3. Fission track dating methods
3.1 Alternative Dating Strategies
In fission track dating many alternative strategies are possible for measuring the ratio of spontaneous to induced track densities, upon which the age depends. Not all of these dating strategies are equally reliable in every case, and care is required to ensure that an appropriate method is selected. In the optimum case spontaneous and induced tracks would have identical properties and could be measured under identical conditions of registration, etching and counting over areas having exactly the same uranium concentrations. A procedure where these conditions were met might be described as an ideal dating method. But such a method is only rarely possible and usually some compromise is demanded by the nature of the material being dated. In practice a variety of factors such as the registration geometry of the etched surface, accumulated radiation damage, anisotropic etching and uranium inhomogeneity must be considered when selecting a suitable method for a particular sample.
The various fission track dating methods differ importantly in the registration geometries of the etched surfaces used to count spontaneous and induced tracks, and the corrections required if these are not equivalent. Other important differences between different dating methods concern whether or not the track densities are measured on single grains or averaged over a large number of grains, and whether an annealing step is required to remove spontaneous tracks prior to neutron irradiation. Figure 3.1 gives an outline of the procedures used in a variety of methods that have actually been used in different fission track studies. All of these methods require that the uranium-bearing mineral grains be physically separated from their host rock. In practice greatest use is made of the population and external detector methods and these remain the best alternatives in most dating situations. Only these two techniques will be considered here and further discussion of other sample handling methods can be found in Gleadow (1981) and Hurford and Green (1982).
Figure 3.1: Schematic representation of different fission track dating procedures (Hurford and Green 1982). Of these alternatives, only the Population and External Detector methods have gained wide currency.
3.2 Registration Geometry
Registration geometry describes the spatial relationship between the track recording surface and the source of the tracks. There are three major types of surface used for track etching and these are illustrated in Figure 3.2. An internal surface is one which is cut artificially through the track producing material after the tracks have been formed. Tracks are recorded on an internal surface which have crossed it from both sides (4π irradiation geometry). Most dating methods use an internal surface for recording spontaneous tracks.
External surfaces are those which were already in existence at the time the tracks were produced, e.g. a polished surface for induced tracks or a crystal face for spontaneous tracks. Such surfaces will receive tracks from one side only (2π geometry). An external detector surface is one where tracks produced in a uranium-bearing material are recorded in an adjacent track recording material, such as a sheet of mica. For a given number of fission events both these surfaces will only record half as many latent (i.e. unetched) tracks as an internal surface.
Figure 3.2: Different types of surfaces which can be used for the registration of fission tracks.
In many dating procedures spontaneous tracks are measured on an internal surface and induced tracks on an external, or external detector surface. In such cases a correction factor must be applied to the ratio of track densities to adjust for the different registration efficiencies of the recording surfaces. Ideally, such correction factors should have a value of 0.5 and this may be closely approximated in certain techniques. A full discussion of geometry factors and their departures from the ideal values can be found in Reimer et al. (1972), Gleadow and Lovering (1977), Green and Durrani (1978) and Wagner and van den Haute (1992).
3.3 The Population Method
In the population method the spontaneous and induced track densities are measured on two aliquots of the separated mineral grains. The induced tracks are revealed in grains from which the spontaneous tracks have been removed by laboratory annealing prior to neutron irradiation. This annealed aliquot is then irradiated together with one or more standard glasses to monitor the neutron fluence. After irradiation separate polished mounts are made for spontaneous and induced tracks that are then etched together to reveal the tracks on internal surfaces. The track densities are measured as averages by counting the same area on a large number of grains in each mount. The number of grains counted for each track density measurement should preferably be several hundred, and no less than about 100. The sequence of steps involved in the Population method is illustrated diagrammatically in Figure 3.3.
Figure 3.3: The sequence of steps involved in the population method of fission track dating. This method has previously been extensively used for apatite but has now largely been replaced by the external detector method.
The major advantages of the population method are that identical etching conditions are used for both spontaneous and induced tracks and that they are revealed on surfaces with identical registration geometries. It is assumed that the two aliquots possess statistically equivalent uranium concentrations, an assumption which may be invalid if there is appreciable variation in uranium content between and within grains. In such cases of variable uranium concentration the precision of ages obtained by the population method is less than that from some other methods, such as the external detector method. Also, if some grains have a track density too high to count, or serious interference from artifacts such as lattice dislocations, then this method breaks down since it requires a representative selection of grains to be counted. An important disadvantage of the method is that it gives no information on the variation of apparent ages of individual mineral grains. Such information is proving to be of great importance in many applications, especially those on sedimentary rocks.
The population method is only useful for apatite, amongst the commonly dated minerals. The method works well for this mineral and can be usefully applied even where the track densities are very low by permitting the counting of relatively large aggregate areas. If the population method is used for sphene and zircon, however, the accumulated radiation damage is erased, along with the spontaneous tracks, during the laboratory annealing step. The etching properties of induced tracks in the annealed grains are then quite different, and less uniform than those of the spontaneous tracks with their accompanying radiation damage. The anisotropically etched induced track density can be seriously underestimated relative to the spontaneous track density, giving a spurious age. Applying identical etching conditions to the two sets of tracks, the main advantage of the population method, is not possible for these two minerals. In effect, the two sets of tracks are being etched in different materials and the relationship between them is variable and uncertain. Another problem for these two minerals is that variable uranium concentration means that the etching rate can vary so greatly from grain to grain (Figure 2.8) that only some of the grains will be well etched after a given etching time. The measurement of a representative average track density under such conditions is quite impossible. The population method is quite inappropriate for sphene and zircon, therefore, and they should be dated by the external detector method.
3.4 The External Detector Method
In the external detector method the spontaneous tracks are measured on an internal surface of the mineral whilst the induced tracks are measured on an external detector irradiated in contact with it, and subsequently etched. In this method spontaneous and induced tracks are measured in exactly matching areas from the same planar surface of an individual crystal. Inhomogeneity in uranium concentration between grains, and even within grains, is therefore not a problem with this technique. Also, because ages can be measured on individual grains, a careful selection of grains can be made to avoid those which may be badly etched or contain dislocations. Typically about 20 grains are counted and the results combined to give an age for the sample.
The external detector is usually a cleaved sheet of low-uranium muscovite. Suitable high quality mica external detectors can even be bought commercially already prepared and pre- cut to the correct size. Mica is more suitable as a detector in dating applications than plastics, such as lexan, because its track registration and etching properties are much more like those of the minerals with which it is being compared. A major advantage of the use of external detectors is the simplicity of handling required after irradiation. All that is necessary is to remove the mica detectors and etch them, nothing further being required for the mineral mount. In particular, no grinding and polishing of radioactive materials is required, in contrast to the population method. The sequence of steps involved in the External Detector Method is shown in Figure 3.4.
consequence of anisotropic etching rates in a mineral such as sphene or zircon is that some grain surfaces in a mount may have a low etching efficiency. Comparison of spontaneous tracks on such surfaces with induced tracks in an adjacent mica detector, where the etching efficiency approaches 100%, will clearly give erroneous results. A very careful selection of only the highest etching efficiency surfaces, as identified by sharp polishing scratches, is therefore essential for the external detector method. For such grains, the ideal geometry factor of 0.5 can be applied to the observed track density ratio to correct for the different registration geometries of the two counting surfaces. Care is also necessary when dealing with low track densities in anisotropically etching minerals to ensure that etching has been sufficient for even the most weakly etched tracks to be revealed. In some materials, such as young sphenes, it is extremely difficult to reveal tracks at all in certain orientations. This effect is moderated by the accumulating radiation damage so that most sphenes and zircons with track densities above about \(10^{5} cm^{-2}\) can be analysed very well by the external detector method (Gleadow 1981).
The external detector method is also the preferred dating method for apatite because of the information gained on the distribution of single grain ages. With care, therefore, the external detector method is the most universally applicable, precise and practically convenient of the varied fission track dating methods now available for minerals.
Figure 3.4: The sequence of steps involved in the external detector method of fission track dating. This method is now the dominant procedure used in most fission track dating laboratories for apatite because of its ease of handling, suitability for automation and its provision of single grain age information.
3.5 The Laser-Ablation ICP-Mass Spectrometry Method
During the 1990s, Laser-Ablation Indictively-Coupled-Plasma Mass Spectrometry (LA- ICP-MS) emerged as a rapid, precise and routine method for trace element and isotopic analysis in minerals at the micro-scale. This is the first technique that has been able to compete with the traditional neutron irradiation methods in terms of high spatial resolution and less than parts per million sensitivity. This analytical development has added a new approach to fission track analysis, whereby \(^{238}U\) can be determined directly in mineral grains, rather than using \(^{235}U\) fission tracks as a proxy, as required by both the Population and External Detector Methods. LA-ICP-MS facilities are now becoming widely available and this mode of analysis has considerable advantages over the conventional methods as it no longer requires neutron irradiations, and the long delays in sample processing (typically several months) that they require. Other advantages of this approach are that it eliminates the need for handling radioactive materials and reduces the overall requirement for fission track counting. Only one track density (the spontaneous track density) is required for this method.
The first detailed study of LA-ICP-MS for fission track analysis was carried out by Hasebe et al. (2004) and more recent studies by Donelick et al. (2005) and Noriko et al. (2009) have provided additional experimental details. These authors have demonstrated the effectiveness of this approach and other research groups are now also adopting this method. The University of Melbourne Group switched to this new technique from 2011.
The sequence of steps in the analysis using LA-ICP-MS is illustrated in Figure 3.5. The first three steps are the same as for the EDM, in that a grain mount is prepared, polished and then etched to reveal the spontaneous fission tracks. The spontaneous tracks are then counted or a set of digital images captured using the automated image capture system (see 4.7 below). The grain coordinates and the slide are then transferred to the laser-ablation stage, and analysed by LA-ICP-MS. Most studies conducted so far have used a single ablation spot of around 20-30 μm diameter, centred in the middle of the area where the tracks were counted. As all of the tracks which are etched on a surface in apatite originate within ~ 8 μm of the surface, it is important to ablate the surface only to about this depth, in case there is zoning of the uranium concentration in the vertical dimension. The \(^{238}U\) concentration is determined relative to a suitable standard of accurately known, and uniform uranium abundance.
The LA-ICP-MS method for direct deterination of \(^{238}U\) is usually calibrated using a variant of the zeta calibration approach described in section 5.2 below, or can be calculated as an absolute age using explicit values for all of the constants in the age equation.
Figure 3.5: The sequence of steps involved in the LA-ICP-MS analysis method. This method is now being used in several major fission track dating laboratories and is likely to become of increasing importance and gradually replace the External Detector Method. The first three steps are the same as for the EDM, but the uranium analysis is conducted by ablating a pit on the polished surface and carrying the resulting plume into the plasma torch of the ICP-MS In an inert gas flow (usually Argon), indicated by the arrows.