TEKTITE COOLING RATES: CALORIMETRIC RELAXATION GEOSPEEDOMETRY APPLIED TO A NATURAL GLASS

Abstract

Enthalpy relaxation of a reheated natural tektite glass has been used to determine the cooling-rate of the glass in nature. The relaxation parameters for the tektite samples were obtained by internal calibration through a series of thermal treatments applied to the sample following the first excursion into the liquid field. The tektite fulfills the criteria required for the use of calorimetric relaxation geospeedometry in that it is chemically homogeneous and pristine as well as being metastable above the glass transition during experimental reheating. Using calorimetric relaxation geospeedometry, the thermal history of the tektite has been constrained to cooling rates of a few °C s-1 at temperatures in the region of the glass transition (ca. 780°C). The tektite, a spheroid of ~40 mm diameter, exhibits a higher cooling rate on the exterior (10°C s-1) than the more massive interior (1°C s-1). This difference in cooling rate is compatible with cooling of the exterior by surface radiation and a cooling of the interior by conduction through a glassy rind.