The circulation of water and its interaction with crystalline phases and melts play an important role on the cycling of mass and energy between the Earth’s mantle, lithosphere, and hydrosphere. To better understand the water cycle, however, it is essential to constrain the distribution of chemical components such as hydrogen and deuterium that constitute the water dissolved in volcanic glasses and minerals. Here, we investigated the relationship between deuterium/hydrogen partitioning and speciation of C-O-H volatiles in silicate melts and fluids at pressures and temperatures reflecting lower crust and upper mantle conditions. We studied the relative distribution of H-D isotopologues of methane, hydrogen, and water dissolved in melts and coexisting fluids in-situ by Raman and infrared spectroscopy in a series of hydrothermal diamond-anvil cell experiments at high temperature and pressure. These experiments were complemented by the use of ¹H/²H and ²⁹Si Nuclear Magnetic Resonance on quenched melts.

 Experiments were conducted to constrain the D/H molar ratios of methane developed under gas and condensed-phase (supercritical water) conditions. Results revealed differences on the equilibrium constant that describe the relationship between the several H/D isotopologues of methane (i.e. CH₃D, CHD₃, CH₂D₂, CD₄) dissolved in supercritical water and present as a homogeneous gas phase. The bulk D/H methane composition in the liquid- system is also twice that of the D/H molar ratios recorded in the gas-bearing system. These observations are attributed to condensed-phase isotope effects induced by differences in the solubility of the isotopic molecules, and driven by the excess energy/entropy developed during mixing of non-polar species in H₂O-D₂O fluids. Our experiments show that isotope fractionation effects need to account for the presence of condensed matter (e.g. melts, magmatic fluids), even at conditions at which theoretical models suggest minimal (or nonexistent) isotope exchange, but comparable to those within the Earth’s crust.

Indeed, in another series of experiments, we find enormous D/H intramolecular fractionation between different molecular environments within silicate glasses quenched from melts synthesized under upper mantle conditions of 1400 °C and 2 GPa. Through experiment and simulation we can rule out kinetic isotope effects and conclude that this fractionation results from molar volume isotope fractionation. Our experimental studies indicate that there is a complex relationship between composition and D-H partitioning. This conclusion is further supported with another set of HDAC experiments designed to determine the effect of the bulk D/H ratio on hydrogen isotope partitioning between water-saturated silicate melts and coexisting silicate-saturated aqueous fluids in the Na₂O–Al₂O₃–SiO₂–H₂O–D₂O system. Data show that the D/H ratios of fluids in equilibrium with melt barely changed with temperature. In contrast, the deuterium content of coexisting melts display strong dependence on temperature. The temperature-dependence of the D/H fractionation factor between melt and fluid is independent of the bulk D/H ratio of the melt-fluid system. In the presence of C-O-H volatiles under upper mantle temperature/pressure conditions, partial melts can have δD values 100% or more lighter relative to coexisting silicate-saturated fluid. This effect is greater under oxidizing than under reducing conditions. Thus, it is suggested that δD variations of upper mantle mid-ocean ridge basalt (MORB) sources, inferred from the δD of MORB magmatic rocks, can be explained by variations in redox conditions during melting. ...

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