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

Enthalpy relaxation of a reheated natural tektite glass has been used to determine the cooling-rate of the glass in nature. The relaxation p arameters for the tektite samples were obtained by internal calibratio n through a series of thermal treatments applied to the sample followi ng the first excursion into the liquid field. The tektite fulfills the criteria required for the use of calorimetric relaxation geospeedomet ry in that it is chemically homogeneous and pristine as well as being metastable above the glass transition during experimental reheating. U sing calorimetric relaxation geospeedometry, the thermal history of th e tektite has been constrained to cooling rates of a few degrees C s(- 1) at temperatures in the region of the glass transition (ca. 780 degr ees C). The tektite, a spheroid of similar to 40 mm diameter, exhibits a higher cooling rate on the exterior (10 degrees C s(-1)) than the m ore massive interior (1 degrees C s(-1)). This difference in cooling r ate is compatible with cooling of the exterior by surface radiation an d a cooling of the interior by conduction through a glassy rind.