Oxygen isotope partitioning between rhyolitic glass/melt and CO_2: An experimental study at 550-950°C and 1 bar

Abstract

Oxygen isotope partitioning between gaseous CO_2 and a natural rhyolitic glass and melt (77.7 wt% SiO_2, 0.16 wt% H_2O_(total)) has been measured at 550-950°C and approximately 1 bar. Equilibrium oxygen isotope fractionation factors (α_((CO_2)-rhyoleit) = (^(18)O/^(16)O)_(CO_2)/(^(18)O/^(16)O)_(rhyolite)) determined in exchange experiments of 100-255 day duration are: T(°C) | 1000 ln α_((CO_2)-rhyolite) 550 | 5.08 ± 0.13 650 | 4.62 ± 0.14 750 | 3.99 ± 0.19 850 | 3.71 ± 0.19 950 | 2.95 ± 0.16 These values agree well with predictions based on experimentally determined oxygen isotope fractionation factors for CO_2-silica glass (Stolper and Epstein, 1991) and CO_2-albitic glass/melt (Matthews et al., 1994), 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 H_2O dissolved in the glass/melt and are three orders of magnitude faster than predicted for rate control solely by diffusion of dissolved molecular CO_2 under the experimental conditions. Additional experiments using untreated and dehydrated (0.09 wt% H_2O_(total)) rhyolitic glass quantitatively supporrt these interpretations. We conclude that diffusive oxygen isotope exchange in rhyolitic glass/melt, and probably other polymerized silicate materials, is 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.

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