THE EQUIVALENCE OF ENTHALPY AND SHEAR-STRESS RELAXATION IN RHYOLITIC OBSIDIANS AND QUANTIFICATION OF THE LIQUID-GLASS TRANSITION IN VOLCANIC PROCESSES

The relaxation of enthalpy and shear stress has been investigated for six silicic volcanic obsidians (calc-alkaline rhyolitic obsidians from Ben Lomond dome, New Zealand, Erevan Dry Fountain, Armenia and Little Glass Butte, USA; peralkaline obsidians from Mayor Island, New Zealan d and Eburru, Kenya and a macusanite obsidian from SE Peru). The tempe rature-dependences of enthalpy and shear stress relaxation are obtaine d from the dependence of the calorimetric heat capacity peak temperatu re on heating rate and the dependence of shear viscosity on temperatur e, respectively. Both processes, enthalpy relaxation and shear stress relaxation, can be approximated to be Arrhenian in the investigated te mperature ranges relevant to volcanological processes. Activation ener gies derived for enthalpy and shear stress relaxation for each sample are equal. This equality permits the calculation of viscosity at the g lass transition as a function of cooling rate of a volcanic melt. The relationship between viscosity at the glass transition and the cooling rate is given by log(10)eta(s)(at T-g) = K-log(10)\q\, where eta(s) t he shear viscosity at the glass transition, q is the cooling rate in d egrees C s(-1) and K is a constant. The six melt compositions investig ated here exhibit a value of K = 10.49 +/- 0.31. For the modeling of v olcanic processes, this equation allows the prediction of the viscosit y at the glass transition temperature for a given value of cooling rat e. Taken together with the Maxwell relation an effective relaxation ti me can be obtained for the cooling rate. Prediction of the glass trans ition temperature permits the allotment of temperature ranges for the liquid and glassy segments of the cooling history of the volcanic melt and thus for the correct assignment of glassy and liquid values of th e derivative thermodynamic properties, such as expansivity and heat ca pacity, in thermodynamic modeling of late-stage volcanic processes.