Apatite Fission Track Thermochronology

2.2.1 Introduction

Apatite fission-track (AFT) analysis includes both the determination of an AFT age as well as modeling of time-temperature paths based on the measured fission-track length distributions. In the last decade, apatite fission-track analysis has become a standard technique and tool for investigating the low-temperature thermal evolution of rocks. It has been used in numerous geoscientific tasks, e.g.

tectonic modeling, landscape development, tectonic geomorphology, mountain building and temperature history of hydrocarbon source rocks, as well as thermal and burial histories of sedimentary basins (e.g. Ravenhurst et al. 1994; House et al. 1999). Additionally, as with the (UTh)/He method, by assuming a geothermal gradient and a surface temperature, AFT data can be used to infer burial depth and denudation rates (see Reiners and Brandon 2006 and references therein).

The main advantage of fission-track analysis over many other thermochronological methods, e.g. the (U-Th)/He method (see Chapter 2.1), is the fact that much more information than just a cooling age

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can be obtained (Fig. 2.3). Especially when several thermochronometers are used to constrain the low temperature thermal evolution of a rock, the combination of fission-track chronology and (UTh)/He dating is heavily recommended (e.g. Brandon and Reiners 2006). On one hand, the closure temperature of the FT system is higher than that of the He system; thus, a more detailed thermal-evolution path can be drawn by using both methods (Fig. 2.3). On the other hand, the FT method can be used to constrain reliable results from the (U-Th)/He method (see also Chapter 2.3).

2.2.2 Principles of Application

The fission-track technique is based on the spontaneous fission of the natural, unstable isotope ²³⁸U in U-bearing minerals such as apatite. Emitted fission products move diametrically to each other, resulting in atomically-scaled linear damage zones within the crystal lattice, which are named fission tracks (FT). The number of tracks within a sample depends on the U-concentration of the crystal and can be described by the decay constant of the ²³⁸U isotope λ = 8.46 x 10^-17/a. If the concentration of U within the crystal is known, the density of spontaneous fission tracks (named s = number of spontaneous tracks/cm²) can be used as a direct indicator of the age of the crystal. In order to determine the fission-track density, the atomically scaled fission tracks are etched with a chemical treatment to make them visible in optical methods. Subsequently, fission tracks are counted at high magnifications under an optical microscope (see Chapter 2.2.3).

The concentration of ²³⁸U is determined indirectly by irradiating the etched crystal with thermal neutrons in a nuclear reactor. Neutron irradiation induces the fission of the isotope ²³⁵U which itself, producing fission tracks which are traced by a U-free external detector atop the crystal (usually muscovite, see Chapter 2.2.3). The density of these induced fission tracks (named i = number of induced tracks/cm²) is obtained analogue to s by etching and counting under the microscope.

Because only the external detector and not the crystal will be etched again after irradiation, the induced fissions do not interfere with the counting of spontaneous tracks. Since the ²³⁵U/²³⁸U ratio is a constant in nature, the initial ²³⁸U content can be calculated from counted s and the known neutron flux used for irradiation.

Spontaneous fission tracks in apatite have approximately the same length. However, because fission tracks are thermally unstable, they are only retained within the crystal lattice below a specific temperature (see below; e.g. Ketcham et al. 1999). If mineral grains are heated above their closure temperatures, both the number and mean length of tracks are subsequently annealed, i.e. they become partially or even totally shortened due to restoration of the crystal lattice. In this respect, AFT ages can be considered as cooling ages, dating the cooling of the sample to a temperature at which the fission tracks became stable. For example, if fission tracks are completely annealing, the apatite fission-track age provides a cooling age recording the time the apatite passed through its

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closure temperature. A fission-track age does not generally reflect a discrete event (Donelick et al.

2005).

The extent of thermal annealing can be expressed by the length-distribution of the fission tracks in the sample. Measurements of the confined track length (horizontal tracks, i.e. tracks interior to the crystal, where both ends can be seen and are parallel to the cut surface of the crystal) in combination with the AFT age yield additional information about the samples' thermal history. For example, the track-length distribution of an apatite from rapidly cooled rocks (e.g. vulcanites) show characteristic long-track length and small standard deviations of 14-15 ± 0.8-1.3 µm (Gleadow et al. 1987). In contrast, slow cooling will result in shorter track length and higher standard deviation 12-14 ± 1-2 m (Fig. 2.3; Gleadow et al. 1987).

Fig. 2.3: Models of apatite fission-track length distribution (heavy lines in upper panels) and predicted track length parameters (lower panels) for apatite. l/lo = present track length divided by the initial length. The two lines at 60°C and 110°C indicate partial annealing zone (PAZ). A) Linear heating. All tracks have more or less the same length at present, as they all experience the same maximum paleotemperature. The track length distribution is unimodal, symmetrical, and has a short mean length. The fission track age does not relate to any discrete event. B) Linear cooling. Each track experiences a different maximum paleotemperature as cooling progresses, which is reflected in the characteristic negatively skewed length distribution. The age does not relate to any discrete event. C) Rapid cooling. Nearly all tracks are formed after the cooling episode, thus all are relatively long. In this case, the fission track age is a reasonable indication of the timing of cooling, compared with the age estimate for (b). D) Heating and cooling. The tracks formed during the heating period have similar lengths, while those formed after reflect the progressive cooling. The track length distribution is typically bimodal, and once again, the fission track age does not relate directly to the timing of cooling or timing of maximum paleotemperature (modified, from Gallagher et al. 1998).

28 2.2.3 Principles of Application

The temperature interval between the partial or complete annealing of spontaneous fission tracks in the apatite crystal is referred to as the apatite partial-annealing zone (PAZA; e.g. Gleadow and Duddy 1981). The temperature range of the PAZA depends on two major factors: (a) the crystallographic orientation of the tracks and (b) the chemical composition of the apatite.

Green and Durrani (1977) were the first to describe a connection between the annealing behavior and the crystallographic orientation of fission tracks. Tracks which are oriented perpendicularly to the crystallographic c-axis are more rapidly annealed than those parallel to the c-axis (e.g. Green 1988; Laslett et al., 1984; Donelick et al., 1990, 1999; Galbraith et al., 1990; Donelick, 1991, Barbarand et al. 2003b).

In addition to the crystallographic orientation, the annealing of fission tracks strongly depends on the chemical composition of the dated apatite crystal, specifically, the concentration of F and Cl, respectively the F/Cl-ratio. In general, F-rich apatites show more annealing than Ch-rich samples at the same time and temperature conditions (Gleadow and Duddy 1981; Green et al. 1985, 1986).

Differences in the annealing behavior can be significant, as expressed by the total annealing temperature of the Fl-rich Durango apatite (90-100°C), in contrast to a Cl-rich apatite (110°C-150°C;

see Burtner 1994, O’Sullivan and Parrish 1995).

Conclusions for the Cl and F contents can be drawn from the etching behavior of a sample, which is expressed by the kinetic parameter Dpar. The Dpar is defined as the mean diameter of the etch figures (geometrical figures formed by the intersection of an etch pit, i.e. the etched fission track and the etched surface) parallel to the crystallographic c-axis. The Dpar is measured on a polished and etched surface and is specified in µm. (e.g. Carlson et al. 1999; Donelick et al. 2005 and references therin). The Dpar has been shown to be an effective expression of the apatite annealing kinetics (e.g.

Ketcham et al. 1999). Values of Dpar observed in nature range from approximately 1.50 µm for apatites that are least resistant to annealing to approximately 5.00 µm or higher for apatites that are most resistant to annealing (e.g. Ketcham 2005).

Experimental studies on the annealing behavior of fission-tracks in apatite have yielded the basis for numerical algorithms accounting for both mixed-compositional apatite and crystallographic effects (see Ketcham et al. 2000 and references therein). Numerical computer modeling (e.g. AFTSolve software; Ketcham et al. 2000) can be used to combine the information derived from track-length distribution, age determination and annealing kinetics of a sample and, thus, yield valuable information on the thermal history of a sample by constraining the low temperature cooling history (see also Chapter 2.3).

29 2.2.4 Analytical Procedure

Following standard density and magnetic mineral separation techniques, the apatite samples were mounted on a glass slide with epoxy. According to Donelick et al. (1999), the mounts were etched at 21°C for 20 s using 5.5 M nitric acid after grinding and polishing procedures in order to reveal spontaneous tracks within the apatite crystals. The external detector method described by Gleadow (1981) was used, while low-uranium muscovite sheets (Goodfellow mica) represent the external detector for induced tracks. For age determination, the zeta calibration approach was adopted (Hurford and Green 1983), and 25 good-quality grains per sample were randomly selected and dated.

The fission-track ages were calculated using the software TRACKKEY version 4.2 (Dunkl 2002).

Additionally, for track length analysis, about 50–60 horizontal confined tracks of each sample were measured considering their angle to the c-axis (Donelick et al. 1999). In order to account for all the effects influencing the apatite age as well as annealing behavior and, thus, the rack length distribution, AFT data are modeled using the numerical simulation software AFTSolve (Ketcham et al.

2000), which accounts for crystallographic effects as well as mixed compositions in apatite (see Chapter 2.2.2). For more details on the analytical procedures and AFT modeling applied, the reader is referred to Löbens (2012).