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1. Phase Relations of a Depleted Peridotite Fluxed by a CO ₂ ‐H ₂ O Fluid—Implications for the Stability of Partial Melts Versus Volatile‐Bearing Mineral Phases in the Cratonic Mantle

   https://doi.org/10.1029/2019JB017653  

   Saha, Sriparna; Dasgupta, Rajdeep (October 2019, Journal of Geophysical Research: Solid Earth)

   Abstract We present phase‐equilibria experiments of a K‐bearing, depleted peridotite (Mg# 92) fluxed with a mixed CO₂‐H₂O fluid (0.5 wt.% CO₂and 0.94 wt.% H₂O in the bulk) to gain insight into the stability of volatile‐bearing partial melts versus volatile‐bearing mineral phases in a depleted peridotite system. Experiments were performed at 850–1150 °C and 2–4 GPa using a piston‐cylinder and a multianvil apparatus. Olivine, orthopyroxene, clinopyroxene, and spinel/garnet are present at all experimental conditions. Textural confirmation of partial melt is made at temperatures as low as 1000 °C at 2 GPa, 950 °C at 3 GPa, and 1000 °C at 4 GPa marking the onset of melting at 900–1000 °C at 2 GPa, 850–950 °C at 3 GPa, and 950–1000 °C at 3 GPa. Phlogopite and magnesite breakdown at 900–1000 °C at 2 GPa, 950–1000 °C at 3 GPa, and 1000–1050 °C at 4 GPa. Comparison with previously published experiments in depleted peridotite system with identical CO₂‐H₂O content introduced via a silicic melt show that introduction of CO₂‐H₂O as fluid lowers the temperature of phlogopite breakdown by 150–200 °C at 2–4 GPa and stabilizes partial melts at lower temperatures. Our study thus, shows that the volatile‐bearing phase present in the cratonic mantle is controlled by bulk composition and is affected by the process of volatile addition during craton formation in a subduction zone. In addition, volatile introduction via melt versus aqueous fluid, leads to different proportion of anhydrous phases such as olivine and orthopyroxene. Considering the agent of metasomatism is thus critical to evaluate how the bulk composition of depleted peridotite is modified, leading to potential stability of volatile‐bearing phases as the cause of anomalously low shear wave velocity in mantle domains such as mid lithospheric discontinuities beneath continents. 

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2. Phase transition kinetics of superionic H2O ice phases revealed by Megahertz X-ray free-electron laser-heating experiments

   https://doi.org/10.1038/s41467-024-52505-0  

   Husband, R_J; Liermann, H_P; McHardy, J_D; McWilliams, R_S; Goncharov, A_F; Prakapenka, V_B; Edmund, E.; Chariton, S.; Konôpková, Z.; Strohm, C.; et al (September 2024, Nature Communications)

   Abstract H₂O transforms to two forms of superionic (SI) ice at high pressures and temperatures, which contain highly mobile protons within a solid oxygen sublattice. Yet the stability field of both phases remains debated. Here, we present the results of an ultrafast X-ray heating study utilizing MHz pulse trains produced by the European X-ray Free Electron Laser to create high temperature states of H₂O, which were probed using X-ray diffraction during dynamic cooling. We confirm an isostructural transition during heating in the 26-69 GPa range, consistent with the formation of SI-bcc. In contrast to prior work, SI-fcc was observed exclusively above ~50 GPa, despite evidence of melting at lower pressures. The absence of SI-fcc in lower pressure runs is attributed to short heating timescales and the pressure-temperature path induced by the pump-probe heating scheme in which H₂O was heated above its melting temperature before the observation of quenched crystalline states, based on the earlier theoretical prediction that SI-bcc nucleates more readily from the fluid than SI-fcc. Our results may have implications for the stability of SI phases in ice-rich planets, for example during dynamic freezing, where the preferential crystallization of SI-bcc may result in distinct physical properties across mantle ice layers. 

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3. Branching Ratio for O + H ₃ ⁺ Forming OH ⁺ + H ₂ and H ₂ O ⁺ + H

   https://doi.org/10.3847/1538-4357/ac41ce  

   Hillenbrand, Pierre-Michel; Ruette, Nathalie de; Urbain, Xavier; Savin, Daniel W. (March 2022, The Astrophysical Journal)

   Abstract The gas-phase reaction of O + H 3 + has two exothermic product channels: OH + + H 2 and H 2 O + + H. In the present study, we analyze experimental data from a merged-beams measurement to derive thermal rate coefficients resolved by product channel for the temperature range from 10 to 1000 K. Published astrochemical models either ignore the second product channel or apply a temperature-independent branching ratio of 70% versus 30% for the formation of OH + + H 2 versus H 2 O + + H, respectively, which originates from a single experimental data point measured at 295 K. Our results are consistent with this data point, but show a branching ratio that varies with temperature reaching 58% versus 42% at 10 K. We provide recommended rate coefficients for the two product channels for two cases, one where the initial fine-structure population of the O( 3 P J ) reactant is in its J = 2 ground state and the other one where it is in thermal equilibrium. 

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4. Selective Catalytic Combustion of Hydrogen under Aerobic Conditions on Na ₂ WO ₄ /SiO ₂

   https://doi.org/10.1002/anie.202412932  

   Kipp, Elijah R; Garcia‐Barriocanal, Javier; Bhan, Aditya (December 2024, Angewandte Chemie International Edition)

   Abstract Na₂WO₄/SiO₂, a material known to catalyze alkane selective oxidation including the oxidative coupling of methane (OCM), is demonstrated to catalyze selective hydrogen combustion (SHC) with >97 % selectivity in mixtures with several hydrocarbons (CH₄, C₂H₆, C₂H₄, C₃H₆, C₆H₆) in the presence of gas‐phase dioxygen at 883–983 K. Hydrogen combustion rates exhibit a near‐first‐order dependence on H₂partial pressure and are zero‐order in H₂O and O₂partial pressures. Mechanistic studies at 923 K using isotopically‐labeled reagents demonstrate the kinetic relevance of H−H dissociation and absence of O‐atom recombination. In situ X‐ray diffraction (XRD) and W L_III‐edge X‐ray absorption spectroscopy (XAS) studies demonstrate, respectively, a loss of Na₂WO₄crystallinity and lack of second‐shell coordination with respect to W⁶⁺cations below 923 K; benchmark experiments show that alkali cations must be present for the material to be selective for hydrogen combustion, but that materials containing Na alone have much lower combustion rates (per gram Na) than those containing Na and W. These data suggest a synergy between Na and W in a disordered phase at temperatures below the bulk melting point of Na₂WO₄(971 K) during SHC catalysis. The Na₂WO₄/SiO₂SHC catalyst maintains stable combustion rates at temperatures ca. 100 K higher than redox‐active SHC catalysts and could potentially enable enhanced olefin yields in tandem operation of reactors combining alkane dehydrogenation with SHC processes. 

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5. Coexistence of vitreous and crystalline phases of H ₂ O at ambient temperature

   https://doi.org/10.1073/pnas.2117281119  

   Shargh, Ali K.; Picard, Aude; Hrubiak, Rostislav; Zhang, Dongzhou; Hemley, Russell J.; Deemyad, Shanti; Abdolrahim, Niaz; Saffarian, Saveez (July 2022, Proceedings of the National Academy of Sciences)

   Formation of vitreous ice during rapid compression of water at room temperature is important for biology and the study of biological systems. Here, we show that Raman spectra of rapidly compressed water at greater than 1 GPa at room temperature exhibits the signature of high-density amorphous ice, whereas the X-ray diffraction (XRD) pattern is dominated by crystalline ice VI. To resolve this apparent contradiction, we used molecular dynamics simulations to calculate full vibrational spectra and diffraction patterns of mixtures of vitreous ice and ice VI, including embedded interfaces between the two phases. We show quantitatively that Raman spectra, which probe the local polarizability with respect to atomic displacements, are dominated by the vitreous phase, whereas a small amount of the crystalline component is readily apparent by XRD. The results of our combined experimental and theoretical studies have implications for detecting vitreous phases of water, survival of biological systems under extreme conditions, and biological imaging. The results provide additional insight into the stable and metastable phases of H 2 O as a function of pressure and temperature, as well as of other materials undergoing pressure-induced amorphization and other metastable transitions. 

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