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 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 T_g 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 (T_g^onset). 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 T_g^onset, but CO₂ appears to have little or no effect. For the entire range of H₂O contents, T_g^onset 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 T_g^onset 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 T_g.