16.2 Track etching

Several different types of geological material are suitable for the determination of fission track ages. Fleischer and Price (1964a) tested them with different acid or alkali leaching solutions to determine the most effective for track observation. The precise progress of the etching process depends on the composition of the matrix and the nature, concentration and temperature of the acid. This can give rise to a surprising variation in the appearance of etched tracks in different materials (e.g. Fig. 16.3), and may affect the accuracy of track counting. These problems were discussed by Fleischer and Price (1964b) in an assessment of the fission track dating of glass.

Fig. 16.3. Drawings of etched fission tracks induced by the same source (²⁵²Cf) in different materials: a) K feldspar; b) soda-lime glass; c) Lexan polycarbonate. Width of each field is 40 :m. From photographs by Fleischer et al. (1968).

The geometry of an etched track depends on the rate of etching down the axis of the track (from its intersection with the surface), relative to the general rate of attack of the polished surface (Fig. 16.4a). One problem in accurate track counting is to distinguish etched tracks from other features. For example, track pits in glass are at first pointed, but with increased etching time they round out. The optimal etching time is then a compromise between the need to make large enough pits to count quickly, and the tendency for large round-bottomed pits to be confused with etched porosity. However, this is not such a problem in mineral phases.

Fig. 16.4. Schematic illustration of the progress of track etching. a) Perpendicular to surface; b) tangential. After Fleischer and Price (1964b).

Another source of uncertainty for both glass- and mineral-dating is caused by tracks which barely register in the etched surface. For example, tracks which are almost tangential to the surface may be completely erased by etching (Fig. 16.4b). Other tracks may not have intersected the original polished surface, but are exposed by the general attack of the surface during etching. These discrepancies will average out statistically if large numbers of tracks are counted with identical spatial geometry (see below), but may cause large errors when spatial geometry varies. A more detailed discussion of track formation and track etching is given by Fleischer et al. (1975).

Fleischer and Price (1964a) estimated the dating range of fission track analysis with different types of material. Using the criterion that dates of reasonable precision can only be determined when the track density is at least 100 per cm², the lower end of the dating range can be estimated for different types of material according to uranium content (Fig. 16.5).

Fig. 16.5. Diagram to show the dating range for fission track analysis of different kinds of geological material according to uranium content. After Wagner (1978).

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