- © The Mineralogical Society Of America
Several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method. Of these, apatite [Ca5(PO4)3(F,Cl,OH)], has proved pre-eminently suitable for this purpose for reasons discussed below. Apatite was one of the first minerals, amongst many others, to be investigated for fission track dating by Fleischer and Price (1964) and has subsequently become by far the most important of all the minerals used for dating by this method. The usefulness of apatites for fission track dating arises from their near-universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups.
Price and Walker (1963) first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238U within their lattices could be used for geological dating. They also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope (Price and Walker 1962b, 1963). This simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses (e.g., Fleischer et al 1975), the most important of which was fission track dating. Fleischer et al. (1964) also showed that spontaneous, or fossil, fission tracks provided an explanation for various ‘anomalous’ etch pits in apatite which had puzzled crystallographers over many years. The essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present. Knowing the rate of spontaneous fission decay, a geological age can be calculated.