Solution behavior of reduced C-O-H volatiles in silicate melts at high pressure and temperature

The solubility and solution mechanisms of reduced C-O-H volatiles in Na(2)O-SiO(2) melts in equilibrium with a (H(2) + CH(4)) fluid at the hydrogen fugacity defined by the iron-wustite + H(2)O buffer [f(H2)(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 degrees C. The solubility, calculated as CH(4), increases from similar to 0.2 wt% to similar to.5 wt% in the melt NBO/Si-range similar to 0.4 to similar to 1.0. The solubility is not significantly pressure-dependent, probably because f(H2)(IW) in the 1-2.5 GPa range does not vary greatly with pressure. Carbon isotope fractionation between methane-saturated melts and (H(2) + CH(4)) fluid varied by similar to 14%. in the NBO/Si-range of these melts. The (C center dot center dot H) and (O center dot center dot H) speciation in the quenched melts was determined with Raman and (1)H MAS NMR spectroscopy. The dominant (C center dot center dot H)-bearing complexes are molecular methane, CH(4,) and a complex or functional group that includes entities with C C-H bonding. Minor abundance of complexes that include Si-O-CH(3) bonding is tentatively identified in some melts. There is no spectroscopic evidence for Si-C or Si-CH(3). Raman spectra indicate silicate melt depolymerization (increasing NBO/Si). The [CH(4)/C C-H](melt) abundance ratio is positively correlated with NBO/Si, which is interpreted to suggest that the (C C-H)-containing structural entity is bonded to the silicate melt network structure via its nonbridging oxygen. The similar to 14%, carbon isotope fractionation change between fluid and melt is because of the speciation changes of carbon in the melt. (C) 2008 Elsevier Ltd. All rights reserved.