Spontaneous fission decay of 238U present in apatite produces minute damage tracks in the crystal lattice that can be seen using an optical microscope. The number of these fission tracks reveals the time over which the tracks have been accumulating. Sensitivity of fission tracks to temperature results in significant track shortening (annealing) at temperatures above ~60°C over geological time. Measuring track lengths provides an estimation of temperature through use of the empirically derived track shortening-to-temperature relationship. In combination, FT apatite age and track-length data can provide a quantitative estimate of the low-temperature (<120°C) thermal history of a rock.
Apatite, where present, was separated from the core samples by conventional crushing, sieving, and magnetic and heavy-liquid separation techniques. Apatite grain mounts were prepared by using epoxy resin on a glass slide and then polishing using carbide papers and alumina. Tracks from spontaneous fission of 238U were revealed by etching with 5-N HNO3 at 20°C for 20 s. Uranium contents of the samples were assessed by irradiating with thermal (low-energy) neutrons, which induce fission in a proportion of 235U sample atoms. The induced tracks were recorded in an external detector of mica, clamped against the sample during irradiation (Gleadow, 1981). Samples were irradiated in the D-3 thermal facility of the Risø Reactor, National Research Center, Roskilde, Denmark, where the cadmium ratio (thermal/epithermal + fast neutrons) is >400, which is well-suited to FT use (Green and Hurford, 1984). Neutron fluence (or time-integrated neutron flux) was monitored using a Corning glass dosimeter CN-5, with a known uranium content of 11 ppm (Hurford and Green, 1982). After irradiation, sample and dosimeter mica detectors were etched in 40% hydrofluoric acid at 20°C for 45 min.
Spontaneous 238U and induced 235U fission track densities were counted for individual apatites in the crystal and its mirror-imaged impression in the mica detector, respectively. Zeiss Axioplan microscopes with 100x dry objectives and a total magnification of 1250x were used for counting. Only crystals with prismatic sections parallel to the c-crystallographic axis, and hence high etching efficiencies, were analyzed: the appropriate geometry factor between spontaneous and induced track densities in this case is 0.5. To avoid a bias in the results through selection of apatite crystals, each sample was systematically scanned, and each crystal that was encountered with the correct orientation was analyzed, regardless of the track density.
The IUGS-recommended zeta calibration approach was used to calculate the ages, whereby a proportionality or calibration constant zeta is evaluated by multiple analysis of mineral standards of known age (Hurford and Green, 1983). The zeta value of 339 ± 5 for dosimeter glass CN-5 has been derived by analyst Andrew Carter from 33 determinations of apatites from the Fish Canyon Tuff, Colorado; the Durango deposit, Cerro de Mercado, Mexico; and the Mount Dromedary banatite, New South Wales, Australia (Hurford, 1990).
A Houston Instruments digitizing tablet was used to measure horizontally confined FT lengths in the apatite crystals, with a cursor equipped with a high-intensity, red-light-emitting diode (LED). The image of the LED, viewed in the microscope via a drawing tube attachment, was superimposed on either end of the track in turn, and the X and Y coordinates for each track end recorded on the tablet. Calibration of the tablet against a stage micrometer provided a direct measure of each track length with an overall precision of about ± 0.1 µm.