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Abstract Earth's accretion was highly energetic and likely involved multiple global melting events. Following the Moon‐forming giant impact, extensive mantle melting and the separation of solids and melts under deep mantle pressures likely produced a basal magma ocean (BMO) beneath the solidified mantle. The presence and evolution of the BMO have been proposed to explain key geophysical and geochemical features of the lowermost mantle. Understanding the evolution of the BMO is crucial for testing these hypotheses, but its interaction with the core presents a significant challenge, as the mechanism of this exchange remains unclear. In this study, we develop a theoretical framework to assess the regime of BMO‐core exchange based on the compositions of the BMO and the core. We propose that during solidification, the BMO may evolve into a regime where the reaction at the BMO‐core interface drives compositional convection in liquids on both sides, if the core has a high enough Si content (–, under the assumption that the O content is –). In this scenario, the BMO‐core exchange would be much more efficient than previously estimated, buffering the tendency of FeO enrichment during crystallization and shortening the lifetime of the BMO. 

Talc is expected to be an important water carrier in Earth's upper mantle, and understanding its electrical and seismic properties under high pressure and temperature conditions is required to detect possible talc‐rich regions in subduction zones imaged using geophysical observations. We conducted acoustic and electrical experiments on natural talc aggregates at relevant pressure‐temperature conditions. Compressional wave velocity (Vp) was measured using ultrasonic interferometry in a Paris‐Edinburgh press at pressures up to 3.4 GPa and temperatures up to 873 K. Similar Vp values are obtained regardless of the initial crystallographic preferred orientation of the samples, which can be explained by talc grain reorientation during the experiment, with the (001) plane becoming perpendicular to the uniaxial compression axis. Electrical conductivity of the same starting material was determined using impedance spectroscopy in a multi‐anvil press up to 6 GPa and 1263 K. Two conductivity jumps are observed, at ∼860–1025 K and ∼940–1080 K, depending on pressure, and interpreted as talc dehydroxylation and decomposition, respectively. Electrical anisotropy is observed at low temperature and decreases with increasing pressure (∼10 at 1.5 GPa and ∼2 at 3.5 GPa). Comparison of acoustic and electrical results with geophysical observations in central Mexico supports the presence of a talc‐bearing layer atop the subducted Cocos plate. 

Abstract Metasomatized mantle xenoliths containing hydrous minerals, such as amphiboles, serpentine, and phlogopite, likely represent the potential mineralogical compositions of the metasomatized upper mantle, where low seismic velocities are commonly observed. This study presents the first experimentally determined single‐crystal elasticity model of an Fe‐free near Ca, Mg‐endmember amphibole tremolite at high pressure and/or temperature conditions (maximum pressure 7.3(1) GPa, maximum temperature 700 K) using Brillouin spectroscopy. We found that sound velocities of amphiboles strongly depend on the Fe content. We then calculated the sound velocities of 441 hydrous‐mineral‐bearing mantle xenoliths collected around the globe, and quantitatively evaluated the roles that amphiboles, phlogopite and serpentine played in producing the low velocity anomalies in the metasomatized upper mantle. 

The cratonic lithosphere is a vast host for deep recycled carbon, trapping up to several weight percent CO₂ at depths overlapping the seismic mid-lithospheric discontinuities (MLDs). However, the role of carbonates, especially for the latest discovered amorphous calcium carbonate (CaCO₃), is underestimated in the formation of MLDs. Using the pulse-echo-overlap method in a Paris-Edinburgh press coupled with synchrotron X-ray diffraction, we explored the acoustic velocities of CaCO₃ under high pressure-temperature (P-T) conditions relevant to the cratonic lithosphere. Two anomalous velocity drops were observed associated with the phase transition from aragonite to amorphous phase and with the pressure-induced velocity drop in the amorphous phase around 3 GPa, respectively. Both drops are comparable with approximately 35% and 52% reductions for compressional (V_P) and shear (V_S) wave velocities, respectively. The V_P and V_S values of the amorphous CaCO₃ above 3 GPa are about 1/2 and 1/3 of those of the major upper-mantle minerals, respectively. These velocity reductions caused by the presence of CaCO₃ would readily cause MLDs at depths of 70–120 km dependent on the geotherm even if only 1–2 vol.% CaCO₃ is present in the cratonic lithosphere. 

The roles of unforgiving H 2 SO 4 solvent in CH 4 activation with molecular catalysts have not been experimentally well-illustrated despite computational predictions. Here, we provide experimental evidence that metal-bound bisulfate ligand introduced by H 2 SO 4 solvent is redox-active in vanadium-based electrocatalytic CH 4 activation discovered recently. Replacing one of the two terminal bisulfate ligands with redox-inert dihydrogen phosphate in the pre-catalyst vanadium (V)-oxo dimer completely quenches its activity towards CH 4 , which may inspire environmentally benign catalysis with minimal use of H 2 SO 4 . 

Abstract As a major nominally anhydrous mineral (NAM) in the Earth’s upper mantle, orthopyroxene could host up to several hundred parts per million H2O in its crystal structure and transport the H2O to the deep Earth. To study the effect of structural H2O on the elasticity of orthopyroxene, we have measured the single-crystal elasticity of Mg1.991Al0.065Si1.951O6 with 842–900 ppm H2O and 1.64 ± 0.20 wt% Al2O3 at ambient conditions using Brillouin spectroscopy. The best-fit single-crystal elastic moduli (Cijs), bulk (KS0), and shear (G0) modulus of the hydrous Al-bearing orthopyroxene were determined as: C11 = 235(2) GPa, C22 = 173(2) GPa, C33 = 222(2) GPa, C44 = 86(1) GPa, C55 = 82(1) GPa, C66 = 82(1) GPa, C12 = 75(3) GPa, C13 = 67(2) GPa, and C23 = 49(2) GPa, KS0 = 111(2) GPa, and G0 = 78(1) GPa. Systematic analysis based on the results presented in this and previous studies suggests that the incorporation of 842–900 ppm H2O would increase C13 by 12.0(7)% and decrease C23 by 8.6(8)%. The effects on C11, C22, C33, C44, C66, KS0, and VP are subtle if not negligible when considering the uncertainties. The C55, C12, G0, and VS are not affected by the presence of structural H2O. Although laboratory experiments show that Fe,Al-bearing orthopyroxenes can host up to 0.8 wt% H2O in its structure, future high-pressure-temperature elasticity measurements on orthopyroxene with higher H2O content are needed to help better quantify this effect. 

Abstract The mantle transition zone connects two major layers of Earth’s interior that may be compositionally distinct: the upper mantle and the lower mantle. Wadsleyite is a major mineral in the upper mantle transition zone. Here, we measure the single-crystal elastic properties of hydrous Fe-bearing wadsleyite at high pressure-temperature conditions by Brillouin spectroscopy. Our results are then used to model the global distribution of wadsleyite proportion, temperature, and water content in the upper mantle transition zone by integrating mineral physics data with global seismic observations. Our models show that the upper mantle transition zone near subducted slabs is relatively cold, enriched in wadsleyite, and slightly more hydrated compared to regions where plumes are expected. This study provides direct evidence for the thermochemical heterogeneities in the upper mantle transition zone which is important for understanding the material exchange processes between the upper and lower mantle.