Oxygen isotope partitioning between rhyolitic glass/melt and CO2: An experimental study at 550-950 degrees C and 1 bar

Oxygen isotope partitioning between gaseous CO2 and a natural rhyolitic glass and melt (77.7 wt% SiO2, 0.16 wt% H2Ototal) has been measured at 550-950 degrees C and approximately 1 bar. Equilibrium oxygen isotope fractionation factors (αCO2-rhyolite = (18O/16O)rhyolite) determined in exchange experiments of 100-255 day duration. These values agree well with predictions based on experimentally determined oxygen isotope fractionation factors for CO2-silica glass and CO2-albitic glass/melt, if the rhyolitic glass is taken to be a simple mixture of normative silica and alkali feldspar components. The results indicate that oxygen isotope partitioning in felsic glasses and melts can be modeled by linear combinations of endmember silicate constituents. Rates of oxygen isotope exchange observed in the partitioning experiments are consistent with control by diffusion of molecular H2O dissolved in the glass/melt and are three orders of magnitude faster than predicted for rate control solely by diffusion of dissolved molecular CO2 under the experimental conditions. Additional experiments using untreated and dehydrated (0.09 wt% H2Ototal) rhyolitic glass quantatively support these interpretations. We conclude that diffusive oxygen isotope exchange in rhyolitic glass/melt, and probably other polymerized silicate materials, it controlled by the concentrations and diffusivities of dissolved oxygen-bearing volatile species rather than diffusion of network oxygen under all but the most volatile-poor conditions. 25 refs., 6 figs., 1 tab