The influence of H₂O and CO₂ on the glass transition temperature: Insights into the effects of volatiles on magma viscosity

CO₂ can play an important role in eruptive processes; in particular, it has the potential to reach saturation at lower concentrations than H₂0 and initiate degassing. The effect of such CO₂ loss on magma viscosity is not well constrained, especially compared to the established effects of H₂0 loss. In terms of understanding the C0₂ solubility mechanism, recent spectroscopic studies have shown that C0₂ speciation is strongly temperature dependent and that C0₂ speciation preserved in quenched glasses below T_g is different from the true C0₂ speciation observed in the melts. However, the effect of C0₂ on the glass transition temperature, and by inference the viscosity, has not been previously established. In this study, calorimetric measurements were conducted on synthetic H₂0- and C0₂-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 (7_g^°nset). 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.% C0₂ and up to 5.49 wt.% H₂0. For both compositions, H₂0 has a large effect in reducing 7_g^°nset, but C0₂ appears to have little or no effect. For the entire range of H₂0 contents, 7 _g^°nset decreases exponentially with H₂0 content from 870 to 523 K and 1036 to 636 K for phonolite and jadeite, respectively, regardless of the C0₂ content. No measurable effect of C0₂ on 7_g^°nset was observed. These results suggest that compared to H₂0, C0₂ contributes little to changes in the physical properties of the melt. They also provide strong evidence for the decoupling of C0₂ speciation from the bulk silicate melt structural relaxation process at T_g.