Structure–property relationships of COHN-saturated silicate melt coexisting with COHN fluid: A review of in-situ, high-temperature, high-pressure experiments

Abstract

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 (H2O˚) and OH-groups exist in silicate- and aluminosilicate-saturated fluids and coexisting water-saturated melts above ~400°C and ~0.5GPa with their OH/H2O˚-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 ∆H 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 ∆H 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.