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.
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. 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 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 U 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.