ANNEALING KINETICS OF FISSION TRACKS IN ZIRCON - AN EXPERIMENTAL-STUDY

A series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350-750 degrees C for 10(-1)-10(3) hr (i.e. 4.5 min- similar to 40 days). Variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from Nisatai Dacite. The fading contours of normalized mean track length (r) on the Arrhenius diagram showed as sets of straight lines. We performed a series of model fittings, called the parallel and fanning models, in order to describe the decrease in r with increasing temperature or heating time. The lowest temperature limit of the zircon partial annealing zone (ZPAZ) was defined as r approximate to 0.95, and the highest as r approximate to 0.4, which approximately corresponded to the total fading of surface tracks. Extrapolation of the results of the laboratory experiments to the geological timescale gives, for a heating duration of 10(6) yr, estimated values of the ZPAZ of similar to 210-320 degrees C (+/- 60 degrees C, 2 sigma) with the parallel model; similar to 190-350 degrees C (+/- 50 degrees C, 2 tau) with the fanning model (critical temperature, T-o = infinity); and similar to 170-390 degrees C (+/- 50 degrees C 2 sigma) with the fanning model (T-o not equal infinity). The temper atures of the ZPAZ decrease by similar to 20 degrees C for an annealing duration that is an order of magnitude longer. Because the estimated closure temperature of zircon fission-track analysis approximately corresponds to the middle of the ZPAZ, these results support the previously estimated closure temperature of similar to 240 degrees C. BY varying the etching time it was revealed that significant removal of alpha-radiation damage occurs at r approximate to 0.93.