Speciation of reduced C-O-H volatiles in coexisting fluids and silicate melts determined in-situ to ∼1.4GPa and 800°C

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.