EVALUATION OF A RELAXATION GEOSPEEDOMETER FOR VOLCANIC GLASSES

The cooling of silicate melts influences the character of volcanic processes, primarily through the effect of temperature on melt viscosity but also through the derivative thermal properties, expansivity and heat capacity. If the cooling rates of volcanic facies can be established then the undoubtedly complex processes of erupting, cooling and degassing magmas can be more readily understood. The frozen structure of every glass, volcanic or synthetic, is influenced by the previous cooling history. Moreover, the relaxation of the glass structure back to equilibrium during reheating across the glass transition is also dependent on the quenched-in structure and thus can be used to determine the cooling rate history. In this study differential scanning calorimetry (DSC) measurements have been used to determine the enthalpic relaxation of four volcanic glasses. Each glass has been heated at known heating rates in the calorimeter to temperatures above the glass transition interval. The first heating of the glass involves the relaxation of the structure frozen in during the volcanic cooling. Subsequent controlled heating and cooling in the calorimeter enables the determination of a set of kinetic parameters which can be used to model the heat capacity curves. By modeling the transient values of heat capacity across the glass transition interval the relaxation of enthalpy, and the volcanic cooling rate across the glass transition interval can be obtained. From our study, we can demonstrate four different cooling rates for volcanic glasses over a range of three orders of magnitude, from 4 degrees C min(-1) to 2 degrees C day(-1). We propose that the enthalpic relaxation of silicate melts can be applied as a geospeedometer to the study of volcanic rocks from a variety of occurrences.