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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. 

Abstract Understanding the natural petrologic and tectonic processes, such as slab subduction, requires accurate determination of the metamorphic pressure (P) and temperature (T) conditions of the exhumed rocks. Recent development of elastic thermobarometry based on the difference between the thermoelastic properties of mineral inclusions and their hosts allows constraining the formation P-T conditions of the inclusion/host system without assuming thermodynamic equilibrium. However, the reliability of elastic thermobarometry heavily depends on the accurate determination of the high P-T single-crystal elastic properties of relevant minerals. In this study, we performed the first high P-T single-crystal elasticity measurements of zircon, which is one of the most common inclusion/host minerals in metamorphic rocks, using Brillouin spectroscopy at pressures up to 7 GPa and temperatures up to 700 K. The single-crystal elastic properties of zircon at ambient conditions were determined as: ρ0 = 4.6674(4) g/cm3; KS = 231(3) GPa; G = 108(11) GPa; C11 = 428(2) GPa; C33 = 484(3) GPa; C44 = 113.5(8) GPa; C66 = 46.9(8) GPa; C12 = 75(2) GPa; and C13 = 151(1) GPa. We also obtained the P and T derivatives of the elastic moduli as ∂KS0/∂P = 5.0(2), ∂KS0/∂T = −0.019(2) GPa/K, ∂G0/∂P = 0.73(6), and ∂G0/∂T = −0.011(1) GPa/K. Compared with other common rock-forming minerals (e.g., quartz, pyroxene, olivine), zircon is extremely incompressible (high KS) and its stiffness (KS) has relatively small T dependence, making it an ideal mineral for the application of elastic thermobarometry. Finally, we utilized the high P-T single-crystal elasticity data of zircon and garnet to evaluate the potential applications of zircon-in-garnet in elastic thermobarometry via elastic modeling. 

Geophysical detection of subducted mid–ocean ridge basalt (MORB) in the lower mantle is hindered by uncertainties in the elasticity of Fe,Al,Mg,Ti–bearing davemaoite, a key MORB component. Using Brillouin spectroscopy and x-ray diffraction, we determined the elasticity of a Ca_0.906(1)Fe²⁺_0.027(1)Fe³⁺_0.042(1)Mg_0.033(1)Al_0.072(1)Ti_0.020(1)Si_0.912(1)O₃davemaoite up to 113 gigapascals and 2294 K. We found that it exhibited a shear wave velocity 10 to 20% slower than end-member davemaoite, making it the slowest phase among major lower-mantle minerals. Our models show that MORB, containing 20 to 25 volume percent davemaoite, potentially contributes to large low-shear-velocity provinces (LLSVPs), whereas a cumulate layer enriched in davemaoite crystallized from basal magma ocean may comprise ultralow-velocity zones (ULVZs). Davemaoite’s ability to host incompatible and heat-producing elements possibly links LLSVPs and ULVZs to mantle plume initiation and geochemical signatures of ocean island basalts. 

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. 

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. 

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 .