Time filter

Source Type

Carnegie, Washington, United States

The structure of H2O-saturated silicate melts and of silicate-saturated aqueous solutions, as well as that of supercritical silicate-rich aqueous liquids, has been characterized in-situ while the sample was at high temperature (to 800°C) and pressure (up to 796MPa). Structural information was obtained with confocal microRaman and with FTIR spectroscopy. Two Al-bearing glasses compositionally along the join Na2O•4SiO2-Na2O•4(NaAl)O2-H2O (5 and 10mol% Al2O3, denoted NA5 and NA10) were used as starting materials. Fluids and melts were examined along pressure-temperature trajectories of isochores of H2O at nominal densities (from PVT properties of pure H2O) of 0.85g/cm3 (NA10 experiments) and 0.86g/cm3 (NA5 experiments) with the aluminosilicate+H2O sample contained in an externally-heated, Ir-gasketed hydrothermal diamond anvil cell.Molecular H2O (H2O°) and OH groups that form bonds with cations exist in all three phases. The OH/H2O° ratio is positively correlated with temperature and pressure (and, therefore, fugacity of H2O, fH2O) with (OH/H2O°)melt>(OH/H2O°)fluid at all pressures and temperatures. Structural units of Q3, Q2, Q1, and Q0 type occur together in fluids, in melts, and, when outside the two-phase melt+fluid boundary, in single-phase liquids. The abundance of Q0 and Q1 increases and Q2 and Q3 decrease with fH2O. Therefore, the NBO/T (nonbridging oxygen per tetrahedrally coordination cations), of melt is a positive function of fH2O. The NBO/T of silicate in coexisting aqueous fluid, although greater than in melt, is less sensitive to fH2O.The melt structural data are used to describe relationships between activity of H2O and melting phase relations of silicate systems at high pressure and temperature. The data were also combined with available partial molar configurational heat capacity of Qn-species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H2O content. © 2010 Elsevier Ltd. Source

Mysen B.O.,Geophysical Laboratory | Yamashita S.,Okayama University
Geochimica et Cosmochimica Acta | Year: 2010

The structure of silicate melts in the system Na2O·4SiO2 saturated with reduced C-O-H volatile components and of coexisting silicate-saturated C-O-H solutions has been determined in a hydrothermal diamond anvil cell (HDAC) by using confocal microRaman and FTIR spectroscopy as structural probes. The experiments were conducted in-situ with the melt and fluid at high temperature (up to 800°C) and pressure (up to 1435MPa). Redox conditions in the HDAC were controlled with the reaction, Mo+H2O=MoO2+H2, which is slightly more reducing than the Fe+H2O=FeO+H2 buffer at 800°C and less.The dominant species in the fluid are CH4+H2O together with minor amounts of molecular H2 and an undersaturated hydrocarbon species. In coexisting melt, CH3 - groups linked to the silicate melt structure via Si-O-CH3 bonding may dominate and possibly coexists with molecular CH4. The abundance ratio of CH3 - groups in melts relative to CH4 in fluids increases from 0.01 to 0.07 between 500 and 800°C. Carbon-bearing species in melts were not detected at temperatures and pressures below 400°C and 730MPa, respectively. A schematic solution mechanism is, Si-O-Si+CH4Si-O-CH3+H-O-Si. This mechanism causes depolymerization of silicate melts. Solution of reduced (C-O-H) components will, therefore, affect melt properties in a manner resembling dissolved H2O. © 2010 Elsevier Ltd. Source

The behavior of melts and fluids is at the core of understanding formation and evolution of the Earth. To advance our understanding of their role, high-pressure/-temperature experiments were employed to determine melt and fluid structure together with carbon isotope partitioning within and between (CH4+H2O+H2)-saturated aluminosilicate melts and (CH4+H2O+H2)-fluids. The samples were characterized with vibrational spectroscopy while at temperatures and pressures from 475° to 850 °C and 92 to 1158 MPa, respectively.The solution equilibrium is 2CH4+Qn=2CH3 -+H2O+Qn+1 where the superscript, n, in the Qn-notation describes silicate species where n denotes the number of bridging oxygen. The solution equilibrium affects the carbon isotope fractionation factor between melt and fluid, αmelt/fluid. Moreover, it is significantly temperature-dependent. The αmelt/fluid<1 with temperatures less than about 1050 °C, and is greater than 1 at higher temperature.Methane-bearing melts can exist in the upper mantle at fO2≤fO2(MW) (Mysen et al., 2011). Reduced (CH)-species in present-day upper mantle magma, therefore, are likely. During melting and crystallization in this environment, the δ13C of melts increases with temperature at a rate of ~0.6‰/°C. From the simple-system data presented here, at T≤1050°C, melt in equilibrium with a peridotite-(CH4+H2O+H2)-bearing mantle source will be isotopically lighter than fluid. At higher temperatures, melts will be isotopically heavier. Degassing at T≤1050°C will shift δ13C of degassed magma to more positive values, whereas degassing at T≥1050°C, will reduce the δ13C of the degassed magma. © 2016 Elsevier B.V. Source

The C-O-H-N solubility and solution mechanisms in silicate melts and C-O-H-N speciation in coexisting fluid to upper mantle temperatures and pressures and with redox conditions from the MH to the IW buffer are discussed. Focus is on in-situ structural characterization of coexisting melt and fluid. In fluid+melt-COH, fluid+melt-NOH, and fluid+melt-OH systems, volatiles are dissolved in molecular form (CO2, CH4, NH3, N2, H2O, H2) and as complexes that form chemical bonding with the silicate network (CO3, CH3, NH2, OH).In silicate-OH systems molecular H2O (H2OÚ) and OH-groups exist in silicate- and aluminosilicate-saturated fluids and coexisting water-saturated melts above ~400°C and ~0.5GPa with their OH/H2OÚ-ratio positively correlated with temperature. The extent of hydrogen bonding in both fluids and melts diminishes with temperature so that above ~400°C it cannot be detected. The incrementH of hydrogen bonding in aqueous fluid (22±1kJ/mol) is about twice that in silicate melts (10±2kJ/mol). Silicate speciation in silicate-saturated fluid and hydrous silicate melts comprises similar Q-species with incrementH of the solution reactions in silicate-saturated fluid, water-saturated melt, and supercritical fluid ~400kJ/mol.In COH-silicate systems methane solubility in melt increases from 0.2wt.% to ~0.5wt.% in the melt NBO/Si range from 0.4 to 1.0 at 1-2.5GPa and 1400°C. The solubility increases by ~150% between the redox conditions of the IW and MH buffers. At the NNO buffer conditions and more oxidizing, carbon exists as carbonate complexes in melts and as CO2 in fluid. Reduced (C+H)-bearing species in melts (CH3-groups and molecular CH4) are stable at fH2(MW) and more reducing conditions, whereas the species in coexisting fluid are CH4, H2, and H2O.In NOH-silicate systems, the N solubility in melt decreases from 0.98 to 0.28wt.% in the melt NBO/Si-range from 0.4 to 1.18 at the redox conditions of the IW buffer. The solubility decreases by about 50% between the redox conditions of the IW and MH buffers. At IW, nitrogen occurs in silicate melts amine groups, NH2, bonded to the silicate network, and as molecular NH3, whereas in coexisting NOH fluids the dominant species are NH3, N2, H2 and H2O. The NH2 -/NH3 abundance ratio varies by ~55 between melt compositions with NBO/Si=1.18 and 0.4. In fluids and melts, decreasing hydrogen fugacity leads to oxidation of nitrogen to form molecular N2 so that at the MH redox conditions, the dominant N-bearing species is N2.The redox-dependent solution mechanisms of COHN volatile components in silicate melts affect their structure differently, which results in redox-dependent thermodynamic and transport properties of magmatic liquids in the interior of the Earth and terrestrial planets. These properties include mineral/melt minor and trace element partitioning, melt/fluid isotope fractionation, and transport and thermodynamic properties of melt saturated with variably-oxidized COHN volatile components. © 2012 Elsevier B.V. Source

Roskosz M.,Lille University of Science and Technology | Bouhifd M.A.,University of Oxford | Bouhifd M.A.,CNRS Magmas and Volcanoes Laboratory | Jephcoat A.P.,University of Oxford | And 2 more authors.
Geochimica et Cosmochimica Acta | Year: 2013

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 18GPa and temperatures to 2850±200K. Multi-anvil experiments were also performed at 5 and 10GPa and 2390±50K. 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 18GPa. Henry's law applies to nitrogen-saturated silicate melts up to 4-5GPa. Independently of these solubility models, it is shown that the partition coefficient of nitrogen between metal and silicate melts changes from almost 104 at ambient pressure to about 10-20 for pressures higher than 1GPa. The pressure-dependence of the nitrogen partitioning can explain the recently suggested depletion of nitrogen relative to other volatiles in the bulk silicate Earth. © 2013 Elsevier Ltd. Source

Discover hidden collaborations