Fission Track Thermochronology and its Application to Geology
This chapter reviews the background, beginnings and early development of fission track (FT) thermochronology. In the 1930s, it was discovered that uranium would break into two lighter products when bombarded with neutrons and, subsequently, that uranium was capable of natural, spontaneous fission. The fission process produced damage tracks in solid state detectors, which could be revealed by chemical etching and observed by electron and, later, by optical microscopy. Fleischer, Price and Walker at the General Electric R&D laboratories developed diverse track etching procedures, estimates of track registration and stability in different materials, track formation models, uranium determination in terrestrial, lunar and meteorite samples, neutron dosimetry and mineral dating using U spontaneous fission. Application to dating of natural and man made glass was frustrated by low uranium content and relative ease of track fading (annealing) In the 1970s 1980s, most FT analyses used apatite, zircon and titanite (sphene) to date tephra and acid intrusive rocks with the recognition of differing sensitivities of track annealing in each mineral. Studies in the Alps showed apatite with its greater susceptibility to annealing could provide estimates of the timing and rate of exhumation. The landmark 1980 Pisa FT Workshop highlighted problems with FT system calibration and emphasised the value of annealing in apatite to reveal thermal history. System calibration eventually reached a consensus agreement in 1988 at the Besancon FT Workshop with the majority of analysts adopting the zeta comparative approach. Multiple laboratory and borehole studies have determined the conditions for track annealing in apatite leading to widespread applications in exhumation, sedimentary basin, hydrocarbon exploration and other areas. 1.1 FT Thermochronology: The Fundamentals If a crystal of apatite (Ca5(PO4)3(F, Cl, OH) is immersed in dilute nitric acid at room temperature for about 20 s, minute etch figures are revealed which can be observed and counted under an optical microscope at high magnification (Fig. 1.1a) Similar etch figures are found if zircon (ZrSiO4) and titanite (sphene) (CaTiSiO5) are immersed in different chemical etchants for appropriate times and temperatures (Fig. 1.1b, c) see Chap. 2, (Kohn et al. 2018) for specific conditions. These etched tracks have accumulated over time and result from the natural, spontaneous fission of U atoms in the crystal lattice of the minerals. Each track results from the fission of a single atom. The number of these spontaneous tracks also depends upon the amount of uranium in the apatite, which is determined conventionally by irradiating the sample with low energy neutrons. Irradiation induces a proportion of the less abundant U isotope to fission, giving a second generation of induced tracks that are revealed and counted in the apatite itself or, more usually, in a detector held against the mineral. The ratio of spontaneous to induced tracks provides a measure of the atoms decayed by fission to the total uranium remaining. Allowance has to be made in calculation for the additional decay of uranium by alpha emission. This spontaneous to induced track ratio, when usedwith the rate of spontaneous fission decay, a physical constant, yields the time during which fission tracks have been accumulating in the mineral which in some circumstances may equate to the age of the sample. This method is known as fission track (FT) dating. A. J. Hurford 32 Tempest Ave, Potters Bar, Hertfordshire EN65JX, UK e mail: tony.hurford@btinternet.com Spontaneous fission also occurs in U, U and Th, but their spontaneous fission half lives are too long and/or abundances too low to produce significant numbers of natural tracks compared to U. (c) Springer International Publishing AG, part of Springer Nature 2019 M. G. Malusa and P. G. Fitzgerald (eds. Fission Track Thermochronology and its Application to Geology, Springer Textbooks in Earth Sciences, Geography and Environment, https:/doi.org/10.1007/978 3 319 89421 8_1 3 Spontaneous tracks heal or fade naturally at temperatures above ambient values, a process known as annealing. Early applications to natural and man made glasses found annealing of tracks (along with low uranium contents) to be major problems. Apatite is more resistant to track fading than glass, but more susceptible than zircon or titanite, especially at temperatures typically found in the upper few kilometres of the Earth's crust. Thus, calculated FT ages may not relate to the age of formation of a sample but to its subsequent history; measured ages should be considered alongside all available geological information. Understanding the annealing parameters of fission tracks has provided the means for interpreting sample thermal history and interpreting the measured FT age. The particular sensitivity of apatite to annealing has made it especially useful in reconstructing thermal history, in particular by relating the reduction of the lengths of spontaneous tracks (specifically confined track lengths see below) to temperature and time. Since each track is formed at a different time in the life of a sample, it is exposed to a different portion of that sample's thermal history. The overall track length distribution thus preserves the integrated temperature record of (a) (b) (c) (d) Fig. 1.1 Fission tracks etched in commonly used materials (scale bars approx. 50 mm) a Spontaneous U tracks in apatite from Tioga ash bed, Pennsylvania, USA (Roden et al. 1990) Etched 5 M HNO3 at 20 deg C for 20 s. b Spontaneous U tracks in zircon from Fish Canyon Tuff. Etched in 100 N NaOH at 220 degC for 6 h (these samples predate use of the NaOH KOH eutectic etchant) c Spontaneous U tracks in titanite (sphene) from the Thorr Granite in Donegal, Ireland. Etched in 1HF:2HCl:3HNO3:6H2O acid mixture at 20 degC for 6 min. d Induced U tracks in a nineteenth century man made U bearing glass from Robert Brill at Corning (Brill et al. 1964) Etched in 40% HF at 20 degC for 5 s. Images courtesy of Andy Gleadow and Andy Carter 4 A. J. Hurford