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Wadsleyite can store significant amounts of H2O in its crystal structure as hydroxyl. However, H2O solubility in Fe‐bearing wadsleyite remains poorly constrained at mantle transition zone temperatures. Previous studies (e.g., Demouchy et al., 2005 [https://doi.org/10.2138/am.2005.1751]; Litasov et al., 2011 [https://doi.org/10.1007/s00269-010-0382-3]) focused primarily on Fe‐free systems, which do not represent the Earth's interior because Fe may affect the H2O solubility. Here, we investigated the temperature dependence of H2O solubility in Fe‐bearing and Fe‐free wadsleyite at 1500–2100 K. The results indicate that H2O solubility in Fe‐bearing wadsleyite is higher than in Fe‐free samples at 1800–1900 K, corresponding to transition zone geotherm, but there is no clear Fe content dependence in the Fe‐bearing samples. Wadsleyite can contain approximately 1.0 wt.% H2O at transition zone temperatures. The H2O solubility in wadsleyite is lower than ringwoodite along a plume geotherm, which may result in dehydration melting at the 520‐km discontinuity by upwelling flow in plumes. Plain Language Summary: The mantle transition zone at 410–660 km depth is a H2O sponge because wadsleyite and ringwoodite can contain large amounts of H2O in their crystal structures as hydroxyl defects. However, the solubility of H2O in Fe‐bearing wadsleyite is poorly constrained compared with ringwoodite and Fe‐free wadsleyite, which have been extensively investigated. The exact H2O storage capacity of the mantle transition zone, therefore, remains unknown. Here, we investigated the solubility of H2O in Fe‐bearing wadsleyite as a function of temperature. The results indicate that wadsleyite can store ∼1.0 wt.% H2O at transition zone temperatures and ∼0.65 wt.% along a plume geotherm. H2O solubility in wadsleyite is lower than that in ringwoodite in mantle plumes. A dehydration melting layer at the 520‐km discontinuity near plumes can, therefore, form via the phase transition from ringwoodite to wadsleyite under H2O‐saturated conditions driven by upwelling flow in mantle plumes. Key Points: H2O solubility in Fe‐bearing wadsleyite decreases with increasing temperatureFe‐bearing wadsleyite can contain ∼1.0 wt.% H2O at mantle transition zone temperatureDehydration melting could occur at the 520‐km discontinuity by upwelling flow near plumes [ABSTRACT FROM AUTHOR]

PERSPECTIVES EARTHSCIENCES SpecialTopic:KeyProblemsoftheDeepEarth Asthenosphere dynamics based on the H 2 O dependence of element diffusivity in olivine TomooKatsura 1,2,* andHongzhanFei 1 The oceanic asthenosphere is one of the most enigmatic regions in the Earth's interior. Because the C H 2 O exponent of Si grain boundary self-diffusivity is identical to that of Si lattice diffusivity within error (0.26 ± 0.07), that of pipe diffusivity should be identical to these values. [Extracted from the article]

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]

We discuss the experimental results of silicon and oxygen self-diffusion coefficients in forsterite and iron-bearing olivine from the perspective of defect chemistry. Silicon diffusion is dominated by V-associated V″″, whereas oxygen diffusion is dominated by hopping of V under anhydrous conditions, and by (OH) under hydrous conditions. By considering the charge neutrality condition of [(OH)] = 2[V″] in hydrous forsterite and iron-bearing olivine, we get D ∝ ( $$C_{{{\text{H}}_{2} {\text{O}}}}$$ ) and D ∝ ( $$C_{{{\text{H}}_{2} {\text{O}}}}$$ ), which explains the experimental results of water effects on oxygen and silicon self-diffusion rates (Fei et al. in Nature 498:213-215, ; J Geophys Res 119:7598-7606, ). The $$C_{{{\text{H}}_{2} {\text{O}}}}$$ dependence of creep rate in the Earth's mantle should be close to that given by Si and O self-diffusion coefficients obtained under water unsaturated conditions. [ABSTRACT FROM AUTHOR]

Fe,Al-bearing MgSiO3 perovskite (bridgmanite) is considered to be the most abundant mineral in Earth's lower mantle, hosting ferric iron in its structure as charge-coupled (Fe2O3 and FeAlO3) and vacancy components (MgFeO2.5 and Fe2/3SiO3). We examined concentrations of ferric iron and aluminum in the perovskite phase as a function of temperature (1700–2300 K) in the MgSiO3-FeAlO3-MgO system at 27 GPa using a multi-anvil high-pressure apparatus. We found a LiNbO3-structured phase in the quenched run product, which was the perovskite phase under high pressures and high temperatures. The perovskite phase coexists with corundum and a phase with (Mg,Fe3+,□)(Al,Fe3+)2O4 composition (□= vacancy). The FeAlO3 component in the perovskite phase decreases from 69 to 65 mol% with increasing temperature. The Fe2O3 component in the perovskite phase remains unchanged at ~1 mol% with temperature. The A-site vacancy component of Fe2/3SiO3 in the perovskite phase exists as 1–2 mol% at 1700–2000 K, whereas 1 mol% of the oxygen vacancy component of MgFeO2.5 appears at higher temperatures, although the analytical errors prevent definite conclusions. The A-site vacancy component might be more important than the oxygen vacancy component for the defect chemistry of bridgmanite in slabs and for average mantle conditions when the FeAlO3 charge-coupled component is dominant. [ABSTRACT FROM AUTHOR]

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]

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: 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]

Inelastic X-ray scattering (IXS) measurements have been performed on an MgO single crystal in order to evaluate IXS as a methodology for accurate and precise determination of elastic constants and sound velocities. By performing the IXS experiment using a 12-analyzer array, the complete set of single-crystal elastic constants of MgO were determined to a precision better than 0.8% (sound velocities to better than 0.2%). The results are consistent with values in the literature. The precision and accuracy of this work, which is significantly better than other published work to date, demonstrates the potential of IXS in determining elastic properties. [ABSTRACT FROM AUTHOR]

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]

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]

• 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]

The Clapeyron slope is the slope of a phase boundary in P–T space and is essential for understanding mantle dynamics and evolution. The phase boundary is delineating instead of balancing a phase transition's normal and reverse reactions. Many previous high pressure–temperature experiments determining the phase boundaries of major mantle minerals experienced severe problems due to instantaneous pressure increase by thermal pressure, pressure drop during heating, and sluggish transition kinetics. These complex pressure changes underestimate the transition pressure, while the sluggish kinetics require excess pressures to initiate or proceed with the transition, misinterpreting the phase stability and preventing tight bracketing of the phase boundary. Our recent study developed a novel approach to strictly determine phase stability based on the phase equilibrium definition. Here, we explain the details of this technique, using the post-spinel transition in Mg2SiO4 determined by our recent work as an example. An essential technique is to observe the change in X-ray diffraction intensity between ringwoodite and bridgmanite + periclase during the spontaneous pressure drop at a constant temperature and press load with the coexistence of both phases. This observation removes the complicated pressure change upon heating and kinetic problem, providing an accurate and precise phase boundary. [ABSTRACT FROM AUTHOR]

• Temperature and Fe content dependences H 2 O solubility in ringwoodite are obtained. • Both Fe-free and Fe-bearing ringwoodite can store 0.8∼1.2 wt.% H 2 O at 2000 K. • The high H 2 O solubility is compatible with the H 2 O-rich mantle transition zone. The mantle transition zone, a potential water reservoir in the Earth's interior, is suggested to contain more than 1 wt.% of H 2 O, at least locally, by the natural water-rich ringwoodite inclusion (Pearson et al., 2014) and the mineral viscosity (Fei et al., 2017). It is against the low H 2 O solubility of ringwoodite (<0.3 wt.%) at mantle transition zone temperature based on laboratory experiments (Ohtani et al., 2000). We revisited the H 2 O solubility in ringwoodite at 1600-2000 K, showing that both iron-free and iron-bearing ringwoodite can store about 0.8-1.2 wt.% of H 2 O under mantle transition zone conditions. Temperature has a relatively small effect on the H 2 O solubility in iron-bearing system, but much larger in iron-free one. The high H 2 O solubility of ringwoodite is compatible with the water-rich mantle transition zone. The relatively low values reported previously is probably due to insufficient H 2 O source or unbuffered by SiO 2. [ABSTRACT FROM AUTHOR]

Eulerian finite strain of an elastically isotropic body is defined using the expansion of squared length and the post-compression state as reference. The key to deriving second-, third- and fourth-order Birch–Murnaghan equations-of-state (EOSs) is not requiring a differential to describe the dimensions of a body owing to isotropic, uniform, and finite change in length and, therefore, volume. Truncation of higher orders of finite strain to express the Helmholtz free energy is not equal to ignoring higher-order pressure derivatives of the bulk modulus as zero. To better understand the Eulerian scheme, finite strain is defined by taking the pre-compressed state as the reference and EOSs are derived in both the Lagrangian and Eulerian schemes. In the Lagrangian scheme, pressure increases less significantly upon compression than the Eulerian scheme. Different Eulerian strains are defined by expansion of linear and cubed length and the first- and third-power Eulerian EOSs are derived in these schemes. Fitting analysis of pressure-scale-free data using these equations indicates that the Lagrangian scheme is inappropriate to describe P-V-T relations of MgO, whereas three Eulerian EOSs including the Birch–Murnaghan EOS have equivalent significance. [ABSTRACT FROM AUTHOR]

The role of stishovite in transporting water in subducting slabs has been a subject of debate for several decades. Here we investigated stishovite's water solubility and its potential role in water transportation as a function of temperature from 1300 to 2100°C at 22 GPa. Under water‐saturated conditions, high‐quality, Al‐free stishovite single crystals were synthesized with multi‐anvil press, and their water contents were measured using infrared spectroscopy. The H2O solubility in stishovite increases from 128(20) to 521(47) wt. ppm with increasing temperature from 1300 to 1700°C and decreased to 145(26) wt. ppm at 2100°C. The maximum H2O content was about seven times larger than earlier high‐temperature multi‐anvil studies but significantly lower than recent laser‐heated diamond anvil cell and low‐temperature multi‐anvil studies. We suggest that Al‐free stishovite may not be a significant H2O carrier in subducting slabs, at least at the topmost lower mantle corresponding to our experimental conditions. Plain Language Summary: Stishovite is a high‐pressure phase of silicon dioxide (SiO2). It was recently reported that stishovite could contain a substantial amount of water based on diamond anvil cell experiments and, consequently, transport water into the lower mantle through subducting slabs. However, the water contents in their samples lack precise constrain due to the limited size of the recovered crystals, making them inaccessible for measurements using reliable FTIR spectroscopy or secondary‐ion mass spectroscopy. To settle this debate, we produced high‐quality single crystals of stishovite under water‐saturated conditions at high pressures and temperatures and measured their water contents using infrared spectroscopy. We discovered that stishovite contained only several hundred ppm of water under conditions at the top of the lower mantle. Therefore, it is likely that stishovite does not contain much water in the upper region of the lower mantle and does not transport water into the deep lower mantle. Key Points: H2O content in Al‐free stishovite increases with temperature up to 1700°C and decreases beyond this temperatureThe maximum H2O content in Al‐free stishovite at 22 GPa is 521 (47) wt. ppmAl‐free stishovite cannot be the major H2O carrier at least up to the uppermost lower mantle conditions [ABSTRACT FROM AUTHOR]

As evidenced by isotope geochemistry, the persistence of primitive reservoirs indicates that the earth's lower mantle is likely to be heterogeneous. Such heterogeneity could be a legacy from magma‐ocean (MO) solidification. The viscosity of MO is a key parameter to constrain the solidification type of MO. Here we directly measure the viscosity of peridotite (an analog of MO composition) melt at the pressure‐temperature conditions of the deep mantle, using the in situ falling sphere method. The viscosity of peridotite melt along liquidus is in the range of 38–17 mPa s at pressures from 7 to 25 GPa, which is 0.9–0.4 times of the estimation based on the viscosity of endmember compositions. Low viscosity favors fractional solidification and chemically layering of the early mantle at least to the top lower mantle, which could be a source of heterogeneity for the present mantle. Plain Language Summary: The earth experiences a large‐scale melting and forms a deep magma ocean in its early history. The viscosity of peridotite melt is a key parameter for understanding the solidification type of magma ocean, which leads to the primitive mantle structure. This study directly measured the viscosity of peridotite melt to deep mantle conditions and revealed that peridotite melt has a lower value of viscosity than expected. The low viscosity of peridotite melt suggests a fractional solidification of the magma ocean, which supports a heterogeneous primitive mantle. Key Points: The viscosity of peridotite melt has been measured down to lower‐mantle conditions by in situ falling sphere viscometryPeridotite melt has a lower viscosity than expected from the viscosity of the endmembersThe low viscosity of peridotite melt suggests a fractional solidification in the magma ocean, supporting a heterogeneous primitive mantle [ABSTRACT FROM AUTHOR]

As evidenced by isotope geochemistry, the persistence of primitive reservoirs indicates that the earth's lower mantle is likely to be heterogeneous. Such heterogeneity could be a legacy from magma‐ocean (MO) solidification. The viscosity of MO is a key parameter to constrain the solidification type of MO. Here we directly measure the viscosity of peridotite (an analog of MO composition) melt at the pressure‐temperature conditions of the deep mantle, using the in situ falling sphere method. The viscosity of peridotite melt along liquidus is in the range of 38–17 mPa s at pressures from 7 to 25 GPa, which is 0.9–0.4 times of the estimation based on the viscosity of endmember compositions. Low viscosity favors fractional solidification and chemically layering of the early mantle at least to the top lower mantle, which could be a source of heterogeneity for the present mantle. Plain Language Summary: The earth experiences a large‐scale melting and forms a deep magma ocean in its early history. The viscosity of peridotite melt is a key parameter for understanding the solidification type of magma ocean, which leads to the primitive mantle structure. This study directly measured the viscosity of peridotite melt to deep mantle conditions and revealed that peridotite melt has a lower value of viscosity than expected. The low viscosity of peridotite melt suggests a fractional solidification of the magma ocean, which supports a heterogeneous primitive mantle. Key Points: The viscosity of peridotite melt has been measured down to lower‐mantle conditions by in situ falling sphere viscometryPeridotite melt has a lower viscosity than expected from the viscosity of the endmembersThe low viscosity of peridotite melt suggests a fractional solidification in the magma ocean, supporting a heterogeneous primitive mantle [ABSTRACT FROM AUTHOR]

Presents measurements of the electrical conductivity of perovskite in a multi-anvil press at conditions appropriate for the uppermost lower mantle. Explanation for the change in slope; Factors that may affect lower-mantle conductivity; Experimental results.

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]

High-temperature and high-pressure equations of state (EOSs) of Pt have been developed using measured shock compression data up to 290 GPa and volume thermal expansion data between 100 and nearly 2000 K and 0 GPa. The lattice thermal pressures at high temperatures have been estimated based on the Mie–Grüneisen relation with the Debye thermal model and the Vinet isothermal EOS. The contribution of electronic thermal pressure at high temperatures has also been included here. The optimized EOS parameters of Pt are K0T=273 GPa, K0T′=5.20, γ0=2.70, and q=1.10 with Θ0=230 K, where the subscript 0 refers to the ambient conditions. The temperature-pressure-volume (T-P-V) data of Pt have also been measured up to 1600 K and 42 GPa, using synchrotron powder x-ray diffraction experiments combined with a Kawai-type multianvil high pressure apparatus and sintered diamond anvils. We find that the newly developed T-P-V EOS of Pt is fully consistent with not only the shock compression data up to 290 GPa and volume thermal expansion data up to near 2000 K, but also the present measured synchrotron T-P-V data and recently measured T-P-V data of Pt up to 1900 K and 80 GPa. Thus we find that there is no need to include a volume dependence of q over a wide pressure range up to more than 300 GPa. The present EOS has been developed without any pressure scale. Such excellent consistency between the EOS and experimental values over wide temperature and pressure ranges shows that the present EOS can be used as a reliable primary pressure standard for static experiments up to 300 GPa and 3000 K. [ABSTRACT FROM AUTHOR]

Abstract The oxygen vacancy (OV, MgAlO 2.5) contents in bridgmanite with periclase and/or a calcium ferrite-type MgAl 2 O 4 –Mg 2 SiO 4 phase (CF-phase) were investigated at temperatures of 1700, 2000, and 2400 K and a pressure of 27 GPa using an ultrahigh-pressure multi-anvil press. The OV content increases significantly with increasing temperature by consuming coexisting periclase and the charge-coupled (CC, AlAlO 3) component in bridgmanite. The increase in OV component at high temperature is promoted by its high entropy. The CC components increase largely with temperature compared with the OV content in bridgmanite coexisting with CF-phase and periclase because the partition coefficient of Mg and Al between bridgmanite and CF-phase approaches unity because of the mixing entropy. The partial molar volumes of OV and CC bridgmanite endmembers at ambient conditions were constrained to be 26.64 and 25.79 cm3/mol, respectively, with densities of 3.428 and 3.955 g/cm3. By assuming linear relationships between wave velocities and density, the OV component may therefore cause a velocity reduction by one order of magnitude larger than the CC component at constant Al content in bridgmanite. The concomitant increase and decrease in OV and CC contents with temperature enhance V P and V S reductions by 0.7% and 2.5% in addition to anharmonic effects. The OV component in bridgmanite in subducted slabs is zero because of low temperature, and the transport of water and argon trapped by the OV component into the lower mantle by bridgmanite is therefore not expected. The storage capacity of these volatiles in subducted slabs will thus be significantly lower than the warm ambient mantle. Since the OV component is expected to enhance diffusion creep, the present results may explain slab stagnation in the mid-mantle due to low OV contents at low temperature, and vertically straight plumes in the lower mantle due to high OV contents at high temperature. Highlights • The oxygen vacancy in bridgmanite increases with temperature. • The increase of oxygen vacancy is promoted by its high entropy at high temperature. • Explanation of the volatile storage induced by bridgmanite in the lower mantle. • Explanation of slab stagnation and vertically straight plumes in the lower mantle. [ABSTRACT FROM AUTHOR]

The dissolution of water changes the chemical and physical properties of melts. The effect of water dissolution depends on the bulk H 2 O content and dissolution mechanisms. Previous research mainly focused on relatively polymerized glasses, whereas dissolution mechanisms of water in highly depolymerized glasses have not been well studied. We investigated three sets of hydrous (up to 5 wt% H 2 O), highly depolymerized glasses using Fourier transform infrared spectroscopy. The three suites of glasses can be classified as tephritic (non-bridging oxygens to tetrahedral cations, NBO/T ≈ 0.9), basanitic (NBO/T ≈ 1.5), and peridotitic (NBO/T ≈ 2.5). The comparison of infrared spectra indicated four prominent features. First, the combination band related to molecular H 2 O at ∼ 5200 cm−1 decreases in relative intensity with increasing depolymerization. At 2 wt% bulk H 2 O, the proportion of H 2 O molecules decreases from the tephritic through basanitic to peridotitic glasses, where the proportion of H 2 O molecules is below 10 %. At 5 wt% bulk H 2 O, the proportion of H 2 O molecules in the peridotitic glass is around 20 %, whereas it is around 50 % in the basaltic glass with the same H 2 O content. Second, a second peak appears at 4270–4180 cm−1, next to the (Si,Al)OH combination peak (network hydroxyl), with increasing CaO and MgO contents in basanitic (∼25 wt% CaO) and peridotitic (∼40 wt% MgO) glasses, respectively. We assign this peak to the (Mg,Ca)OH group (free hydroxyl); this is the first identification of this group based on infrared spectroscopy. This assignment is also supported by IR data on Mg(OH) 2 and Ca(OH) 2 crystalline phases from the literature. Third, the ratio of (Si,Al)OH and (Mg,Ca)OH peak absorbances remains nearly constant with increasing water content in basanitic and peridotitic glasses, suggesting no effect of water on polymerization of extremely depolymerized melts. Fourth, the (Si,Al)OH peak shifts to lower wavenumbers with increasing depolymerization. As a result, the (Si,Al)OH peak position could potentially be used to evaluate the degree of melt polymerization in general, and the effect of water on melt polymerization in particular. [ABSTRACT FROM AUTHOR]

Phase relations in the MgSiO3–MgAl2O4–Al2O3 system are investigated at 27 GPa and 2000–2600 K using a multi‐anvil apparatus. The AlAlO3 content in the bridgmanite increases from 8.6 to 20.0 mol% with increasing temperature from 2000 to 2600 K, while the MgAlO2.5 content remains nearly constant, that is, 3–4 mol% at these temperatures. Therefore, the presence of an additional Al‐bearing phase suppresses the oxygen vacancy substitution for Al3+ in bridgmanite. Conversely, significant amounts of the Al8/3O4 component can be dissolved into the calcium‐ferrite type MgAl2O4–Mg2SiO4–Al8/3O4 phase (CF‐phase), implying that the CF‐phase likely contains some amount of vacancies. Therefore, the CF‐phase could also be a candidate mineral for transporting volatiles into the lower mantle. Our results, combined with previous studies on Al‐bearing bridgmanite, indicate that, once the Al per formula unit exceeds 0.12 in bridgmanite, the MgAlO2.5 content remains nearly constant and the AlAlO3 component becomes dominant. Plain Language Summary: Bridgmanite, the most abundant mineral in the Earth's mantle, can contain Al3+ in the forms of MgAlO2.5 and AlAlO3. To constrain the substitution mechanism of Al3+ in bridgmanite, we investigate the MgAlO2.5 and AlAlO3 contents in bridgmanite coexisting with a calcium‐ferrite type phase and corundum at different temperatures. Our results demonstrate that the MgAlO2.5 content reaches saturation (3–4 mol%) when an additional Al‐bearing phase exists in the system. Key Points: The MgAlO2.5 content in bridgmanite remains constant with temperature when bridgmanite coexists with corundum and the CF‐phaseThe MgAlO2.5 content in bridgmanite reaches a solubility of 3–4 mol% when bridgmanite coexists with an additional Al‐bearing phaseA significant amount of the Al8/3O4 component can be dissolved into the CF‐phase [ABSTRACT FROM AUTHOR]

Penetrating, high‐energy synchrotron X‐rays are in strong demand, particularly for high‐pressure research in physics, chemistry and geosciences, and for materials engineering research under less extreme conditions. A new high‐energy wiggler beamline P61 has been constructed to meet this need at PETRA III in Hamburg, Germany. The first part of the paper offers an overview of the beamline front‐end components and beam characteristics. The second part describes the performance of the instrumentation and the latest developments at the P61B endstation. Particular attention is given to the unprecedented high‐energy photon flux delivered by the ten wigglers of the PETRA III storage ring and the challenges faced in harnessing this amount of flux and heat load in the beam. Furthermore, the distinctiveness of the world's first six‐ram Hall‐type large‐volume press, Aster‐15, at a synchrotron facility is described for research with synchrotron X‐rays. Additionally, detection schemes, experimental strategies and preliminary data acquired using energy‐dispersive X‐ray diffraction and radiography techniques are presented. [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]

Bridgmanite may contain a large proportion of ferric iron in its crystal structure in the forms of FeFeO3 and MgFeO2.5 components. We investigated the pressure dependence of FeFeO3 and MgFeO2.5 contents in bridgmanite coexisting with MgFe2O4‐phase and with or without ferropericlase in the MgO‐SiO2‐Fe2O3 ternary system at 2,300 K, 33 and 40 GPa. Together with the experiments at 27 GPa reported in Fei et al. (2020, https://doi.org/10.1029/2019GL086296), our results show that the FeFeO3 and MgFeO2.5 contents in bridgmanite decrease from 7.6 to 5.3 mol % and from 2 to 3 mol % to nearly zero, respectively, with increasing pressure from 27 to 40 GPa. Accordingly, the total Fe3+ decreases from 0.18 to 0.11 pfu. The formation of oxygen vacancies (MgFeO2.5 component) in bridgmanite is therefore dramatically suppressed by pressure. Oxygen vacancies can be produced by ferric iron in Fe3+‐rich bridgmanite under the topmost lower mantle conditions, but the concentration should decrease rapidly with increasing pressure. The variation of oxygen‐vacancy content with depth may potentially affect the physical properties of bridgmanite and thus affect mantle dynamics. Plain Language Summary: Bridgmanite is the most abundant mineral in the Earth's lower mantle. Although its basic chemical formula is MgSiO3, large amounts of Fe3+ can be added in the following two ways: (1) Two Fe3+ replace Mg2+ and Si4+ and form the FeFeO3 component. This is called charge‐coupled substitution because the overall charge does not change. (2) One Fe3+ replaces one Si4+ and forms the MgFeO2.5 component. Here the loss of positive charge is compensated by a loss of oxygen and is therefore called oxygen‐vacancy substitution. In this study, we measured the effect of pressure on the abundance of these two components of bridgmanite. We found that the MgFeO2.5 content decreases greatly with increasing pressure. Some oxygen sites may therefore be vacant in bridgmanite at the top of lower mantle, but the concentration of oxygen vacancies should decrease rapidly in deeper regions. The decrease of oxygen‐vacancy concentration in bridgmanite will change the nature of the lower mantle, for example, rocks will become harder, and electrical conductivity will decrease with increasing depth. Key Points: MgFeO2.5, FeFeO3, and total Fe3+ contents in bridgmanite decrease with increasing pressureFe3+‐linked oxygen vacancies in bridgmanite are destabilized by increasing pressureMgFeO2.5 can be formed in Fe3+‐rich bridgmanite under the topmost lower mantle conditions [ABSTRACT FROM AUTHOR]

We report a new rapid-quench technique for the Kawai-type multi-anvil press: several important improvements were made to our previous design. As a result, we are able to routinely quench melts with low glass-forming ability and form glasses. Owing to the use of 3D-printed parts to supply the coolant, the new design is easier to assemble and demonstrates better temperature stability and cooling rate. It was also found that the cooling rate is both pressure- and temperature-dependent. The cooling rate increases with increasing pressure from 6700 °C/s at 1 GPa to 8200 °C/s at 5.5 GPa and decreases with increasing temperature at a rate of 550 °C s−1/100 °C. Taking these dependencies into account, the new rapid-quench design produces more than 15% higher cooling rate compared to the previous design. Moreover, enhancing coolant circulation, which was achieved by using tapered inner anvils with holes, additionally increases the cooling rate by about 4%. As the structure of the rapid-quench assembly differs dramatically from other existing designs, pressure calibration and temperature distribution in the experimental cell and sample capsule were determined for the first time. It was found that the first 0.6 MN of press load is not used to generate pressure due to the hard tungsten components in the assembly. At the current state-of-the-art, it is possible to routinely reach a pressure of 9 GPa and a temperature of 2200 K with the temperature variation not exceeding 70 K within the sample capsule. [ABSTRACT FROM AUTHOR]

We found a new nickel sulfide that is isostructural with Fe3S. The synthesized nickel sulfide is a non-stoichiometric compound with a Ni deficiency and its composition is Ni2.90±0.5S. In situ synchrotron X-ray observations indicate that Ni3-xS forms above 5.1 GPa and melts incongruently into Ni+liquid up to 10 GPa. The bulk modulus of Ni3-xS at 300 K was determined to be 140 ± 2 GPa with a fixed K' = 4 by static compression with a liquid pressure medium. The eutectic point of the Ni-NiS system lies between Ni3S and Ni3S2 up to 10 GPa and its composition changes from Ni66.6S33.4 at 0.1 MPa to Ni70.7S29.3 at 10 GPa. The eutectic melting temperature of the Ni-NiS system decreases to 5.1 GPa and 720 K as the pressure increases where Ni and α-Ni3S2 are the eutectic solids. On the other hand, the eutectic melting temperature increases with a positive slope above 5.1 GPa where Ni3S+α-Ni3S2 is stable under subsolidus conditions. Intermediate compounds appear at a lower pressure in the Ni-NiS system than that for the Fe-FeS system. Ni3S2 is stable at atmospheric pressure and Ni3S forms at 5.1 GPa, whereas Fe3S2 and Fe3S form at 14 and 20 GPa, respectively. The addition of Ni complicates the melting relationship in the Fe-FeS system at high pressure because of the wider stability field of Ni3S. The low-melting temperature of the Fe-Ni-S system plays an important role in the percolative core-formation of planetesimals during planetary accretion. [ABSTRACT FROM AUTHOR]

To determine the phase boundary between the perovskite and post-perovskite structures in MnGeO3, in situ X-ray observations were carried out at pressures of 57-68 GPa and temperatures of 1000-1900 K using the Kawai-type high-pressure apparatus equipped with sintered diamond anvils interfaced with synchrotron radiation. The phase boundary was determined to be P (GPa) = 39.2 + 0.013T (K) based on Tsuchiya's (2003) gold pressure scale. The Clapeyron slope, dP/dT, of 13(+12/-5) MPa/K, determined in the present study is larger that of MgGeO3 and MgSiO3. [ABSTRACT FROM AUTHOR]

Abstract: By applying the numerical tensor analysis proposed by Yoneda and Kubo [Simultaneous determination of mean pressure and deviatoric stress based on numerical tensor analysis: a case study for polycrystalline X-ray diffraction of gold enclosed in a methanol–ethanol mixture. J. Phys.: Condens. Matter 18(2006)S979], we have determined deviatoric stress and mean pressure of polycrystalline MgO compressed in the Kawai-type apparatus. After the compression at room temperature, the mean pressure and the deviatoric stress in MgO were determined as 34.5 and 3.2GPa, respectively. The mean pressure is significantly lower than the nominal pressure of 37.2GPa determined by the conventional pressure determination method. By heating the sample to 1850K with the constant press load, the deviatoric stress dramatically decreased to 0.1GPa with a mean pressure of 34.2GPa at room temperature after the heating. These results show both the importance of stress analysis to determine pressure more accurately and the effectiveness of annealing to reduce deviatoric stress in the sample. [Copyright &y& Elsevier]

We investigated phase relations in pyrolite at ∼33-40 GPa and ∼1800-2150 K by in situ X-ray diffraction using Kawai-type apparatus with sintered diamond anvils. The results demonstrated that MgSiO3-rich orthorhombic perovskite (mpv), CaSiO3-rich cubic perovskite (cpv) and (Mg,Fe)O ferropericlase (fp) are the stable phases in pyrolite bulk composition at the conditions corresponding to the lower mantle. However, chemical analyses of a run product recovered from ∼ 34 GPa by an analytical transmission electron microscope showed the coexistence of metallic iron particles with mpv, fp, and SiO2-rich amorphous phase. Also, Fe/Mg partitioning coefficient between mpv and fp was found to be 0.66(31), which is consistent with previous results for pyrolite bulk composition at 26-30 GPa and ∼1900 K. These results indicate that iron-rich metallic particles can exist in the lower mantle as a stable phase to the depth of at least ∼900 km. [ABSTRACT FROM AUTHOR]

Pressure–temperature conditions of tourmaline breakdown in a metapelite were determined by high-pressure experiments at 700–900°C and 4–6 GPa. These experiments produced an eclogite–facies assemblage of garnet, clinopyroxene, phengite, coesite, kyanite and rare rutile. The modal proportions of tourmaline clearly decreased between 4.5 and 5 GPa at 700°C, between 4 and 4.5 GPa at 800°C, and between 800 and 850°C at 4 GPa, with tourmaline that survived the higher temperature conditions appearing corroded and thus metastable. Decreases in the modal abundance of tourmaline are accompanied by decreasing modal abundance of coesite, and increasing that of clinopyroxene, garnet and kyanite; the boron content of phengite increases significantly. These changes suggest that, with increasing pressure and temperature, tourmaline reacts with coesite to produce clinopyroxene, garnet, kyanite, and boron-bearing phengite and fluid. Our results suggest that: (1) tourmaline breakdown occurs at lower pressures and temperatures in SiO2-saturated systems than in SiO2-undersaturated systems. (2) In even cold subduction zones, subducting sediments should release boron-rich fluids by tourmaline breakdown before reaching depths of 150 km, and (3) even after tourmaline breakdown, a significant amount of boron partitioned into phengite could be stored in deeply subducted sediments. [ABSTRACT FROM AUTHOR]

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 phase relations of iron-rich olivine and its high-pressure polymorphs are important for planetary science and meteoritics because these minerals are the main constituents of terrestrial mantles and meteorites. The olivine–ahrensite binary loop was previously determined by thermochemical calculations in combination with high-pressure experiments; however, the transition pressures contained significant uncertainties. Here we determined the binary loop of the olivine–ahrensite transition in the (Mg,Fe)2SiO4 system at 1740 K in the pressure range of 7.5–11.2 GPa using a multi-anvil apparatus with the pressure determined using in situ X-ray diffraction, compositional analysis of quenched run products, and thermochemical calculation. Based on the determined binary loop, a user-friendly software was developed to calculate pressure from the coexisting olivine and ahrensite compositions. The software is used to estimate the shock conditions of several L6-type chondrites. The obtained olivine–ahrensite phase relations can also be applied for precise in-house multi-anvil pressure calibration at high temperatures. [ABSTRACT FROM AUTHOR]

We attempted to generate ultrahigh pressure and temperature simultaneously using a multi-anvil apparatus by combining the technologies of ultrahigh-pressure generation using sintered diamond (SD) anvils, which can reach 120 GPa, and ultrahigh-temperature generation using a boron-doped diamond (BDD) heater, which can reach 4000 K. Along with this strategy, we successfully generated a temperature of 3300 K and a pressure of above 50 GPa simultaneously. Although the high hardness of BDD significantly prevents high-pressure generation at low temperatures, its high-temperature softening allows for effective pressure generation at temperatures above 1200 K. High temperature also enhances high-pressure generation because of the thermal pressure. We expect to generate even higher pressure in the future by combining SD anvils and a BDD heater with advanced multi-anvil technology. [ABSTRACT FROM AUTHOR]

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]

The tungsten isotope composition of meteorites indicates that core formation in planetesimals occurred within a few million years of Solar System formation. But core formation requires a mechanism for segregating metal, and the 'wetting' properties of molten iron alloy in an olivine-rich matrix is thought to preclude segregation by permeable flow unless the silicate itself is partially molten. Excess liquid metal over a percolation threshold, however, can potentially create permeability in a solid matrix, thereby permitting segregation. Here we report the percolation threshold for molten iron-sulphur compounds of approximately 5 vol.% in solid olivine, based on electrical conductivity measurements made in situ at high pressure and temperature. We conclude that heating within planetesimals by decay of shortlived radionudides can increase temperature sufficiently above the iron-sulphur melting point (∼1,000 °C) to trigger segregation of iron alloy by permeable flow within the short timeframe indicated by tungsten isotopes. We infer that planetesimals with radii greater than about 30 km and larger planetary embryos are expected to have formed cores very early, and these objects would have contained much of the mass in the terrestrial region of the protoplanetary nebula. The Earth and other terrestrial planets are likely therefore to have formed by accretion of previously differentiated planetesimals, and Earth's core may accordingly be viewed as a blended composite of preformed cores. [ABSTRACT FROM AUTHOR]

• The grain size exponent for growth of bridgmanite in post-spinel is 5.2 ± 0.3. • The activation enthalpy for grain growth of bridgmanite is 260 ± 20 kJ/mol. • Grain size of bridgmanite is ∼30-45 μm in cold subducted slabs with 1-10 Myr age. The grain size of minerals is an important parameter that controls mantle dynamics. We investigated the grain growth kinetics of bridgmanite that coexists with ferropericlase by multi-anvil experiments at 27 GPa, 1400-2400 K, corresponding to the topmost lower mantle conditions. The experimental results indicate that the grain size of bridgmanite systematically increases with increasing duration, with a grain size exponent of 5.2 ± 0.3. The grain growth rate increases with increasing temperature with an activation enthalpy of 260 ± 20 kJ/mol. The grain size of bridgmanite is ∼30-45 μm in young subducted slabs at the topmost lower mantle conditions. The small grain size in subducted slabs may significantly reduce their creep strength in relatively to ambient mantle, and may cause the slab stagnation at the topmost lower mantle. [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]

• Oxygen fugacity exhibits minimal variation in the upper lower mantle. • Bridgmanite Fe3+/∑Fe ratios decrease with increasing temperature. • Oxygen vacancies in Fe-Al bearing bridgmanite increase with increasing temperature. Bridgmanite, the most abundant mineral in the lower mantle, readily incorporates Fe3+ even under very reducing conditions where it is in equilibrium with metallic iron. This has led to the proposal that charge disproportionation of Fe2+ to produce Fe3+ and metallic iron may occur in the lower mantle. The amount of Fe3+ in bridgmanite is important for understanding the oxidation state of the mantle, as charge disproportionation of Fe2+ to form Fe3+ and metallic iron, followed by loss of metallic iron to the core, could have raised the Fe3+ content of the mantle as a whole after core formation. Here, the Fe oxidation state in bridgmanite was studied as a function of temperature at 27 GPa and an oxygen fugacity of ∼2.0 log units above the iron-wüstite oxygen buffer (ΔIW+2) using multi-anvil techniques. Bridgmanite Fe3+/∑Fe ratios were determined by Mössbauer spectroscopy and decrease from ∼80% at 1800 K to 50% at 2600 K. A decreasing Fe3+/∑Fe ratio with increasing temperature indicates that the level of charge disproportionation would have been lower under the higher temperature conditions of the early Earth. However, if the redox state of the upper lower mantle is buffered by coexisting iron and ferropericlase, then the relative oxygen fugacity will have changed very little during secular cooling. [ABSTRACT FROM AUTHOR]

We have developed a high-pressure furnace assembly with a commercially available chemical-vapor-deposition synthesized boron-doped diamond heater consisting of four strips for large-volume multi-anvil presses (LVPs). This assembly successfully generated temperatures up to 2990 K at 15 GPa. It also has highly reproducible power–temperature relations, enabling us to estimate temperature from power reliably. It can be used for experiments above 9 GPa and is particularly useful for synchrotron x-ray experiments because of the x-ray transparency. It is also competitive in price. This technique is, thus, practical in various LVP experiments in the diamond-stability field. [ABSTRACT FROM AUTHOR]

We tested boron-doped diamond (BDD) synthesized by chemical vapor deposition (CVD) as a heating element in a multi-anvil apparatus. We succeeded in manufacturing BDD into a tubular shape by laser cutting and electric discharging machining. The BDD tube shaped by the electric discharging machining was contaminated by discharging electrode materials (Mo and W), which affected the heating performance. The laser-cut BDD tube has a clean surface and, therefore, had a good heating performance. We succeeded in generating temperature as high as 2670 K at a pressure around 30 GPa with laser-cut heater. Heating reproducibility was confirmed through repeated heating and cooling cycles. The recovered sample shows that a higher temperature generation above 2670 K was prevented by eutectic melting of ZrO2 thermal insulator and Al2O3 sample. Owing to the commercial availability with a reasonable price, CVD–BDD heaters are more practical than a high-pressure synthesized BDD heaters for wide applications. [ABSTRACT FROM AUTHOR]