Etchable fission track length reduction in apatite : experimental observations, theory and geological applications

Abstract

Induced fission tracks in apatite, and probably natural tracks as well, exhibit mean etchable lengths that depend in part on time, temperature, and track orientation in the host crystal. A model is developed whereby mean etchable track length variec elliptically with track angle to the crystallographic c-axis. Using this model and the proposed statistical treatment, it is possible to measure the mean etchable lengths parallel (cB) and perpendicular (aB) to the c-axis for single populations of fission tracks in apatite. Experiments conducted over a range of heat treatment conditions indicate the following: (1) the shape of the etchable track length distribution is characteristically normal with standard deviation 0.75 μm over all orientations when aB ≥ 8 μm, (2) both aB and cB undergo measureable reduction of order 0.5 μm during the first three weeks after irradiation and (3) the "activation energy" for annealing is, at most, only weakly dependent upon orientation and the values of either aB or cB. The latter observation is a requirement for the mean etchable track length at fixed orientation to be an unique reflection of the state of the fission tracks, independent of the shape of the time-temperature path followed to reach that state. Given this notion, a method is developed that permits an objective estimate of the time-temperature path experienced by a natural apatite to be obtained given knowledge of its fission track age and etchable track length distribution. Several applications to natural apatites indicate that this method is capable of distinguishing between different thermal histories and, therefore, that it can be useful for elucidating the low-temperature thermal histories of rocks.