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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]

The Earth’s mantle transition zone could potentially store a large amount of water, as the minerals wadsleyite and ringwoodite incorporate a significant amount of water in their crystal structure. The water content in the transition zone can be estimated from the electrical conductivities of hydrous wadsleyite and ringwoodite, although such estimates depend on accurate knowledge of the two conduction mechanisms in these minerals (small polaron and proton conductions), which early studies have failed to distinguish between. Here we report the electrical conductivity of these two minerals obtained by high-pressure multi-anvil experiments. We found that the small polaron conductions of these minerals are substantially lower than previously estimated. The contributions of proton conduction are small at temperatures corresponding to the mantle transition zone and the conductivity of wadsleyite is considerably lower than that of ringwoodite for both mechanisms. The dry model mantle shows considerable conductivity jumps associated with the olivine–wadsleyite, wadsleyite–ringwoodite and post-spinel transitions. Such a dry model explains well the currently available conductivity–depth profiles obtained from geoelectromagnetic studies. We therefore conclude that there is no need to introduce a significant amount of water in the mantle transition to satisfy electrical conductivity constraints. [ABSTRACT FROM AUTHOR]

In situ observations of the zircon-reidite transition in ZrSiO4 were carried out using a multianvil high- pressure apparatus and synchrotron radiation. The phase boundary between zircon and reidite was determined to be P (GPa) = 8.5+0.0017×(T-1200) (K) for temperatures between 1100–1900 K. When subducted slabs, including igneous rocks and sediments, descend into the upper mantle, the zircon in the subducted slab transforms into reidite at pressures of about 9 GPa, corresponding to a depth of 270 km. Reidite found in an upper Eocene impact ejecta layer in marine sediments is thought to have been transformed from zircon by a shock event. The peak pressure generated by the shock event in this occurrence is estimated to be higher than 8 GPa. [ABSTRACT FROM AUTHOR]

A substantial amount of subducted basaltic crusts may exist in the lower mantle. It features distinct chemical composition from the peridotitic mantle and plays important roles in the chemical and dynamic evolution of Earth's interior. However, the chemical composition of mineral phases present in basaltic crust in the lower mantle is still poorly constrained. Here, we determined phase relations of normal mid-ocean ridge basalt (MORB) up to 52 GPa at 2000 K by multi-anvil press. Throughout our experiments, the mineral assemblages consist of five phases including bridgmanite, stishovite, calcium perovskite (davemaoite), calcium ferrite and new hexagonal aluminous phases. The density of MORB calculated from our phase assemblages and previously published thermoelastic data is 2-3% higher than that of the average mantle, which is consistent with literature. The new hexagonal aluminous phase is a host of potassium but only occupies 1-2 vol.% due to limited abundance of potassium in normal MORB. This indicates that the new hexagonal aluminous phase doesn't affect the elastic properties of basalt. Electron energy-loss spectra of recovered basaltic Al-rich bridgmanite show significant enrichment of ferrous iron (75-85%) compared with peridotitic Al-poor bridgmanite (∼30%), which is against previous studies showing that ferric iron ratio in bridgmanite increases with Al content. Ferric iron exhibits strong partitioning into the new hexagonal aluminous phase (41-64%), whereas bridgmanite (14-28%) and calcium ferrite phase (5-27%) remain Fe2+-enriched. The oxygen vacancy component of MgAlO 2.5 in bridgmanite is ∼11% up to 40 GPa, which is much higher than that in peridotitic bridgmanite (2-3%), possibly producing a viscosity contrast in the mid-mantle that would explain slab stagnation and plume thinning between 660 km and 1000 km depth. The presence of ferrous iron-rich bridgmanite in the deep lower mantle may contribute to seismic features of large low-shear-velocity provinces. • High pressure-temperature phase relations of MORB were determined up to 52 GPa. • Basaltic bridgmanite has a distinctly low Fe3+/ΣFe of less than 28%. • Basaltic bridgmanite contains much larger MgAlO 2.5 than peridotitic one below 40 GPa. • MORB has higher density than the average mantle in the lower mantle. • Seismological observations around 1000 km depth may be explained by presence of MORB. [ABSTRACT FROM AUTHOR]

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]

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]