Graphite capsules are commonly used in high-temperature, high-pressure experiments, particularly for nominally anhydrous experiments and iron-bearing silicate samples. Due to the presence of graphite in the sample assembly, the oxygen fugacity for these experiments is thought to be relatively low, typically at or below the graphite-CO-CO2 buffer (CCO). The detailed mechanism and kinetics of redox control in graphite capsule experiments are, however, poorly understood. This is especially problematic for short duration experiments (e.g. kinetic experiments), because it is uncertain whether the experimental product will preserve its initial oxygen fugacity, or become reduced during the experiment. In this study, a set of basaltic glasses after high-temperature experiments in graphite capsules were analyzed by micro X-ray absorption near-edge structure (mu-XANES) to obtain their Fe3+/sigma Fe profiles near the graphite-melt interface. The results show rapid reduction of ferric iron in the basaltic melt, reaching near-equilibrium in half an hour for samples of 2 mm diameter and 1.3-1.9 mm thickness. Even for a "time-zero" experiment, which was quenched immediately after reaching the target temperature, the reduction profile is over 100 mu m in length. By comparing experiments at the same temperature and pressure but with different durations, the reduction reaction progress is found to be linear to the square root of duration, indicating that the reduction process is diffusion-controlled. Such a rapid reduction of the basaltic melt requires a mechanism that is significantly faster than divalent cation diffusion or oxygen diffusion, and is best explained by molecular hydrogen diffusion. It has been shown by previous studies that nominally anhydrous high-pressure experiments could contain significant amounts of water. Thousands of ppm of H2O could remain in the graphite capsule even after drying at 120 degrees C for an extended time period. At high temperatures, H2O reacts with graphite to produce molecular hydrogen, which then diffuses into the basaltic glass and causes reduction. This mechanism is also supported by a compensating H2O profile of equivalent length in the basaltic glass, showing evidence for H2O produced by molecular hydrogen reacting with ferric iron. A quantitative model is proposed and it successfully reproduces the Fe3+/sigma Fe profiles in our experiments. The model helps explain the kinetics of the reduction process and demonstrates that for a basaltic glass with reasonable initial FeO* content, Fe3+/sigma Fe ratios, and thicknesses, the equilibrium oxidation state can usually be reached in one hour at similar to 1300 degrees C and similar to 0.5 GPa. Although extrapolating our conclusion to the large range of graphite capsule experiments requires knowledge on how H-2 solubility and diffusivity varies as a function of silicate composition, temperature, and pressure, the reduction process is expected to be rapid in general because H-2 diffusivity is high in silicate melts. Our study elucidates the mechanism and rate of oxygen fugacity change in graphite capsule experiments. Based on thermodynamic calculations, the reaction between graphite capsule and H2O is expected to produce a C-O-H fluid with an intrinsic oxygen fugacity of CCO -0.

High-pressure COH fluids have a fundamental role in a variety of geological processes. Their composition in terms of volatile species can control the solidus temperature and carbonation/decarbonation reactions, as well as influence the amount of solutes generated during fluid-rock interaction at depth. Over the last decades, several systems have been experimentally investigated to unravel the effect of COH fluids at upper-mantle conditions. However, fluid composition is rarely tackled as a quantitative issue, and rather infrequently fluids are analyzed in the same way as the associated solid phases in the experimental assemblage. A comprehensive characterization of carbon-bearing aqueous fluids in terms of composition is hampered by experimental difficulties in synthetizing and analyzing high-pressure fluids without altering their composition upon quenching.

Several characteristics of a planet, including its internal dynamics, hinge on the composition and crystallization regime of the core, which, in turn, depends on the phase relations, melting behaviour and thermodynamic properties of constituent materials. The Fe-Si-C ternary system can serve as a proxy for core composition and formation processes under reducing conditions. We conducted laser-heated diamond anvil cell experiments coupled with in situ X-ray diffraction and electron microscopy analysis of the recovered samples, on four different starting compositions in the Fe-Si-C ternary system. Phase relations up to 200 GPa and up to 4000 K were determined. An FeSi phase with a B2 structure and iron carbides with different stoichiometries (i.e., Fe3C and Fe7C3) are the main observed phases, along with pure C (diamond) that has an extended stability field in the subsolidus regime. Carbon is largely soluble in B2-structured FeSi, whereas Si does not partition into the carbides. The melting curve determined for the starting material containing the least amount of light elements is consistent with the one for the Fe-C system. The other starting materials display higher melting temperatures than that of Fe-C, suggesting the existence of at least two different invariant points in the Fe-Si-C system. Applied to planetary interiors, our observations highlight how a small variation in light elements content would deeply affect the solidification style of a core. Bottom-up (Fe-enriched systems) and top-down regimes (C-rich systems), as well as solidification of a crystal mush (Si-enriched systems). These three crystallization regimes influence significantly the possibility of starting and sustaining a dynamo. Our results provide new insights into the differentiation of terrestrial planets in the Solar System and beyond, contributing to the study of planetary diversity. (C) 2022 Elsevier Ltd. All rights reserved.

The electrical resistivity of solid and liquid Cu and Au were measured at high pressures from 6 up to 12 GPa and temperatures similar to 150 K above melting. The resistivity of the metals was also measured as a function of pressure at room temperature. Their resistivity decreased and increased with increasing pressure and temperature, respectively. With increasing pressure at room temperature, we observed a sharp reduction in the magnitude of resistivity at similar to 4 GPa in both metals. In comparison with 1 atm data and relatively lower pressure data from previous studies, our measured temperature-dependent resistivity in the solid and liquid states show a similar trend. The observed melting temperatures at various fixed pressure are in reasonable agreement with previous experimental and theoretical studies. Along the melting curve, the present study found the resistivity to be constant within the range of our investigated pressure (6-12 GPa) in agreement with the theoretical prediction. Our results indicate that the invariant resistivity theory could apply to the simple metals but at higher pressure above 5 GPa. These results were discussed in terms of the saturation of the dominant nuclear screening effect caused by the increasing difference in energy level between the Fermi level and the d-band with increasing pressure.

On compression of alpha-cristobalite SiO2 to pressures above approximately 12 GPa, a new polymorph known as cristobalite X-I forms. The existence of cristobalite X-I has been known for several decades; however, consensus regarding its exact atomic arrangement has not yet been reached. The X-I phase constitutes an important step in the silica densification process, separating low-density tetrahedral framework phases from high-density octahedral polymorphs. It is the only nonquenchable high-density SiO(2 )phase, which reverts to the low-density form on decompression at ambient temperature. Recently, an experimental study proposed an octahedral model of SiO2 X-I with intrinsic structural defects involving partial Si site occupancies. In contrast, our new single-crystal synchrotron X-ray diffraction experiments have shown that the ideal structure of this phase should instead be described by a defect-free model, which does not require partial occupancies. The structure of cristobalite X-I consists of octahedral chains with a 4-60 degrees-2 zigzag chain geometry. This geometry has not been previously considered but is closely related to post-quartz, stishovite, and seifertite. In addition to the ideal, defect-free crystal structure, we also present a description of the defects that are most likely to form within the X-I phase. Density functional theory calculations support our observations, confirming the dynamic stability of the X-I geometry and reasonably reproducing the pressure of the phase transformation. The enthalpy of cristobalite X-I is higher than stishovite and seifertite, but X-I is favored as a high-pressure successor of cristobalite due to a unique transformation pathway. Elastic and lattice dynamical properties of the X-I phase show intermediate values between stable tetrahedral and octahedral polymorphs, confirming the bridge-role of this phase in SiO2 densification.

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.

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.

Titanium (Ti) isotopes are emerging as a power tool for studying magmatic processes on the Earth and other planets. Pioneering studies carried out bulk-rock Ti isotopic measurements by conventional solution nebulization multi-collector inductively coupled plasma mass spectrometry (SN-MC-ICP-MS) and in situ Ti isotopic analysis via secondary ionization mass spectrometry (SIMS), which sacrificed spatial resolution and had relatively low analytical precision, respectively. In this work, a novel and robust method for in situ Ti isotopic analysis of titanium-bearing minerals (i.e., rutile and ilmenite) was presented, based on a femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometer (fs-LA-MC-ICP-MS). Very stable isotopic signals can be ensured after careful optimization of the parameters of fs-LA (e.g., fluence, spot size, and frequency), and thus a high analytical precision has been obtained for Alfa-Ti (an ultrapure Ti metal rod) under a high spatial resolution of spot diameter = 30 mu m. The within run and external reproducibility for delta Ti-49/47 (Ti-49/Ti-47 isotopic ratio, reported as delta Ti-49/47 notation, in parts in 10(3)) measurement on Alfa-Ti are 0.05 parts per thousand (2SE, internal precision of within-spot analysis) and 0.07 parts per thousand (2SD, external reproducibility of spot-to-spot analysis), respectively. A series of titanium-bearing minerals, including five potential rutile U-Pb chronological standards and four potential ilmenite Fe isotope standards, were assessed for Ti isotopic homogeneity on a 30 mu m-scale and precision of the measurement. The most homogeneous minerals were subsequently used to comprehensively evaluate the analytical accuracy and potential matrix effect. Our results show that in situ Ti isotopic analysis is susceptible to matrix effects when using fs-LA and accurate delta Ti-49/47(OL-Ti) values (calibrated against the OL-Ti reference material developed in the Origins Laboratory of the University of Chicago) can be obtained when a matrix-matched reference material is used as a bracketing standard. Therefore, well characterized matrix-matched reference materials are necessary for in situ Ti isotopic analysis. KNW rutile and PZH12-15 ilmenite characterized in this study show potential as suitable reference materials for micro-beam Ti isotopic analysis.