Fission track dating

Fission track dating ( fission track dating ) is a radiometric dating method for the cooling history of rocks in terms of specific, contained in rocks minerals ( thermochronology ).

Fission tracks are micrometer-sized defect lines (English tracks) in the crystal structure of minerals ( lattice defects ), the case of spontaneous radioactive decay (English fission ) of uranium -238 ( 238U ) arise. The fission tracks created by the diametrically apart moving fission products of uranium. The resulting fission track can be made visible by etching the polished crystal surfaces and has a length of about 10 to 20 microns and a diameter of about 1-5 microns. The length and the density of fission tracks is used to determine the cooling age of rocks. For this purpose one can use all uranium-bearing minerals in principle, but most are apatite and zircon. With their different closure temperatures, the cooling history within the upper 10 to 2 km of the Earth's crust can be represented. The fission track dating is a standard method in the geosciences and is used to reconstruct orogenic processes to determine erosion and uplift rates of rocks and to illustrate the thermal history of sedimentary basins. Due to this last point, the fission track method is also frequently used in the field of hydrocarbon exploration.

Since the gradual decay of the uranium atoms is carried out at a constant rate, the number of fission tracks is proportional to the uranium content and the age of the examined crystal. To determine the proportion of decayed atoms of 238U, the number of fission tracks is measured. Subsequently, the sample is exposed to thermal neutron radiation so that the atoms of the other isotope of uranium, uranium -235 ( 235U ) decompose. These decays are mapped to a uranium - free external detector, which usually consists of white mica (muscovite ). The induced by the neutron bombardment fission tracks are counted on the external detector. Since the ratio of 235U and 238U is constant in nature, the initial content of 238U can be calculated on the basis of the examined Minerals of the induced fission track density at a known neutron flux. On the basis of the relationship between natural and artificially produced fission tracks can now be closed to the age of the crystal.

Methodological weaknesses and solutions

In the literature, however, different decay rates are given for the spontaneous decay of 238U. Therefore, one used for the calculation of fission tracks - aging called a zeta factor to each Datierer determined individually by repeated dating of samples with known ages (standards). These standards are minerals from volcanic deposits whose age was determined by independent dating methods, such as the Ar - Ar thermochronology. For a zeta calibration usually several months are needed, which makes the incorporation of relatively time-consuming in the fission track dating.

In the past it was often viewed as a methodological flaw is that fission track dating of how all thermochronological methods, so-called for slowly cooled samples " apparent cooling age " provide that are not directly related to a specific geological event. Since by the shortening pattern of fission tracks the Abkühlpfad the samples tested are presented in detail, however, precisely this perceived weakness is the major benefit and the unique feature of this method is that instead of giving only a punctual information about the geological history of the sample, the thermal history on a longer period can be reconstructed from several millions of years.