THERMAL ANNEALING CHARACTERISTICS OF SPONTANEOUS FISSION TRACKS IN ZIRCON

One-hour isochronal annealing experiments were carried out on zircon crystals KT06, separated from a rhyolitic welded tuff of the Koto Rhyolites, southwest Japan, in order to study thermal annealing characteristics of spontaneous fission tracks in zircon. Variations of track density, horizontal confined track length and track angular distribution were measured at various temperatures between 400° and 750°C. Track density and track length begin to decrease around 450°C and reach zero around 750°C. Both time and anisotropy of track etching increase drastically for the samples annealed above 500°C because of the loss of accumulated radiation damage derived from α-decay of U and Th. Angular distributions of etched tracks in those samples suggest no significant apparent reduction of etching efficiency due to increased etching anisotropy, so long as adopted etching criteria are strictly maintained. Angular distribution measurements also reveal anisotropic annealing characteristics of tracks, which may produce an additional sampling bias in track length measurement. © 1990.