Speciation of D-O-H-C-N volatiles in alkali aluminosilicate melts and of silicate in D-O-H-C-N fluid has been determined in situ to 800 degrees C and >2 GPa under reducing and oxidizing conditions by using an externally heated hydrothermal diamond-anvil cell with Raman spectroscopy as the structural probe. Reducing conditions were near those of the IW oxygen buffer, whereas oxidizing conditions were obtained by conducting the experiments with oxidized components only and with Pt as a catalyst. Raman bands assigned to C-H stretching in CHxDy isotopologues and CH4 groups (including CH3) were employed to determine the CH4/CHxDy ratio in fluids and melts. This ratio decreases from 1.5-2 at 500 degrees C to between 1.2 and 1 with 800 degrees C with Delta H-values of 13.6 +/- 2.1 and 5.5 +/- 1.1 kJ/mol for melt and fluid, respectively. The CHx/CHxDy fluid/melt partition coefficient ranges between similar to 16 and similar to 3 with Delta H= 33 +/- 6 kJ/mol assuming no pressure effect. This behavior of deuterated and protonated complexes is ascribed to speciation of volatile and silicate components in fluids and melts in a manner that is conceptually similar to D/H partitioning among complexes and phases in brines and hydrous silicate systems.

Nitrogen is the dominant gas in Earth atmosphere, but its behavior during the Earth' differentiation is poorly known. To aid in identifying the main reservoirs of nitrogen in the Earth, nitrogen solubility was determined experimentally in a mixture of molten CI-chondrite model composition and (Fe, Ni) metal alloy liquid. Experiments were performed in a laser-heated diamond-anvil cell at pressures to 18 GPa and temperatures to 2850 +/- 200 K. Multi-anvil experiments were also performed at 5 and 10 GPa and 2390 +/- 50 K. The nitrogen content increases with pressure in both metal and silicate reservoirs. It also increases with the iron content of the (Fe, Ni) alloy. Sieverts' formalism successfully describes the nitrogen solubility in metals up to 18 GPa. Henry's law applies to nitrogen-saturated silicate melts up to 4-5 GPa. Independently of these solubility models, it is shown that the partition coefficient of nitrogen between metal and silicate melts changes from almost 10(4) at ambient pressure to about 10-20 for pressures higher than 1 GPa. The pressure-dependence of the nitrogen partitioning can explain the recently suggested depletion of nitrogen relative to other volatiles in the bulk silicate Earth. (c) 2013 Elsevier Ltd. All rights reserved.

Hydrogen-rich and water-rich fluids exert different control on dissolution mechanism of oxide and silicate minerals in the Earth's interior. With Mg-silicate-H-2 fluids, dissolution tends to be incongruent with the (Mg/Si)(fluid) < (Mg/Si)(Mg-silicate) with formation of SiOH and SiH4 complexes in the fluid (Shinozaki et al. 2013, 2014). In contrast, in Mg-silicate-H2O systems, Mg-silicate minerals in the mantle (pyroxene and forsterite) dissolve stoichiometrically (congruently) in aqueous fluids to at least 10 GPa pressure. Metasomatic alteration by H-2-rich fluids enriches, therefore, the mantle in SiO2 compared with alteration by H2O fluid. This difference becomes increasingly important with mantle depth because the environment becomes more reducing, which results in an increase of H-2/H2O fluids (Shinozaki et al. 2014, this issue). Chemical gradients with depth of the Earth could be affected by increased H-2/H2O of mantle fluids whereby Mg/Si ratios, for example, will become variable. Silicate-H-2 alteration processes likely also played major roles during the early, core-forming stages of the Earth. Such a process could be responsible for Mg/Si changes in the early silicate Earth.

Experiments to determine silicate structural species in silicate-saturated aqueous fluids in equilibrium with silica polymorphs (quartz and coesite), enstatite, and enstatite+forsterite in the SiO2-H2O and MgO-SiO2-H2O systems have been carried out in situ in the 0.4-5.4GPa and 700-900 degrees C pressure and temperature ranges, respectively. MicroRaman spectroscopy was the structural probe. In the SiO2-H2O system (1.6-5.4GPa/700-900 degrees C), the detected silicate species are Q(0) (SiO44-), Q(1) (0.5 Si2O76-), and Q(2) (SiO32-). The expression 2Q(1)Q(0)+Q(2) describes the equilibrium among these species with H and V values from the isochoric temperature and isothermal pressure dependence of its equilibrium constant, K=XQ0XQ2/(X-Q1)(2), range from -23 to -69kJ/mol and -1 to -2cm(3)/mol, respectively. In the system MgO-SiO2-H2O the calculated silica solubility, using literature algorithms, is approximately 50% of that in the SiO2-H2O system at similar temperature and pressure. Only Q(1) and Q(0) species were detected in the MgO-SiO2-H2O fluids, whether in equilibrium with enstatite+forsterite (P<3GPa) or enstatite only (P>3GPa). The temperature and pressure dependence of the equilibrium constant, K=X-Q1/X-Q0, for this system yields average values of H=405kJ/mol and V=-2.30.4cm(3)/mol. The speciation of silicate in aqueous fluids resembles that in hydrous melts as a function of temperature and pressure at deep crustal and upper mantle temperature and pressure conditions, and they become increasingly similar with depth. As the silicate speciation and solubility in the aqueous fluid depend on silicate composition, the pressure and temperature at which complete miscibility occurs will also vary with silicate composition. The structural similarity between fluids and melts will also lead to fluid/melt element partition coefficients trending toward 1 and mineral/fluid partition coefficients trending toward mineral/melt values in the upper mantle as the silicate-H2O systems approach complete miscibility with increasing temperature and pressure.

Oxygen fugacity (f(O2)) is a fundamental parameter that controls carbon mobility in aqueous fluids in geological environments such as subduction zones, where reduced serpentinite fluids have the potential to infiltrate oxidized carbonate-bearing lithologies. Using experiments and calculations, we describe how mineral-fluid equilibria evolve as f(O2) decreases in the model Ca-C-O-H system at forearc conditions (300-700 degrees C and 2-10 kbar). Experimental calcite solubility was constant at f(O2) values from quartz-fayalite-magnetite (QFM) to hematite-magnetite (HM). At lower f(O2) values of iron-magnetite (IM) or wustite-magnetite (WM), calcite reacted with H-2 to form methane plus portlandite or melt. These results were consistent with thermodynamic calculations and indicate that carbon mobility, as parameterized by total aqueous carbon ([C-TOT]), is strongly dependent on f(O2). At constant pressure and temperature, carbon mobility is minimized at oxidizing conditions, where [C-TOT] is controlled by calcite solubility. Carbon mobility is maximized at the most reducing conditions because all the carbon in the system is present as CH4. An intermediate region of carbon mobility exists in which calcite is stable with a CH4-bearing fluid. As pressure increases from 2 to 10 kbar, the f(O2) range over which calcite is stable with a methane-rich fluid shifts to more reducing conditions. The variety of geological conditions with the potential for redox enhancement of carbon mobility becomes more restricted with depth. Reduction melting was observed at 700 degrees C and 6 kbar, and at 650 degrees C and 10 kbar, due to the partial reaction of calcite to portlandite at conditions above the hydrous melting curve of calcite+portlandite. Although likely metastable in the present experiments, reduction melting may occur in nature whenever H-2 partially reduces carbonate minerals at pressures and temperatures above the hydrous melting curve of calcite+portlandite. Whether it causes melting or not, calcite reduction is likely an important mechanism for abiotic methanogenesis in natural systems such as subduction zone forearcs or similar environments with the potential for interaction of reduced fluids with carbonate minerals. Because calcite solubility at oxidized conditions is already known to increase substantially with pressure, the additional increase in carbon mobility provided by calcite reduction implies that subduction zones may host some of the most carbon-rich aqueous fluids on Earth.

Effects of H2O on the solution behavior of fluorine and chlorine in peralkaline sodium aluminosilicate glasses quenched from melts at high temperature (1400 degrees C) and pressure (1.5 GPa) were studied by combining solubility measurements and Raman spectroscopy. With increasing H2O content from 0 to similar to 10 wt%, the fluorine solubility increases from 3.3 to 4.4 mol% in Al-free glasses and from 6.3 to 9.3 mol% in Al-rich glasses (10 mol% Al2O3). In contrast, in the same H2O concentration range the chlorine solubility decreases from 5.7 to 3.4 mol% in Al-free glasses and from 3.6 to 1.7 mol% in Al-rich glasses.

Water can form different chemical bonds with the ionic entities composing silicate melts. Because of that, its influence on the physico-chemical properties of magmas can vary with silicate composition and water content, temperature, and pressure. To further our understanding of how silicate chemical composition governs proton distribution in magmas, the environment of protons in hydrous alkali (Li, Na, K) silicate glasses was varied as a function of the type of alkali metal and total water content. From H-1 MAS NMR spectroscopy, H+ are distributed among five different structural environments in alkali silicate glasses. One of these environments is in the form of H2O molecules (H2Omol). The four others are the proton environments associated with Si-OH bonding, and perhaps also with M-OH bonding (with M = Li, Na, or K). Those environments differ in their O-O distance and extent of hydrogen bonding. H2Omol, species are located in an environment with an O-O distance of similar to 290 pm. OH- groups are in environments with O-O distances from 240 to 305 pm. The ionic radius of the alkalis, and hence their ionic field strength, determines the fraction of water dissolved as H2Omol and OH- groups, as well as the distribution of protons in the various OH- environments. The mean volume of the H+ oxygen coordination sphere was calculated using the H-1(+) NMR signal intensity and the mean O-O distance around H+. Increasing ionic radius of the alkali metal in silicate glasses results in a decrease of this mean volume. The partial molar volume of water in the corresponding melts determined through other technics seems to vary in a comparable way. Therefore, the chemical composition of silicate melts may control the partial molar volume of dissolved water because of its influence on the structural environment of protons. This probably also plays a role in determining water solubility.

Carbon speciation in and partitioning among silicate-saturated C-O-H fluids and (C-0-H)-saturated melts have been determined similar to 1.7 GPa and 900 degrees C under reducing and oxidizing conditions. The measurements were conducted in situ while the samples were at the conditions of interest. The solution equilibria were (1) 204(4) + Q(n) = 2CH(3)(-) + H2O + Q(n+1) and (2) 2CO(3)(2-) + H2O + 2Q(n+1) = HCO3- + 2Q(n), under reducing and oxidizing conditions, and where the superscript, n, in the Q(n)species denotes number of bridging oxygen in the silicate species (Q-species). The abundance ratios, CH3/CH4 and HCO3-/CO32-, increase with temperature. The enthalpy change associated with the species transformation differs for fluids and melts and also for oxidized and reduced carbon [Reducing: Delta H-(1)(fluid) = 16 +/- 5 kJ/mol, Delta H-(1)(melt) = 50 +/- 5 kJ/mol; oxidizing Delta H-(2)(fluid) = 81 +/- 14 kJ/mol]. For the exchange equilibrium of CH4 and CH3 species between fluid and melt, the temperature-dependent equilibrium constant (XCH4/XCH3)(fluid)/(XCH4/XCH3)(melt), yields Delta H = 34 +/- 3 kJ/mol.

Previous studies of hydrous glasses and melts with infrared spectroscopy have led to the conclusion that the IR combination peaks near 4500 and 5200 cm(-1) reflect the existence of OH- (hydroxyl) groups and H2Omol water molecules in those materials. Here, we show that the glass chemical composition can impact profoundly the intensities and frequencies of the fundamental O-H stretching signal and, therefore, potentially those of the 4500 and 5200 cm(-1) combination peaks. In alkali silicate glasses, compositional effects can give rise to peaks assigned to fundamental O-H stretching at frequencies as low as 2300 cm(-1). This expanded range of Raman intensity assigned to O-H stretch is increasingly important as the ionic radius of the alkali metal increases. As a result, the combination of the fundamental O-H stretch in OH- groups with the Si-O-H stretch located near 910 cm(-1) gives rise to a complex combination signal that can extend to frequencies much lower than 4200 cm(-1). This combination signal then becomes unresolvable from the high-frequency limb of the band assigned to fundamental O-H stretch vibration in the infrared spectra. It follows that, when O-H stretch signals from OH- groups extend to below 3000 cm(-1) the 4500 cm(-1) peak does not represent the total OH- signal. Under such circumstances, this infrared peak may not be a good proxy for determining the concentration of OH- hydroxyl groups for glassy silicate materials.

Degassing of water during the ascent of hydrous magma in a volcanic edifice produces dramatic changes in the magma density and viscosity. This can profoundly affect the dynamics of volcanic eruptions. The water exsolution history, in turn, is driven by the water solubility and solution mechanisms in the silicate melt. Previous studies pointed to dissolved water in silicate glasses and melts existing as molecules (H2Omol species) and hydroxyl groups, OH. These latter OH groups commonly are considered bonded to Si4+ but may form other bonds, such as with alkali or alkaline-earth cations, for instance. Those forms of bonding influence the structure of hydrous melts in different ways and, therefore, their properties. As a result, exsolution of water from magmas may have different eruptive consequences depending on the initial bonding mechanisms of the dissolved water. However, despite their importance, the solution mechanisms of water in silicate melts are not clear. In particular, how chemical composition of melts affects water solubility and solution mechanism is not well understood. In the present experimental study, components of such information are reported via determination of how water interacts with the cationic network of alkali (Li, Na, and K) silicate quenched melts. Results from Si-29 single-pulse magic-angle spinning nuclear magnetic resonance (Si-29 SP MAS NMR), infrared, and Raman spectroscopies show that decreasing the ionic radius of alkali metal cation in silicate melts results in decreasing fraction of water dissolved as OH groups. The nature of OH bonding also changes as the alkali ionic radius changes. Therefore, as the speciation and bonding of water controls the degree of polymerization of melts, water will have different effects on the transport properties of silicate melts depending on their chemical composition. This conclusion, in turn, may affect volcanic phenomena related to the viscous relaxation of hydrous magmas, such as for instance the fragmentation process that occurs during explosive eruptions.