Fission Track Thermochronology Lab, Basel University

The fission track (FT) method is a single crystal technique and has several advantages in comparison to other dating methods: (1) any loss of daughter products due to increase of temperature can be detected through track-length measurements and (2) the low temperature range covered by the FT method (~300 – 60°C) is out of detection of most other radiometric dating systems.

FT thermochronology is widely used for reconstruction of low-temperature thermal histories in upper crustal rocks. The method has found particular application in estimating temperature history and long-term denudation rates in orogenic belts, rifted margins and more stable areas, providing a means of assessing the timing and volume of sediment being delivered to sedimentary basins, and as an estimator of hydrocarbon maturity potential.

Fission-track analysis of U-bearing minerals (mainly zircon, apatite and sphene) is based on the natural decay by spontaneous fission of the ²³⁸U isotope. The high relative abundance of ²³⁸U and the longer half-life with respect to fission of other naturally fissioning isotopes (such as ²³⁵U and ²³²Th) infer that all natural tracks in terrestrial minerals are the products of fission of ²³⁸U atoms, located within the mineral itself (Fleischer et al., 1975). Natural fission of the U nucleus is an explosive event during which two highly charged particles fly in opposite direction from each other at high velocity (Fleischer et al., 1975), producing a single damage trail in the crystal that is identified as a spontaneous fission track. The submicroscopic tracks accumulate over time and are revealed by chemical etching of polished internal surfaces of the crystal (Price and Walker, 1962).

For the fission-track system of apatite a transition zone where tracks are essentially unstable is recognized. This is the partial annealing zone and is defined by upper and lower temperature limits. The effective closure lies within these bounds and is dependent on cooling rates. The partial annealing zone for apatite lies between 60°C and 120°C (Green and Duddy, 1989; Corrigan, 1993), with a mean effective closure temperature constrained at 110 ± 10°C.

In apatites, tracks are formed continuously with an approximately uniform length of ca. 16.3mm. Upon heating, tracks are annealed or shortened to a length that is determined by the maximum temperature and the time experienced. For example, at a temperature of 110 – 120°C for a period of 10⁵ – 10⁶ years, tracks are completely annealed. This characteristic allows construction of time-temperature paths of many different rock types by inverse modelling of observed FT age and confined track length data (Gallagher, 1994; Ketcham et al. 2000).

Similar principles apply for the zircons, but our knowledge of zircon annealing is not as advanced as that of apatite. Nevertheless, wide-ranging values for the temperature bounds for the partial annealing zone of zircon have been published. It is well accepted that at high cooling rates the closure of the radiometric system takes place at higher temperatures. The latest estimations of the zircon partial annealing zone suggest temperature limits of ~390 – 170°C (Yamada et al., 1995) and of ~310 – 230 °C (Tagami and Dumitru, 1996; Tagami et al., 1998). Recently, in his overview on the zircon fission track dating method, Tagami (2005) reported temperature ranges for the closure temperature between ~300 – 200 °C. Accordingly, we use a value of 250 ± 50 °C for the mean effective closure temperature and the 200 – 300 °C temperature interval for the partial annealing zone.

References

• Corrigan, J.D. 1993. Apatite fission-track analysis of Oligocene strata in South Texas, U.S.A.; testing annealing models., Chemical-Geology, 104, 227-249.
• Fleischer, R.L., Price, P.B. and Walker, R.M. (eds.), 1975. Nuclear Tracks in Solids: Principles and Applications. University of California Press., Berkeley.
• Foster, D., Kohn, B. and Gleadow, A.J.W. 1996. Sphene and zircon fission track closure temperatures revisited: empirical calibration from 40Ar/39Ar diffusion studies of K-feldspar and biotite. In: B. (eds.), International Workshop on Fission-Track Dating, University of Ghent Ghent, Belgum.
• Gallagher, K. 1994. Genetic algorithms: A powerful new method for modelling fission-track data and thermal histories. In: Lanphere, M.A., Dalrymple, G.B., Turrin, B.D. (eds.), Proceedings International Conference of Geochronology, Cosmochronology and Isotope Geology. US Geological Survey.
• Green, P.F. and Duddy, I.R. 1989. Some comments on paleotemperature estimation from apatite fission track analysis. J. of Petrol. Geol., 12, 111-114.
• Ketcham, R.A., Donelick, R.A. and Donelick, M.B. 2000. AFTSolve: A program for multi-kinetic modeling of apatite fission-track data. Geol. Mater. Res., 2, 1-32.
• Price, P.B. and Walker, R.M. 1963. Chemical etching of charged particle tracks in solids. J. Appl. Phys., 33. 3400-3406.
• Tagami, T., 2005. Zircon Fission-Track Thermochronology and Applications to Fault Studies. Reviews in Mineralogy and Geochemistry, 58, 95-122.
• Tagami, T. and Dumitru, T.A. 1996. Provenance and history of the Franciscan accretionary complex: Constraints from zircon fission track thermochronology. J. Geophys. Res., 101, 8345-8255.
• Tagami, T., Galbraith, R.F., Yamada, R. and Laslett, G.M. 1998. Revised annealing kinetics of fission tracks in zircon and geological implications, In: Van den Haute, P. and de Corte, F. (eds.), Advances in Fission-Track Geochronology,. Solid Earth Sci. Libr. Kluwer Acad., Norwell, Mass., 10, 99-112.