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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: Recent laboratory electrical conductivity measurements of the main mantle constituent minerals have represented considerable efforts to determine the effects of water content on electrical conductivity. However, there are large discrepancies between the results of and those of on hopping conduction and the effects of water on the electrical conductivity of wadsleyite. To investigate the cause of these discrepancies, the electrical conductivity of anhydrous and hydrous wadsleyite were newly measured under low and high temperature conditions by impedance spectroscopy. The conductivity values of dry wadsleyite aggregates with less than 2ppm H2O by weight were similar to those for hopping conduction reported by and distinctly higher than those of . For hydrous wadsleyite, at temperatures below 1000K, the electrical conductivity in an Arrhenius plot was repeatable along the heating–cooling paths and was similar to the results of . The impedance spectrum in the complex impedance plane of hydrous wadsleyite showed a semicircular shape, and the infrared spectrum did not show any shape change after the conductivity measurements. In contrast, when the temperature exceeds 1000K, the electrical conductivity in an Arrhenius plot showed higher activation enthalpy. The impedance spectra were greatly distorted and the impedance arc contained at least two relaxation processes. This shape is similar to those reported by who measured the conductivity above 1000K. The infrared spectra showed a large contribution from molecular water after conductivity measurements, suggesting significant dehydration during the conductivity measurements. In summary, the results obtained from conductivity measurements at higher temperatures (>1000K) do not represent the proton conduction in the grain interior. [Copyright &y& Elsevier]

• Liquidus phase relations in a hydrous silicate system were determined at 24 GPa. • Melts are neutrally buoyant at 24 GPa before ∼98% crystallization of a magma ocean. • A transient melt-rich layer may form atop the lower mantle in a mushy magma ocean. The solidification of a deep magma ocean occurred early in Earth's history. Although the initial amount of H 2 O in Earth's magma ocean is predicted to be low (e.g., <3000 ppm), as an incompatible element it becomes highly enriched (e.g. >10 wt%) in the final few percent of crystallization. In order to understand how a hydrous magma ocean would crystallize at the top of the lower mantle, we determined liquidus phase relations in the MgO-FeO-CaO-Al 2 O 3 -SiO 2 -H 2 O system at 24 GPa. We find that the bridgmanite (brg) + stishovite (st) + melt and bridgmanite (brg) + ferropericlase (fp) + melt cotectic boundary curves trend to Mg-rich melt compositions with decreasing temperature and extend to very high H 2 O contents (∼80 mol% H 2 O). The brg+st+melt curve is a subtraction curve at < ∼18 mol% H 2 O and a reaction curve at higher H 2 O contents, whereas the brg+fp+melt is a subtraction curve throughout its length. The density of melts along the two cotectics leads to neutral buoyancy with respect to shallow lower mantle and transition zone minerals at H 2 O contents up to ∼25 mol%. A transient melt-rich layer can form at the top of the lower mantle during late-stage crystallization in a mushy magma ocean when melt percolation dominates. When crystallization exceeds ∼98%, hydrous melts (>25 mol% H 2 O) become buoyant and can percolate into and hydrate the mantle transition zone. [ABSTRACT FROM AUTHOR]

High‐temperature ionic conductivity in olivine single crystals has been measured in the [100], [010], and [001] crystallographic orientations as a function of pressure from 2 to 10 GPa, temperature from 1450 to 2180 K, and H2O content from 20 to 580 wt. ppm using multianvil presses with in situ impedance analyses. The experimental results yield an activation energy, activation volume, and H2O content exponent of 250–405 kJ/mol, 3.2–5.3 cm3/mol, and 1.3 ± 0.2, respectively, for the high‐temperature ionic conduction regime. Olivine ionic conductivity has negative pressure and positive temperature dependences and is significantly enhanced by H2O incorporation. The [001] direction is more conductive than the [100] and [010] directions. The H2O‐enhanced ionic conductivity may contribute significantly to the electrical conductivity profile in the asthenosphere, especially in the regions under relatively high‐temperature and low‐pressure conditions. Key Points: Pressure, temperature, and water content dependences of olivine ionic conductivity are obtainedOlivine ionic conductivity is dramatically enhanced by water incorporationOlivine ionic conductivity may contribute significantly to the bulk conductivity of olivine in the asthenosphere [ABSTRACT FROM AUTHOR]

We investigated the H2O‐content dependence of Si self‐diffusion coefficient in Fe‐free wadsleyite using multi‐anvil experiments at pressures of 19–20 GPa, temperatures of 1573–1873 K, and H2O‐content ranging from ∼10 to 5,300 wt. ppm by the isotopic thin‐film diffusion‐couple method. The 29Si‐doped diffusion profiles were measured by nanoscale secondary ion mass spectrometry in the depth profiling mode. The H2O contents in the samples were analyzed by Fourier transformation infrared spectroscopy. The experimental results show a H2O enhancement of Si diffusion coefficient with a H2O content exponent of 0.8 ± 0.1. The activation enthalpy was found to be 270 ± 40 kJ/mol. The diffusion coefficients in the [100], [010], and [001] directions are indistinguishable. The temperature and H2O‐content dependences of Si diffusion indicate that H2O incorporation dramatically reduces the rheological strength of wadsleyite, whereas temperature has a relatively small effect. The viscosity in the mantle transition zone could be significantly reduced by H2O incorporation in wadsleyite. The viscosity contrast between mantle plumes and surroundings may control the evolution of plume shapes at 410–660 km depths. Plain Language Summary: Water incorporation in nominally anhydrous minerals may dramatically affect their physical and chemical properties by introducing additional point defects as hydroxyl in the crystal structures. In particular, the atomic diffusivity in minerals, which is controlled by the concentrations of point defects, could be enhanced, therefore, the diffusion‐controlled properties such as rheology could be affected by water incorporation. Because the mantle transition zone is considered to be water‐rich, we investigated the effect of water on silicon self‐diffusion rate in wadsleyite, which is the dominant mineral in the upper part of mantle transition zone at 410–520 km depth. The experimental results show that water‐rich (1.0 wt.% H2O) wadsleyite is rheologically more than 2.4 orders of magnitude weaker than water‐poor (less than 10 wt. ppm) conditions. Therefore, the mantle transition zone is significantly weakened by water incorporation. The viscosity contrast between plumes and surrounding mantle may control the evolution of plume shapes at mantle transition zone depths. Key Points: Silicon self‐diffusion in wadsleyite is enhanced by water incorporation with an exponent of 0.8 ± 0.1Silicon self‐diffusion in wadsleyite is nearly isotropic, and has an activation energy of 270 ± 40 kJ/molViscosity contrast between plume and surrounding mantle is controlled by water and temperature, which may affect the plume shape evolution [ABSTRACT FROM AUTHOR]

• The H 2 O solubility in wadsleyite is independent from oxygen fugacity. • The dissociation of H 2 O to H 2 and O 2 in the mantle transition zone is negligible. • The H 2 O-rich and low- f O2 conditions in the mantle transition zone are compatible. The mantle transition zone is considered as a potential water reservoir in the Earth's interior because of the high H 2 O solubility in wadsleyite and ringwoodite. However, this scenario is based on experiments performed under relatively oxidized conditions. In order to constrain the H 2 O solubility in minerals in the reducing (close to iron-wüstite buffer) mantle transition zone, here we systematically measured the water content in wadsleyite at 17 GPa and 1673 K, as a function of oxygen fugacity in the range of Hf-HfO 2 , Mo-MoO 2 , Re-ReO 2 , and Pt-PtO 2 buffer conditions. The results suggest that the oxygen fugacity has no meaningful effect on H 2 O solubility in wadsleyite. Wadsleyite in the mantle transition zone can store as much as ∼1.0 wt.% H 2 O in spite of the reducing conditions. The dissociation of H 2 O into H 2 and O 2 is negligible, namely, there should be negligible molecular H 2 in the mantle transition zone. [ABSTRACT FROM AUTHOR]

To constrain the effect of water on upper mantle dynamics, we measured the annihilation rate coefficients (k) of [100](010) and [001](100) dislocations in olivine, referred to as a -dislocations and c -dislocations, respectively, as a function of water content. Natural olivine single crystals were doped with 5–800 wt. ppm water and sheared in the [100] or [001] directions along the (010) and (100) planes to produce a - and c -dislocations, respectively, and then annealed under quasi-hydrostatic conditions at a constant pressure and temperature of 5 GPa and 1473 K. The obtained annihilation rate coefficients are fitted to a power-law equation, yielding astonishingly small water-content exponents of 0.0 ± 0.1 and 0.2 ± 0.2 for the a - and c -dislocations, respectively. The overall effect of water on dislocation mobility is therefore small because these two slip systems are considered to be the least and most sensitive to water, respectively. These results imply that water incorporation does not effectively increase the dislocation-creep rate and that a water-induced fabric transition is unlikely. The effects of water on asthenospheric dynamics may thus be limited, and the lateral seismic anisotropy changes observed in the asthenosphere may solely reflect changes of mantle flow geometry. • Water has a small effect on olivine dislocation mobility. • Water effects are identical in the olivine [100](010) and [001](100) slip systems. • Variation of seismic anisotropy is not caused by different water contents in olivine. [ABSTRACT FROM AUTHOR]

Abstract: The electrical conductivity of Ca-free aluminous enstatite with various water contents has been determined at a pressure of 3GPa in a Kawai-type multi-anvil apparatus. Impedance spectroscopy was performed for both hydrogen-doped and -undoped samples in a frequency range from 0.1Hz to 1MHz to examine the effect of water on conductivity. Two conduction mechanisms were identified for hydrogen-undoped samples at temperature of 1000–1723K and for hydrogen-doped samples at relatively lower temperature range of 500–900K to minimize dehydration of samples. For the hydrogen-undoped samples, the activation enthalpy is around 1.9eV at the higher temperatures range (>1300K) suggesting that the dominant charge transfer mechanism is Fe2+−Fe3+ hopping (small polaron) conduction. For the hydrogen-doped samples measured below 900K, the activation enthalpy decreases from 1.11 to 0.70eV, and the conductivity values systematically increase with increasing water content, suggesting that proton conduction is the dominant conduction mechanism. Taking hopping conduction and water content dependence of activation enthalpy for proton conduction into account, all electrical conductivity data were fitted to the formula σ=σ 0h exp(−H h /kT)+σ 0p C w exp[−(H p 0−αC w 1/3)/kT], where σ 0 is pre-exponential factor, C w is the water content in weight percent, H is the activation enthalpy, H p 0 is the activation enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolution temperature and subscripts h and p represent hopping and proton conductions, respectively. Using the present results, a laboratory-based conductivity-depth profile in the Earth''s upper mantle has been constructed as a function of water content. Comparison of our model with the currently available geophysical observations beneath the Eastern Pacific Rise indicates that hydrous aluminous enstatite cannot account for the high conductivity anomaly at the top of the asthenosphere as well as hydrous olivine. [Copyright &y& Elsevier]

The HO content of wadsleyite were measured in a wide pressure (13-20 GPa) and temperature range (1,200-1,900°C) using FTIR method. We confirmed significant decrease of the HO content of wadsleyite with increasing temperature and reported first systematic data for temperature interval of 1,400-1,900°C. Wadsleyite contains 0.37-0.55 wt% HO at 1,600°C, which may be close to its water storage capacity along average mantle geotherm in the transition zone. Accordingly, water storage capacity of the average mantle in the transition zone may be estimated as 0.2-0.3 wt% HO. The HO contents of wadsleyite at 1,800-1,900°C are 0.22-0.39 wt%, indicating that it can store significant amount of water even under the hot mantle environments. Temperature dependence of the HO content of wadsleyite can be described by exponential equation $$ C_{{{\text{H}}_{2} {\text{O}}}} = 6 3 7.0 7 {\text{e}}^{ - 0.00 4 8T} , $$ where T is in °C. This equation is valid for temperature range 1,200-2,100°C with the coefficient of determination R = 0.954. Temperature dependence of HO partition coefficient between wadsleyite and forsterite ( D) is complex. According to our data apparent D decreases with increasing temperature from D = 4-5 at 1,200°C, reaches a minimum of D = 2.0 at 1,400-1,500°C, and then again increases to D = 4-6 at 1,700-1,900°C. [ABSTRACT FROM AUTHOR]

Abstract: The electrical conductivity of San Carlos olivine aggregate of various water content was measured at a pressure of 10GPa in a Kawai-type multi-anvil apparatus. Conductivity measurements were performed on two sets of samples to determine the effect on conductivity of water in olivine: 1) a hydrogen-doped sample and 2) a hydrogen-undoped sample. To minimize water escape from the hydrogen-doped samples, the conductivity measurement was carried out below 1000K. Three conduction mechanisms were identified from the Arrhenian behavior of the undoped samples, which include a small amount of water. A change in the activation enthalpy indicated that the dominant conduction mechanism changed from proton conduction to small polaron conduction with increasing temperature. At temperatures above 1700K, the activation enthalpy exceeds 2eV suggesting that the dominant mechanism of charge transport would be ionic conduction. The conductivity increased with increasing water content. The activation enthalpy for proton conduction tends to decrease slightly with increasing water content. The activation enthalpy determined for each run had similar values (~0.9eV). Taking the water concentration dependence of activation enthalpy into account for proton conduction, all data were fitted to the electrical conductivity formula σ=σ 0Iexp[−E I/kT]+σ 0Hexp[−E H/kT]+σ 0P C Wexp[−(E 0−αC W 1/3)/kT], where σ 0 represents a pre-exponential term, C W is the water content in weight percent, E is the activation enthalpy, E 0 is the activation enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolute temperature and subscripts I, H and P denote ionic, hopping (small polaron) and proton conductions, respectively. The conductivity jump at the 410km discontinuity (olivine–wadsleyite transition) is much smaller than that previously predicted. Since the contribution of proton conduction to the bulk electrical conductivity decreases with increasing temperature the high conductivity anomaly at the top of the asthenosphere cannot be explained by olivine hydration. [Copyright &y& Elsevier]

Abstract: Electrical conductivity of wadsleyite was measured at 16GPa using a KAWAI-type multi-anvil press under controlled oxygen fugacity. Water-doped and -undoped samples were used to examine the effect of water on conductivity. Two water-doped wadsleyite samples used for the conductivity measurements contained 0.3±0.01 and 1.2±0.02wt.% water. The temperature ranges for the conductivity measurements were 500–2000K for water-undoped samples and 500–1000K for water-doped samples. Above 1500K, the electrical conductivity values are essentially the same among different runs with an average activation enthalpy of 1.5eV, suggesting that small polaron conduction should dominate. Below 1000K, the conductivity systematically increases with increasing water content, suggesting proton conduction as a dominant conduction mechanism. Electrical conductivity of anhydrous wadsleyite in the mantle transition zone should be about 3×10−2 S/m in the normal geotherm. Hydration enhances the conductivity of wadsleyite; by containing 0.1wt. % water, the conductivity of wadsleyite increases by 0.3log units. The conductivity jump associated with the dry olivine–wadsleyite transition is only 0.7log units. A dry olivine–wadsleyite conductivity model well agrees with the current semi-global conductivity-depth profiles, thus the concept of a globally hydrated mantle transition zone is not necessary in view of electrical conductivity. The high conductivity observed in the wedge mantle cannot be explained by hydrous wadsleyite. Alternatively, the presence of either phase E or supercritical fluids in the wedge mantle is proposed. [Copyright &y& Elsevier]