CONFINED FISSION-TRACK LENGTH MEASUREMENT OF ZIRCON - ASSESSMENT OF FACTORS AFFECTING THE PALEOTEMPERATURE ESTIMATE

We carried out a series of laboratory annealing experiments in order to understand the factors affecting observed confined track lengths in thermally annealed zircon. The variation of confined fission-track lengths according to both experimental and analytical conditions was determined on zircons from Nisatai Dacite. It was found that etching temperature, track etching criteria, measurement criteria of horizontal confined tracks (HCT's) and the orientation of HCT's significantly control the observed track lengths if samples are highly annealed (i.e. > 600 degrees C for 1-hr annealing). The first three factors may be related to the presence of gap zones in an annealed track, whilst the fourth is caused by anisotropic annealing and etching characteristics. For reliable thermal history reconstruction, therefore, it should be the minimum requirement to document explicitly the experimental and analytical conditions of these factors in zircon fission-track analysis.