Melt Viscosity at the Glass Transition During Volcanic Processes: Predictions From Calorimetric Studies

The viscosity of volcanic melts at the glass transition has been determined for 11 compositions ranging from basanite to rhyolite. The temperature-dependence of viscosity, together with the cooling rate dependence of the glass transition, permits the calibration of the value of the viscosity at the glass transition at a given cooling rate for each melt. We have applied micropenetration techniques to determine viscosities of supercooled melts. In addition, glass transition temperatures have been obtained by rate heating glass samples to supercooled liquid conditions using differential scanning calorimetry. The activation energies obtained from calorimetry and viscometry are identical for each melt composition investigated. This confirms that a simple shift factor can be used for each in order to determine the viscosity at the glass transition for a given cooling rate in nature. The results of this study indicate that there is a subtle but significant compositional dependence of the shift factor by a factor of 10 (in log terms) from 10.8 for rhyolites to 9.6 for basanites. The composition-dependence of the shift factor is presented here in terms of a compositional parameter, the mol% of excess oxides. Using this parameterisation, we obtain a non-linear dependence of the shift factor upon composition that matches all 11 observed values within error. The resulting model permits the prediction of viscosity at the glass transition, during the cooling of glassy volcanic rocks to within 0.1 log units. The resultant shear viscosities vary over five orders of magnitude for published cooling rates in the literature between tens of Kelvins per second and few Kelvins per day across the glass transition.