Glass transition temperatures of natural hydrous melts: a relationship with shear viscosity and implications for the welding process.

Glass transition temperatures (Tg) have been determined for natural multicomponent melts using differential scanning calorimetry. Trachytic, dacitic, phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3.75 wt.%. For each sample Tg has been obtained over a range of cooling/heating rates using the extrapolated onset and the peak temperatures in heat capacity–temperature curves. Tg of all compositions are strongly reduced by increasing water content, particularly for the first 1 wt.% added. Base composition also has an effect, with the lowest Tg occurring in the peralkaline phonolite suite. For all samples a clear dependence on the cooling/heating rate has been recorded. These results have been compared with rheological investigations on the same samples. On the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study, both dry and hydrous. Our determinations of Tg provide a lower limit for the time–temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature. Furthermore, by using the viscosity values predicted at the glass transition, we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at Tg.