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We review the currently available results of laboratory experiments, geochemistry and MT observations and attempt to explain the conductivity structures in the oceanic asthenosphere by constructing mineral-physics models for the depleted mid-oceanic ridge basalt (MORB) mantle (DMM) and volatile-enriched plume mantle (EM) along the normal and plume geotherms. The hopping and ionic conductivity of olivine has a large temperature dependence, whereas the proton conductivity has a smaller dependence. The contribution of proton conduction is small in DMM. Melt conductivity is enhanced by the H 2 O and CO 2 components. The effects of incipient melts with high volatile components on bulk conductivity are significant. The low solidus temperatures of the hydrous carbonated peridotite produce incipient melts in the asthenosphere, which strongly increase conductivity around 100 km depth under older plates. DMM has a conductivity of 10 − 1.2 ~− 1.5 S/m at 100–300 km depth, regardless of the plate age. Plume mantle should have much higher conductivity than normal mantle, due to its high volatile content and high temperatures. The MT observations of the oceanic asthenosphere show a relatively uniform conductivity at 200–300 km depth, consistent with the mineral-physics model. On the other hand, the MT observations show large lateral variations in shallow parts of the asthenosphere despite similar tectonic settings and close locations. Such variations are difficult to explain with the mineral-physics model. High conductivity layers (HCL), which are associated with anisotropy in the direction of the plate motion, have only been observed in the asthenosphere under infant or young plates, but they are not ubiquitous in the oceanic asthenosphere. Although the general features of HCL imply their high-temperature melting origin, the mineral-physics model cannot explain them quantitatively. Much lower conductivity under hotspots, compared with the model plume-mantle conductivity suggests the extraction of volatiles from the plume mantle by the ocean island basalt (OIB) magmatism. [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]

Abstract: This study of the electrical conductivity of metamorphic rocks as a function of temperature highlights the importance of conductivity variations associated with dehydration. We observed conductivity changes for anhydrous rocks and hydrous amphibolites up to 1100K at 0.5GPa. Our experimental results revealed that dehydration takes place and that there is a non-linear variation above metamorphic temperatures. To evaluate the observed changes in conductivity after dehydration, numerical calculations were conducted. The calculation results showed that the bulk conductivity of rock plus pure water cannot account for the conductivity values of reacted rocks if the crust dehydrates. However, the data for 0.1m NaCl solutions are consistent with those for amphibolites between 530 and 1100K. In this temperature range, the estimated volume fractions of dewatered saline solutions vary between 0.04 and 0.36vol.%. These results show that although dehydration is insignificant, and only small amounts of high-salinity fluids are formed in the crust, the total conductivity of amphibolites is high after dehydration. [Copyright &y& Elsevier]

Abstract: Electrical conductivity of ringwoodite with various iron contents [Fe/(Fe+Mg)=0.09, 0.2 and 0.3] was measured at pressure (20GPa) and temperature (up to 1900K) conditions of the lower part of the mantle transition zone in a Kawai-type multi-anvil apparatus. The conductivity increased with increasing total iron content. All electrical conductivity data were fitted to the formula of electrical conductivity σ = σ 0 X Fe exp(−H/kT), where σ 0 is the pre-exponential term, X Fe is the mole fraction of iron content in the Mg site, H is the activation enthalpy, k is the Boltzmann constant and T is absolute temperature. The activation enthalpy becomes higher at a certain temperature. At high temperatures, the activation enthalpy decreased from 1.44 to 0.92eV with increasing total Fe content. At low temperatures less than 1000K, the activation enthalpy also decreases from 1.15 to 0.74eV with total Fe content. Dependence of the activation enthalpy on Fe content suggests that the dominant mechanism of charge transport is Fe2+–Fe3+ hopping (small polaron). Recent electrical conductivity-depth profiles of the transition zone beneath the Pacific Ocean obtained from the electromagnetic induction study shows that the conductivity values between 520 and 660km depths may be explained by ringwoodite with Fe/(Fe+Mg)=0.10. On the other hand, assuming a normal geotherm, conductivity values beneath the continent or stable craton are considerably lower than those of ringwoodite with Fe/(Fe+Mg)=0.10. Taking into consideration results from the global seismic tomographic studies, relatively low conductivity in these regions can be explained by the existence of a cooler region compared with the surrounding mantle, rather than the presence of iron-poor ringwoodite, or a combination of both. [Copyright &y& Elsevier]

Abstract: We conducted electrical conductivity measurements perpendicular and parallel to mineral foliation in dry gneiss at up to 1000 K and a constant pressure of 1 GPa. The analyzed gneisses were obtained from the Higo metamorphic belt, Kyushu, Japan. As the metamorphic conditions of these rocks have been well determined by previous studies, we were able to select samples that were representative of the middle to lower crust. Prior to the conductivity measurements, the samples were maintained at the maximum temperature for a long period, until the electrical conductivity had stabilized. Our experiment results reveal linear and reproducible conductivity data between temperatures of 600 and 1000 K. Conductivity measured perpendicular and parallel to foliation differ by an order of magnitude over the same temperature window. A plausible explanation for this discrepancy in conductivity is the contrasting configuration of minerals in the two sample orientations, as observed by backscattered electron image (BEI) and electron probe microanalysis (EPMA). We evaluated the conductivity and computed activation energy for each of the samples and compared the results with those of previous studies; our results are consistent with the conductivity values reported for other types of rocks. We also compared the experiment results with data derived from electromagnetic (EM) soundings. Electrical conductivity measurements undertaken perpendicular to foliation can account for the subsurface conductivity structure beneath central Kyushu, Japan. [Copyright &y& Elsevier]

The connectivity of molten Fe–S in peridotite has been experimentally investigated by means of in situ electrical conductivity measurements at high temperatures and 1 GPa. Starting materials were powdered mixtures of peridotite KLB-1 with various amounts (0, 3, 6, 13, 19, 24 vol.%) of the 1 GPa eutectic composition in the Fe–FeS binary system. At temperatures above the eutectic point in the Fe–FeS system (∼980 °C) and below the solidus of KLB1 (∼1200 °C), molten Fe–S in a solid silicate matrix interconnects when the volume fraction is over ∼5%. Conductivity–temperature paths indicate that in the presence of partial silicate melting the connectivity of molten Fe–S in a peridotite matrix is inhibited. Based on observations of retrieved samples, the percolation threshold of Fe–S melts in the presence of low to moderate degrees of silicate melt is estimated at 13±2 vol.%. These results indicate that if the volume fraction of Fe-alloy in a planetesimal was initially greater than 5%, and if early heating by decay of radionuclides raised the temperature of the interior above the Fe-alloy melting point, initial metal segregation was controlled by permeable flow of molten iron alloy in a solid silicate matrix. These conditions were likely met by many terrestrial objects in the early solar nebula. Efficient removal of residual Fe-alloy (5 vol.%) from silicate requires high-degree melting of silicate so that metal can segregate as droplets. Giant impacts during the final stage of accretion of large planetary objects could supply the energy required for high-degrees of melting. Alternatively, if initial metal segregation were delayed until a planetary object grew to large size (∼1000 km in diameter), release of gravitational potential energy due to metal segregation could contribute enough heat to form a magma ocean. [Copyright &y& Elsevier]

The presence of a small fraction of basaltic melt is a potential explanation for mantle electrical conductivity anomalies detected near the top of the oceanic asthenosphere. The interpretation of magnetotelluric profiles in terms of the nature and proportion of melt, however, relies on mathematical models that have not been experimentally tested at realistically low melt fractions (<0.01). In order to address this, we have performed in situ electrical conductivity measurements on partially molten olivine aggregates. The obtained data suggest that the bulk conductivity follows the conventional Archie's law with the melt fraction exponents of 0.75 and 1.37 at melt fractions greater and smaller than 0.5 vol.% respectively at 1350 °C. Our results imply multiple conducting phases in melt-bearing olivine aggregate and a connectedness threshold at ∼0.5 vol.% of melt. The model predicts that the conductive oceanic upper asthenosphere contains 0.5 to 1 vol.% of melt, which is consistent with the durable presence of melt at depths over millions years while the oceanic plates spread apart at the mid-ocean ridge. Beneath ridges a minimum permeability may allow mid-ocean ridge basalts to rise out of the mantle, where our model indicates that melt is present in proportions of up to 4 vol.%. [ABSTRACT FROM AUTHOR]

The electrical conductivity of partially molten peridotite was measured during deformation in simple shear at 1 GPa in a DIA type apparatus with a uniaxial deformation facility. To detect development of electrical anisotropy during deformation of partially molten system, the electrical conductivity was measured simultaneously in two directions of three principal axes: parallel and normal to the shear direction on the shear plane, and perpendicular to the shear plane. Impedance spectroscopy measurement was performed at temperatures of 1523 K for Fe-bearing and 1723 K for Fe-free samples, respectively, in a frequency range from 0.1 Hz to 1 MHz. The electrical conductivity of partially molten peridotite parallel to shear direction increased to more than one order of magnitude higher than those normal to shear direction on the shear plane. This conductivity difference is consistent with the magnitude of the conductivity anisotropy observed in the oceanic asthenosphere near the Eastern Pacific Rise. On the other hand, conductivity perpendicular to the shear plane decreased gradually after the initiation of shear and finally achieved a value close to that of olivine. The magnitude and development style of conductivity anisotropy was almost the same for both Fe-bearing and Fe-free melt-bearing systems, and also independent of shear strain. However, such conductivity anisotropy was not developed in melt-free samples during shear deformation, suggesting that the conductivity anisotropy requires a presence of partial melting under shear stress. Microstructural observations of deformed partially molten peridotite samples demonstrated that conductivity anisotropy was attributed to the elongation of melt pockets parallel to the shear direction. Horizontal electrical conductivity anisotropy revealed by magnetotelluric surveys in the oceanic asthenosphere can be well explained by the realignment of partial melt induced by shear stress. [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]

Abstract: In order to investigate the effect of carbonate-content in partial melt on bulk conductivity under high pressure, electrical conductivity measurements were performed on carbonate melt-bearing peridotites using a Kawai-type multi-anvil apparatus. The starting materials were composed of spinel lherzolite (KLB1) with small amounts of dolomite (1 and 3wt.%). To obtain various melt fractions, annealing experiments were performed at different temperatures above 1400K at 3GPa. At low temperatures (⩽1500K), the conductivity was distinctly higher than that of carbonate-free peridotite and close to that of the carbonatite melt-bearing olivine aggregates. Although the sample conductivity increased with increasing temperature, the rate at which the conductivity increases was small and the conductivity approached that of silicate melt-bearing peridotite. CO2 concentration in the partial melt decreased with increasing annealing temperature. Thus, the small increase of the conductivity with annealing temperature is attributed to a decrease of the melt conductivity due to a decrease in carbonate content in the partial melt. As the carbonate concentration in the melt decreases, the estimated melt conductivity approaches that of the basaltic melt. Therefore, conductivity enhancement by the carbonate-bearing melt is very effective at temperature just above that of the carbonate peridotite solidus. [Copyright &y& Elsevier]

We report the first study of electrical conductivities of silicate melts at very high pressures (up to 10 GPa) and temperatures (up to 2,173 K). Impedance spectroscopy was applied to dry and hydrous albite (NaAlSiO) glasses and liquids (with 0.02-5.7 wt% HO) at 473-1,773 K and 0.9-1.8 GPa in a piston-cylinder apparatus, using a coaxial cylindrical setup. Measurements were also taken at 473-2,173 K and 6-10 GPa in two multianvil presses, using simple plate geometry. The electrical conductivity of albite melts is found to increase with temperature and water content but to decrease with pressure. However, at 6 GPa, conductivity increases rapidly with temperature above 1,773 K, so that at temperatures beyond 2,200 K, conductivity may actually increase with pressure. Moreover, the effect of water in enhancing conductivity appears to be more pronounced at 6 GPa than at 1.8 GPa. These observations suggest that smaller fractions of partial melt than previously assumed may be sufficient to explain anomalously high conductivities, such as in the asthenosphere. For dry melt at 1.8 GPa, the activation energy at T > 1,073 K is higher than that at T < 1,073 K, and the inflection point coincides with the rheological glass transition. Upon heating at 6-10 GPa, dry albite glass often shows a conductivity depression starting from ~1,173 K (due to crystallization), followed by rapid conductivity enhancement when temperature approaches the albite liquidus. For hydrous melts at 0.9-1.8 GPa, the activation energies for conductivity at ≥1,373 K are lower than those at <973 K, with a complex transition pattern in between. Electrical conductivity and previously reported Na diffusivity in albite melt are consistent with the Nernst-Einstein relation, suggesting the dominance of Na transport for electrical conduction in albite melts. [ABSTRACT FROM AUTHOR]

Abstract: Electrical impedance measurements were performed on two types of partial molten samples with basaltic and carbonatitic melts in a Kawai-type multi-anvil apparatus in order to investigate melt fraction–conductivity relationships and melt distribution of the partial molten mantle peridotite under high pressure. The silicate samples were composed of San Carlos olivine with various amounts of mid-ocean ridge basalt (MORB), and the carbonate samples were a mixture of San Carlos olivine with various amounts of carbonatite. High-pressure experiments on the silicate and carbonate systems were performed up to 1600K at 1.5GPa and up to at least 1650K at 3GPa, respectively. The sample conductivity increased with increasing melt fraction. Carbonatite-bearing samples show approximately one order of magnitude higher conductivity than basalt-bearing ones at the similar melt fraction. A linear relationship between log conductivity (σ bulk) and log melt fraction (ϕ) can be expressed well by the Archie''s law (Archie, 1942) (σ bulk/σ melt = Cϕ n ) with parameters C =0.68 and 0.97, n =0.87 and 1.13 for silicate and carbonate systems, respectively. Comparison of the electrical conductivity data with theoretical predictions for melt distribution indicates that the model assuming that the grain boundary is completely wetted by melt is the most preferable melt geometry. The gradual change of conductivity with melt fraction suggests no permeability jump due to melt percolation at a certain melt fraction. The melt fraction of the partial molten region in the upper mantle can be estimated to be 1–3% and ∼0.3% for basaltic melt and carbonatite melt, respectively. [Copyright &y& Elsevier]

Abstract: The electrical conductivity of sintered polycrystalline periclase has been measured at 20GPa and temperatures up to 1700K with a KAWAI-type multi-anvil apparatus. Arrhenius plot of the electrical conductivity shows two linear regions, implying a change of charge transport mechanism with temperature in this experiment. At temperatures below 700K small polaron conduction dominates, with an activation energy of 0.2eV. At higher temperatures, the activation energy is determined to be 1.18eV, which is much higher than 0.2eV at low temperature. Our observations suggest the high-temperature conduction is attributed to a large polaron process with magnesium vacancy trapping hole on oxygen. [Copyright &y& Elsevier]

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]

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]