The cooling rate of the El'gygytgyn impact glass.

The thermal history of an impact glass from the El'gygytgyn impact structure, central Chukotka, Russia has been investigated with the method of relaxation geospeedometry. The cooling rate of the melt fragment has been quantified by heat capacity ( c _p) measurements using differential scanning calorimetry. Cycles of thermal treatments applied to the melt fragment sample result in a set of unique kinetic parameters, which were then used to model heat capacity curves. The Tool-Narayanaswamy-Moynihan approach was used to quantify the natural cooling rate from changes in the normalized heat capacity curves. Cooling rates ranging from 0.06 to 0.40 K min⁻¹ were found by modeling the structural relaxation within the glass transition interval. Because of the absence of thermal reheating, a single natural annealing process is thought to have been responsible for the modeled cooling rate, even though a heat buffering process that affects the cooling history might have been involved. The El'gygytgyn impact glass cooled significantly slower than previously studied tektites and faster than most volcanic glasses. The calculated viscosity of log η = 10.75 ± 0.50 Pa s at the glass transition temperature T_g^peak (1020 K) supports this assumption. Consequently, a slow cooling process is accompanied by a high viscosity η. [ABSTRACT FROM AUTHOR]