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The electrical conductivity of Earth's mantle has recently become an interesting topic across diverse Earth science communities. Many electrical conductivity data of mantle phases have been accumulated through the development of high-pressure experiments. These data will provide information on valence states, water concentration, Fe concentration, oxygen fugacity, and the connectivity of the conductive phase in geological materials such as minerals, melts, and rocks. Although several groups have measured the electrical conductivity of mantle materials at high pressure, they have provided inconsistent results, especially with regard to the effect of water. Thus, it is timely to review the problems underlying experimental techniques. We discuss the current understanding of the effect of water on the electrical conductivity of nominally anhydrous mantle minerals, with some speculation on the form of volatile components in Earth's interior. Finally, we consider the role of water in major conductivity anomalies observed in the upper mantle and transition zone. [ABSTRACT FROM AUTHOR]

Abstract: The temperature at the 410-km discontinuity is re-evaluated by comparing the depth of the discontinuity with the olivine–wadsleyite transition pressure obtained using in situ X-ray diffraction experiments by and equation of state (EoS) of MgO by (Tange scale) and . The newly estimated temperature is 1830±48K, 70K higher than that by our previous estimation. The EoSes of the major mantle minerals (olivine, wadsleyite, ringwoodite and perovskite) are also recalculated using the Tange scale. The adiabatic temperature gradient is calculated using the thermal expansion coefficient obtained from these EoSes. The adiabatic temperature gradient gradually decreases with increasing depth without a phase transition, and abruptly increases in association with phase transitions. The adiabatic temperature gradients are found to be 04–0.5 and 0.3K/km in the upper and lower parts of the mantle, respectively. The temperatures at a depth of 200km, the bottom of the mantle transition zone, the top of the lower mantle and a depth of 2700km are found to be 1720±40, 2010±40, 1980±40, and 2730±50K. The mantle potential temperature is found to be 1610±35K. [ABSTRACT FROM AUTHOR]

Abstract: Electrical conductivities of majorite garnet with compositions of pyrolite minus olivine (pyrolite majorite) and mid-ocean ridge basalt (MORB majorite) were measured under physical conditions of the mantle transition zone (18 and 23GPa and temperatures up to 2000K) in a Kawai-type multi-anvil apparatus. The samples with MORB composition are mainly composed of majorite, which has higher Fe and Al contents, and contain a small amount of stishovite. The conductivity of the MORB majorite is more than twice higher than those of the pyrolite majorite at the same temperature. The activation energies of these majorites are both 1.4eV at temperature of 1000–1600K suggesting that the dominant mechanism of charge transportation is Fe2+–Fe3+ hopping (small polaron) conduction. At higher temperatures (>1600K), corresponding to temperature conditions of the transition zone, conduction mechanism of the pyrolite majorite would change from small polaron to ionic conduction. The pyrolite majorite has only slightly higher and lower conductivity than dry wadsleyite and ringwoodite, respectively, and will not largely change the conductivity-depth profile predicted for the dry mantle transition zone. The laboratory-based conductivity profile of the mantle transition zone with pyrolitic composition can explain well the current semi-global conductivity-depth profile obtained from electromagnetic study beneath Pacific. On the other hand, the garnetite originating from the oceanic crust has remarkably higher conductivity than the surrounding mantle because the conductivity of MORB majorite is significantly higher than those of wadsleyite and ringwoodite. Conductivity values of MORB majorite agree with those of the stagnant slab beneath the northeastern China. [Copyright &y& Elsevier]

Water has a strong influence on transport properties in minerals and on the Earth's interior dynamics. However, the small effect of water on upper-mantle rheology was suggested based on Si self-diffusivity because Si is the slowest diffusion species in silicate minerals and believed to control their rheology (Fei et al. 2013). The aim of this study is to examine whether the effect of water on Si self-diffusivity is also small in wadsleyite, which is the major mineral of the upper part of the mantle transition zone. We have experimentally determined Si self-diffusivity as a function of water content and crystallographic orientation Measurement were conducted on single crystals of Fe-free wadsleyite using 29Si enriched thin-film as a diffusing couple. Diffusion profiles were obtained using a NanoSIMS with the depth-profile method. Here, we determined Arrhenius relations for volume diffusion rates parallel to each crystallographic orientation. It was found that water significantly enhances Si self-diffusivity in comparison with previously reported values for wadsleyite and olivine. These results suggest that mantle viscosity significantly decreases across the 410-km discontinuity by assuming high water content in the mantle transition zone. [ABSTRACT FROM AUTHOR]