Solution behavior of reduced COH volatiles in silicate melts at high pressure and temperature

The solubility and solution mechanisms of reduced COH volatiles in Na2OSiO2 melts in equilibrium with a (H2 + CH4) fluid at the hydrogen fugacity defined by the iron-wüstite + H2O buffer [fH2(IW)] have been determined as a function of pressure (1–2.5 GPa) and silicate melt polymerization (NBO/Si: nonbridging oxygen per silicon) at 1400 °C. The solubility, calculated as CH4, increases from ∼0.2 wt% to ∼0.5 wt% in the melt NBO/Si-range ∼0.4 to ∼1.0. The solubility is not significantly pressure-dependent, probably because fH2(IW) in the 1–2.5 GPa range does not vary greatly with pressure. Carbon isotope fractionation between methane-saturated melts and (H2 + CH4) fluid varied by ∼14‰ in the NBO/Si-range of these melts.The (C..H) and (O..H) speciation in the quenched melts was determined with Raman and 1H MAS NMR spectroscopy. The dominant (C..H)-bearing complexes are molecular methane, CH4, and a complex or functional group that includes entities with CCH bonding. Minor abundance of complexes that include SiOCH3 bonding is tentatively identified in some melts. There is no spectroscopic evidence for SiC or SiCH3. Raman spectra indicate silicate melt depolymerization (increasing NBO/Si). The [CH4/CCH]melt abundance ratio is positively correlated with NBO/Si, which is interpreted to suggest that the (CCH)-containing structural entity is bonded to the silicate melt network structure via its nonbridging oxygen. The ∼14‰ carbon isotope fractionation change between fluid and melt is because of the speciation changes of carbon in the melt.