Gap trace dating

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Fission track dating (Fission Track Dating) is a radiometric method for dating the cooling history of rocks based on certain minerals contained in the rocks ( thermochronology ).

Fissure traces are micrometer-sized lines of defects ( tracks ) in the crystal structure of minerals ( lattice defects ) that arise during spontaneous radioactive decay ( fission ) of uranium- 238 ( 238 U). The fission traces arise from the uranium fission products moving diametrically apart. The resulting crack can be made visible by etching the polished crystal surfaces and then has a length of approx. 10 to 20 μm and a diameter of approx. 1 - 5 μm. The length and the density of the fissure tracks are suitable for determining the cooling age of rocks. In principle, all uranium-containing minerals can be used for this, but the most common are apatite and zircon . With their different closing temperatures, the cooling history can be shown within the upper approx. 10 to 2 km of the earth's crust. Fissure trace dating is a standard method in geosciences and is used to reconstruct mountain-forming processes, determine erosion and uplift rates of rocks and to depict the thermal history of sedimentary basins. Because of the latter point, the fissure trace method is also often used in the field of hydrocarbon exploration.

Since the gradual decay of the uranium atoms occurs at a constant rate , the number of fission traces is proportional to the uranium content and the age of the crystal examined. To determine the proportion of decayed atoms of 238 U, the number of cleavage traces is measured. The sample is then exposed to thermal neutron radiation, causing the atoms of another uranium isotope, uranium-235 ( 235 U), to decay. These decays are displayed on a uranium-free external detector, which normally consists of light mica (muscovite). The slit tracks induced by the neutron bombardment are counted on the external detector. Since the ratio of 235 U and 238 U is constant in nature, the initial 238 U content of the investigated mineral can be calculated using the induced fissure trace density if the neutron flux is known . The age of the crystal can now be deduced from the ratio of natural and artificially created cracks.

Methodological weaknesses and possible solutions

In the literature, however, different decay rates are given for the spontaneous decay of 238 U. For this reason, a so-called zeta factor is used for the calculation of cleavage-trace ages, which each dater determines individually by repeated dating of samples with known ages (standards). These standards are minerals from volcanic deposits, the ages of which have been determined by independent dating methods such as Ar-Ar thermochronology. A zeta calibration usually takes several months, which makes it relatively time-consuming to familiarize yourself with the dating of fissure traces.

In the past, it was often seen as a methodological weak point that fissure trace dating, like all thermochronological methods, provides so-called “apparent cooling ages” for slowly cooled samples, which have no direct reference to a specific geological event. Since the cooling path of the examined samples can be shown in detail due to the shortening pattern of the fissure tracks, it is precisely this supposed weak point that is the great advantage and the unique selling point of this method: instead of only providing selective information about the geological past of the sample, the thermal history can be can be reconstructed over a period of several million years .

literature

  • Günther Wagner and Peter van den Haute: Fission-Track Dating (Solid Earth Science Library vol. 6) , Dordrecht, Kluwer Academic Publishers Group, 1992, ISBN 0-7923-1624-X .

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