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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: In order to overcome disadvantages of the DIA type press in squeezing the Kawai-cell, such as uneven compression between the upper and lower anvils and the four surrounding anvils and frictional loss of applied load in the guide block, we have developed a new 6-axis apparatus in which the movements of the six anvils are controlled by a servo mechanism. It is possible to keep the Kawai-cell cubic within an accuracy of ±2μm during compression and decompression. Pressure generation using sintered diamond cubic anvils with edge length of 14.0mm and a truncation of 1.5mm has been carried out up to ca. 60GPa by measuring electrical resistance of GaP, Zr, and Fe2O3. The results are compared with our previous calibration, carried out using an almost the same sample setup for identical anvils at SPring-8, by means of in situ X-ray observation. It is demonstrated that a significant amount of the applied load is lost by friction when the Kawai-cell is squeezed in the DIA type press. The load loss increases with increasing load, or pressure, and amounts to 45% at ca. 60GPa. Therefore the 6-axis apparatus is very advantageous to generate higher pressures in the Kawai-cell. However, individual control of the anvils sometimes induces a runaway advancement of the anvils which brings about an abrupt increase of pressure. [Copyright &y& Elsevier]

Abstract: Volumes of forsterite were measured by in situ X-ray diffraction at simultaneously high pressures and high temperatures up to 14GPa and 1900K in a Kawai-type multi-anvil apparatus. By fitting the present data to the Mie-Grüneisen-Debye thermal equation of state with the results of the previous reliable studies, we obtained γ 0 =1.31(1) and q =3.2(1). The logarithmic volume dependence of the thermal expansion coefficient was found to be δ T =8.4(2). The average temperature derivative of the isothermal bulk modulus in the investigated pressure–temperature ranges was found to be (∂K T /∂T) P =−0.028(1)GPa/K. The isobaric heat capacity and thermal pressure at high pressure and high temperature were estimated using the present volume data. The adiabatic geotherm at the top and base of the uppermost mantle are 0.60(2) and 0.34(2)K/km. The temperature estimated from the 410-km discontinuity depth and the olivine-wadsleyite transition pressure suggests a mantle potential temperature of 1520±40K. [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]

SPEED-Mk.II, the second KAWAI-type high P–T apparatus for in situ X-ray observation that was installed at the bending magnet beam line at SPring-8, is described. The guide block system was designed so that the change of the relative dimension of the cubic compression space with press load can be minimized by repeated adjustments. The hydraulic system was designed so as to enable smooth compression and decompression. These precise controls should be advantageous for high-pressure generation, especially when sintered diamond (SD) anvils are used. An oscillation system was equipped for the first time in a large volume press, making it possible to obtain high-quality diffraction patterns even when the number of sample grains is limited. The use of the oscillation system also reduces errors in pressure determination that may be caused by insufficient averaging of diffraction angles over grains in a limited diffraction volume, because the oscillating grains should sweep through the 2θ range that is allowed by the finite widths of the optical slits. [Copyright &y& Elsevier]

The phase boundary of the post-spinel transition in Mg2SiO4 was re-investigated by means of high P–T in situ X-ray diffractometry with a gold pressure marker in a Kawai-type apparatus. Rapid and continuous temperature changes were conducted to initiate dissociation of spinel, which tends to be inert after long annealing. Isothermal decompression at high temperature was conducted to form spinel from perovskite plus periclase. The phase boundary is located at ca. 22 GPa in the temperature range from 1550 to 2100 K, which is 1–1.5 GPa lower than the 660 km discontinuity. This discrepancy might be explained in terms of the pressure effect of thermocouple emf and inaccurate equation of state (EOS) for the pressure maker. The transition is found to be less sensitive to temperature than reported previously, with a Clapeyron slope ranging from −2 to −0.4 MPa/K. This small Clapeyron slope implies that the post-spinel transition would not be an effective barrier to mantle convection. [Copyright &y& Elsevier]

The single-crystal elastic moduli of spinel-structured γ-(Mg0.91Fe0.09)2SiO4 (ringwoodite) were measured to 16 GPa at room temperature, and to 923 K at ambient pressure by Brillouin spectroscopy. A third-order finite-strain equation of state yields pressure derivatives of 4.1 (2) and 1.3 (1) for the adiabatic bulk modulus (KS) and shear modulus (μ), respectively. The pressure derivatives of aggregate elastic moduli for all three polymorphs (α, β, and γ) of (Mg,Fe)2SiO4 in the Mg-rich end are equal within the mutual experimental uncertainties, with (∂KS/∂P)T=4.1–4.3 and (∂μ/∂P)T=1.3–1.4. A linear decrease of the elastic moduli and sound velocities with temperature adequately describes the data. The temperature derivatives of KS and μ are −0.021 and −0.016 GPa/K, respectively. An extrapolation of our data to transition zone pressures and temperatures indicates that the shear and compressional impedance contrasts associated with β-(Mg,Fe)2SiO4→γ-(Mg,Fe)2SiO4 transition are sufficient to produce a visible discontinuity at 520 km depth even with only a moderate (30–50%) amount of olivine. Moreover, the partitioning of Fe between β-(Mg,Fe)2SiO4 and γ-(Mg,Fe)2SiO4 and majoritic garnet will produce a relatively sharp seismic discontinuity (in the order of 10–15 km) even when the latent heat of reaction is taken into account. The visibility of the discontinuity in regional and global studies could be inhibited by large topography that would accompany any local variations in temperature or Fe content. The high VP contrasts observed in some localities are difficult to explain by the β→γ transition and are suggestive of a chemical boundary. This is consistent with chemical heterogeneity in the transition zone, probably related to subduction. One of the possible explanations for high velocity contrasts near 520 km is a “buried Moho”—a boundary between garnetite (former oceanic crust) and γ-(Mg,Fe)2SiO4-enriched (former peridotite) layers. [Copyright &y& Elsevier]

Oxygen self-diffusion coefficients ( D Ox ) were measured in single crystals of dry synthetic iron-free olivine (forsterite, Mg 2 SiO 4 ) at a temperature of 1600 K and under pressures in the range 10 −4 to 13 GPa, using a Kawai-type multi-anvil apparatus and an ambient pressure furnace. Diffusion profiles were obtained by secondary ion mass spectrometry operating in depth profiling mode. D Ox in forsterite increases with increasing pressure with an activation volume of −3.9 ± 1.2 cm 3 /mol. Although Mg is the fastest diffusing species in forsterite under low-pressure conditions, O is the fastest diffusing species at pressures greater than ∼10 GPa. Si is the slowest throughout the stable pressure range of forsterite. Based on the observed positive and negative pressure dependence of D Ox and D Mg (Mg self-diffusion coefficient), respectively, D Ox  +  D Mg in forsterite decreases with increasing pressure, and then increases slightly at pressures greater than 10 GPa. This behavior is in agreement with the pressure dependence of ionic conductivity in forsterite based on conductivity measurements ( Yoshino et al., 2017 ), and can be used to explain the conductivity increase from ∼300 km depth to the bottom of the asthenosphere. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mg2SiO4 diffusivity in K–Mg carbonatite melt is 2×10−9 m2/s at 16.5GPa and 1700°C. [•] MgSiO3 diffusivity in hydrous K–Mg carbonatite melt is 4×10−9 m2/s at 24GPa, 1500°C. [•] MgSiO3 diffusivity in hydrous-silicate melt is 5×10−8 m2/s at 24GPa and 1500°C. [•] Stress-driven transcrystalline migration rate of TZ and LM melts is 10−1–102 m/year. [ABSTRACT FROM AUTHOR]

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]

Abstract: Rheology of the lower mantle characterizes the dynamics of the earth''s interior and it is often controlled by the textures of the constituting material which are (Mg,Fe)SiO3 perovskite and ferro-periclase aggregate. We conducted high-pressure experiments to synthesize the (Mg,Fe)SiO3 perovskite and ferro-periclase aggregates and measured two important textures of “grain size” and “dihedral angle”. The grain growth rates of perovskite and (ferro-)periclase in two phase aggregates were influenced by the iron content and increased with factors of ∼1.5 in iron-rich system. This difference in grain growth rates indicates that the viscosity of aggregates of iron-rich system is only a few times greater than that of iron-poor system for likely diffusion creep in the lower mantle. In contrast, the change of the dihedral angle of perovskite – periclase – perovskite at triple grain junction with variation of iron content was not observed systematically, but the dihedral angle decreases from ∼110° to ∼105° with an increase of temperature from 1673 to 2273K. The dihedral angle of 105–110° would imply the interconnected network spatially of ferro-periclase in the aggregates and the connectivity increases with temperature. As a result, at higher temperature, ferro-periclase plays more important role for understanding the rheology of the lower mantle because ferro-periclase is a few order of magnitude softer than (Mg,Fe)SiO3 perovskite. [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]

Abstract: The elastic moduli of polycrystalline ringwoodite, (Mg0.91Fe0.09)2SiO4, were measured up to 470K by means of the resonant sphere technique. The adiabatic bulk (K S) and shear (μ) moduli were found to be 185.1(2) and 118.22(6)GPa at room temperature, and the average slopes of dK S/dT and dμ/dT in the temperature range of the study were determined to be −0.0193(9) and −0.0148(3)GPa/K, respectively. Using these results, we estimate seismic wave velocity jumps for a pure olivine mantle model at 520km depth. We find that the jump for the S-wave velocity is about 1.5 times larger than that for the P-wave velocity at this depth. This suggests that velocity jumps at the 520km discontinuity are easier to detect using S-waves than P-waves. [Copyright &y& Elsevier]

In situ X-ray diffraction experiments on FeS up to 22 GPa and 1600 K were carried out using large volume multianvil apparatus, combined with synchrotron radiation at SPring-8. We investigated phase stability relationships of FeS and determined the straight phase boundaries between FeS III (monoclinic phase) and FeS IV (hexagonal phase) to be T (K)=20P (GPa)+170 and between FeS IV and FeS V (NiAs-type phase) to be T (K)=39.6P (GPa)+450. We also found anomalous behavior in the c/a ratio, thermal expansion, and isothermal compression of FeS V as well as FeS IV, in the pressure range 4–12 GPa. These anomalies in FeS can be attributed to the spin-pairing transition of Fe, and divides FeS IV and FeS V into the high-spin low-pressure phase (LPP) and the possibly low-spin high-pressure phase (HPP). In order to investigate the internal structure of Mars, we evaluated the equations of state for FeS IV (HPP) and FeS V (HPP). A least square fit to the experimental data yielded K0T=62.5±0.9 GPa at T=600 K and (dK0/dT)P=-0.0208±0.0028 GPa/K for FeS IV (HPP), and K0T=54.3±1.0 GPa at T=1000 K and (dK0/dT)P=-0.0117±0.0015 GPa/K for FeS V (HPP) with fixed K′=4. Thermal expansion coefficients were α=7.16×10-5+6.08×10-8T for FeS IV (HPP) and α=10.42×10-5 for FeS V (HPP), respectively. Using these equations of state, we examined the internal structure of Mars that has a model mantle composition [Meteoritics 20 (1985) 367] and Fe–FeS core. Our models show that an Mg-silicate perovskite-rich lower mantle is stable only with the Fe-rich core having less than 20 wt.% sulfur. The polar moment of inertia factor C derived from Mars Pathfinder data [Science 278 (1997) 1749] is consistent with any compositions between Fe and FeS for the Martian core, but it excludes the presence of a crust thicker than 100 km. [Copyright &y& Elsevier]