Towards zircon fission-track thermochronology: Reference framework for confined track length measurements

Abstract

Horizontal confined track (HCT) lengths and angles to the crystallographic c-axis were measured for both unannealed spontaneous and induced fission tracks (FT) in zircons to establish a reference framework of HCT length measurement. Induced track lengths were analysed on eleven zircon separates from various rock types, including four age standards, and spontaneous lengths on three zircon standards were analysed. In general, the observed HCT lengths in each sample are slightly anisotropic, with shorter tracks having greater angles to the c-axis. In addition, all samples show large anisotropy in the number of observed HCT's, particularly for induced tracks. As an observed mean HCT length is controlled by the combined influence of these two anisotropies, selective measurement of tracks that have specified azimuth angles to the c-axis (e.g., 60–90°) is recommended in annealing studies or thermochronological applications. Mean spontaneous and induced HCT lengths of 60–90° are approximately concordant for individual age standards, although slightly controlled by the difference in overetching effects due to the different etching criteria adopted. This indicates that natural track annealing at an ambient surface temperature at geological timescales is negligible in zircon, in contrast to the case in apatite. Mean lengths of induced HCT's do not differ significantly between the zircons analysed. Therefore the overall mean and standard deviation values of HCT lengths for 60–90° azimuth angles, i.e. 10.50 ± 0.04 (1 standard error) μm and 0.76 ± 0.03 μm for spontaneous tracks and 10.73 ± 0.04 and 0.63 ± 0.03 μm for induced tracks, respectively, offer reliable references for HCT length measurement in zircon FT thermochronology.