Cooling rate variation in natural volcanic glasses from Tenerife,Canary Islands

 Silicate melts form glasses in a variety of geological environments. The relaxation (equilibration) of the frozen glass structure provides a means of investigating the quench rates of natural glasses, and this cooling history provides an important constraint for models of melt dynamics. Phonolite glasses from the central volcanic edifice of Tenerife, Canary Islands indicate a range of five orders of magnitude cooling rate, determined by modeling the relaxation of the structure-dependent property, enthalpy (H) across the glass transition. The relaxation of enthalpy is determined by heat capacity (c _p = ΔH/ΔT) measurement of natural glass samples by differential scanning calorimetry (DSC). Upon heating, the heat capacity curve in the vicinity of the glass transition has a geometry characteristic of the previous cooling rate. A series of thermal treatments applied to each individual sample results in a set of sample-specific parameters which are used to model the heat capacity curve of the naturally cooled glass. The cooling rate is then derived. The equivalence of shear and enthalpic relaxation enables the relaxation of enthalpy for these volcanic samples to be described by a general term for the evolution of fictive temperature. Quench rates for thirty-one glasses are calculated to be within the range 10°C s^–1 to 7°C per day. The cooling rates quoted are linear approximations across the glass transition. Within different volcanic facies cooling rates depend on several factors. The most rapidly cooled glasses occur where samples lose heat by radiation from the surface. Our analyses indicate that in certain environments, a natural annealing process results in slow quench rates. This is interpreted as either a slow initial cooling process or the reheating of a glass to an annealing temperature within the glass transition interval. The latter results in relaxation to a lower temperature structure. Controls on these processes include the initial temperature and dissipation of thermal energy from the volcanic body. Our results are consistent with an influence of volatiles on quench rates in volcanic bombs where glass adjacent to vesicular layers is relatively rapidly quenched. We interpret this as a rapid quench rate frozen into the glass resulting from a change in viscosity due to volatile degassing. In lava flows, the conduction of heat from the hot flow interior controls the cooling process and diminishes the effect of volatile exsolution. Relaxation geospeedometry can be applied to glass samples from a variety of geological environments where cooling rates cannot be measured directly. Such measurements provide a means of determining cooling rates for a variety of volcanic processes, an independent calibration for existing temperature and time data and a means for testing cooling-rate-dependent models. Received: 9 January 1996 / Accepted: 13 May 1996