerals that are likely caused by the difference in the mobility of hydrogen. A comparison of experimental results with geophysically inferred conductivity shows: (1) Electrical conductivity of the continental lower crust can be explained by a combination of high temperature, high (ferric) iron content presumably associated with dehydration.(2) Electrical conductivity of the asthenosphere can be explained by a modest amount of water (~10 -2 wt% in most regions, less than 10 -3 wt% in the central/western Pacific). ( 3) Electrical conductivity of the transition zone requires a higher water content (~10 -1 wt% in most regions, ~10 -3 wt% in the southern European transition zone, ~1 wt% in the East Asian transition zone). The majority of observations including those on the lower crust and the asthenosphere can be interpreted without partial melting or any fluids. However, experimental studies on electrical conductivity of lower mantle minerals are incomplete and it is not known if hydrogen enhances the conductivity of lower mantle minerals or not. Some discussions are also presented on the electrical conductivity in other planetary bodies including the Moon and Mars.

We investigated the electrical conductivity of amphibole-bearing rocks under the conditions of the middle to lower crust. Alternating current measurements were performed in the frequency range of 10-10 6 Hz in a cubic-anvil high-pressure apparatus at 0.5-1.0 GPa and 373-873 K. The electrical conductivity of these rocks is weakly temperature dependent below *800 K with modest anisotropy and relatively low conductivity (*5 9 10 -3 S/m at *750 K with the activation enthalpy of 64-67 kJ/mol). However, the electrical conductivity starts to increase with temperature more rapidly above *800 K (activation enthalpy of 320-380 kJ/mol). The infrared spectroscopy observations indicate that dehydration occurs in this high temperature regime. The observed high activation enthalpy and the reproducibility suggest that the enhanced conductivity is not due to the direct effect caused by the generation of conductive fluids. Dehydration of amphibole is associated with the oxidation of iron (from ferrous to ferric), and we suggest that the increased conductivity associated with dehydration is caused by oxidation. This effect may explain high electrical conductivity observed in some regions of the continental crust.

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[1] Presence of graphite is one of the mechanisms to explain enhanced electrical conductivity. Because the conductivity of graphite is highly anisotropic and the connectivity of graphite depends strongly on the geometry of the crystals, the key issue is the geometry of graphite in a rock including their crystallographic orientation and the shape of graphite crystals. We explored the role of graphite on electrical conductivity in olivine-rich aggregates. To obtain well-defined results, we conducted an experimental study at high pressure and temperature conditions. Olivine aggregates containing diamonds were annealed to transform diamond to graphite with nearly equilibrium morphology. Graphite formed by the transformation from diamond has thin disk-shape morphology, the plane being the highly conductive (0001) plane. When the concentration of graphite exceeds the percolation threshold (~1 wt%), electrical conductivity is significantly enhanced. Some of the observed high conductivity regions may represent regions of high concentration of graphite.

We present a new global electrical conductivity model of Earth's mantle. The model was derived by using a novel methodology, which is based on inverting satellite magnetic field measurements from different sources simultaneously. Specifically, we estimated responses of magnetospheric origin and ocean tidal magnetic signals from the most recent Swarm and CHAMP data. The challenging task of properly accounting for the ocean effect in the data was addressed through full three-dimensional solution of Maxwell's equations. We show that simultaneous inversion of magnetospheric and tidal magnetic signals results in a model with much improved resolution. Comparison with laboratory-based conductivity profiles shows that obtained models are compatible with a pyrolytic composition and a water content of 0.01 wt % and 0.1 wt % in the upper mantle and transition zone, respectively.

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Regions with high electrical conductivities in subduction zones have attracted a great deal of attention. Determining the exact origin of these anomalies could provide critical information about the water storage and cycling processes during subduction. Antigorite is the most important hydrous mineral within deep subduction zones. The dehydration of antigorite is believed to cause high-conductivity anomalies. To date, the effects of dehydration on the electrical conductivity of antigorite remain poorly understood. Here, we report new measurements of the electrical conductivity of both natural and hot-pressed antigorite at pressures of 4 and 3 GPa, respectively, and at temperatures reaching 1073 K. We observed significantly enhanced conductivities when the antigorite was heated to temperatures beyond its thermodynamic stability field. Sharp increases in the electrical conductivity occurred at approximately 848 and 898 K following the decomposition of antigorite to forsterite, enstatite and aqueous fluids. High electrical conductivities reaching 1 S/m can be explained by the presence of an interconnected network of conductive aqueous fluids. Based on these results for the electrical conductivity of antigorite, we conclude that high-conductivity regions associated with subduction zones can be attributed to dehydration-induced fluids and the formation of interconnected networks of aqueous fluids during the dehydration of antigorite.