Speciation of reduced C–O–H volatiles in coexisting fluids and silicate melts determined in-situ to ∼1.4 GPa 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 1435 MPa). 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 730 MPa, respectively. A schematic solution mechanism is, Si–O–Si + CH4⇌Si–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.