Research Methods - Thermochronology
Apatite fission-track thermochronology
Information on the thermal history of apatite are
stored in two archives: the etch pit areal density at an artificially
polished internal surface, and the length distribution of horizontal
confined tracks. The single grain age and the confined length
distribution are used to numerically model possible t-T paths for
The simulation program (e.G. HeFTy) are based on algorithms, which are derived from a combination of laboratory annealing experiments and FT data from natural key areas where the geological t-T history is reasonably known (Green et al., 1986, 1989; Laslett et al., 1987; Duddy et al., 1988; Crowley et al., 1991; Lutz and Omar, 1991; Gallagher, 1995; Galbraith and Laslett, 1996; Willett, 1997; Ketcham et al., 1999; Carlson et al., 1999; Barbarand et al., 2003a, Ketcham 2005).
The temperature sensitive annealing of fission-tracks in apatite is constraint by two important effects 1.) the crystallographic orientation of the tracks and 2.) the chemical composition of the apatite (e.G. Cl/F-ratio).
The crystallographic effect was first described by Green and Durrani (1977).
Tracks orthogonal to the c-axis anneal more rapid than tracks parallel to the c-axis (Green, 1988).
This anisotropy increases with annealing (Green, 1981; Laslett et al., 1984; Donelick et al., 1990, 1999; Galbraith et al., 1990; Donelick, 1991). Donelick et al. (1990, 1999), and Donelick (1991) further extended the database on crystallographic effects and integrated the results in the recent multikinetic annealing model of Ketcham et al. (1999) and Ketcham (2003). Barbarand et al. (2003b) confirmed the strong influence of the crystallographic orientation by presenting a large annealing dataset of apatites.
The first geological observation elucidating that the chemical composition of apatite might influence the fission-track annealing rate was described by Gleadow and Duddy (1981) for drill-core samples from the Otway basin in Australia. Green et al. (1985, 1986) demonstrated that the annealing of fission-tracks in apatite is dependent on the chlorine/fluorine ratio, where fluorine rich apatites show more annealing then chlorine rich samples at the same t-T history.
The effect of composition has been described for sedimentary and magmatic environments (Burtner, 1994; O’Sullivan and Parrish, 1995). Fluorine-rich apatites such as Durango apatite show a complete annealing in geological environment at temperatures of 90 °C – 110 °C. In contrast, chlorine-rich apatites completely anneal at temperatures of 110 °C – 150 °C. In a recent study, Barbarand et al. (2003a) gave further indications that chlorine has a dominant control on the fission-track annealing.
The multikinetic model that is the base of the numerical simulation software AFTSolve, includes mixed-compositional apatites and accounts for crystallographic effects (Ketcham et al., 2000). The partial annealing zone of fission tracks in apatite range from about 110°C/10Ma (depending on chemical composition) to about 60°C/10Ma.