Detail on the formation and annealing of fission tracks in apatite and zircon.

AFTA^® (Apatite Fission Track Analysis) and ZFTA (Zircon Fission Track Analysis) rely on analysis of radiation damage features ("fission tracks") in detrital apatite and zircon grains, respectively, within sedimentary rocks. Fission tracks are produced continuously through geological time, as a result of the spontaneous fission of ²³⁸U atoms. Once formed, tracks are shortened (annealed) at a rate which depends on temperature, and the final length of each individual track is determined by the maximum temperature which that track has experienced. Therefore as the temperature to which an apatite or zircon grain has been subjected increases, all existing tracks shorten to a length determined by the prevailing temperature, regardless of when they were formed. After the temperature has subsequently decreased, all tracks formed prior to the thermal maximum are "frozen" at the degree of length reduction they attained at that time.

Apatite in sediments which have been heated to a maximum paleotemperature less than ~110^oC (the precise value depends on the chlorine content of the apatite grain) at some time in the past and subsequently cooled will contain two populations of tracks - a shorter component formed prior to the thermal maximum and a longer component representing tracks formed after cooling. The length of the shorter component indicates the maximum paleotemperature while the proportion of short to long tracks indicates the timing of cooling in relation to the total duration over which tracks have been retained. More complex thermal histories result in more complex distributions of track length. If the maximum paleotemperature exceeds ~110^oC, all tracks are totally annealed, and tracks are only retained once the sample cools below this temperature. In zircon, fission tracks are more stable, and the maximum paleotemperature must exceed ~300^oC (corresponding to vitrinite reflectance values of at least 6% Ro(max) before zircons show appreciable annealing effects.

The annealing kinetics of fission tracks in apatite during geological thermal histories is well understood, based on study of the response of fission tracks to elevated temperatures both in the laboratory (Green et al., 1986; Laslett et al., 1982, 1987; Duddy et al., 1988; Green, 1988, Green et al., 1989b) and in geological situations (Gleadow and Duddy, 1981; Gleadow et al., 1986, Green et al., 1989a).

Natural apatites essentially have the composition Ca₅(PO₄)₃(F,OH,Cl). The amount of chlorine in the apatite lattice exerts a subtle compositional control on the degree of annealing, with apatites poorer in chlorine being more easily annealed than those richer in chlorine. The result of this effect is that in a single sediment sample, individual apatite grains may show a spread in the degree of annealing, manifested by a range of fission track ages and lengths between apatite grains.

The data are interpreted using proprietary multi-compositional kinetic equations based both on laboratory annealing studies on a range of apatites with different Cl contents, and on observations of geological annealing in apatites from a large number of samples from exploration wells in which thermal histories are simple and well understood.