CO2 can play an important role in eruptive processes; in particular, it has the potential to reach saturation at lower concentrations than H2O and initiate degassing. The effect of such CO2 loss on magma viscosity is not well constrained, especially compared to the established effects of H2O loss. In terms of understanding the CO2 solubility mechanism, recent spectroscopic studies have shown that CO2 speciation is strongly temperature dependent and that CO2 speciation preserved in quenched glasses below Tg is different from the true CO2 speciation observed in the melts. However, the effect of CO2 on the glass transition temperature, and by inference the viscosity, has not been previously established.

In this study, calorimetric measurements were conducted on synthetic H2O- and CO2-bearing phonolite and jadeite glasses in order to investigate the volatile’s effect on the glass transition interval, by defining a single glass transition temperature (Tgonset). The samples were synthesised in a piston-cylinder apparatus between 1300 and 1550 °C, at 1.0 to 2.5 GPa, and contained up to 2.29 wt.% CO2 and up to 5.49 wt.% H2O.

For both compositions, H2O has a large effect in reducing Tgonset, but CO2 appears to have little or no effect. For the entire range of H2O contents, Tgonset decreases exponentially with H2O content from 870 to 523 K and 1036 to 636 K for phonolite and jadeite, respectively, regardless of the CO2 content. No measurable effect of CO2 on Tgonset was observed.

These results suggest that compared to H2O, CO2 contributes little to changes in the physical properties of the melt. They also provide strong evidence for the decoupling of CO2 speciation from the bulk silicate melt structural relaxation process at Tg.

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