Preprints
https://doi.org/10.5194/gchron-2021-8
https://doi.org/10.5194/gchron-2021-8

  02 Mar 2021

02 Mar 2021

Review status: this preprint is currently under review for the journal GChron.

Age distribution of horizontal fission tracks

Peter Klint Jensen1 and Kirsten Hansen2 Peter Klint Jensen and Kirsten Hansen
  • 1DTU Civil Engineering, Technical University of Denmark, Kgs. Lyngby, DK–2800, Denmark
  • 2Natural History Museum of Denmark, University of Copenhagen, Copenhagen, DK–1350, Denmark

Abstract. Equations for the distribution of age versus length of partially annealed horizontal fission tracks in apatite is presented. Probabilistic least–squares inversion corrects natural track length histograms for observational biases considering the variance of data, modelization, and prior information. The corrected histogram is validated by its variance–covariance matrix. It is considered that horizontal track data can be with or without measurements of angles to the c–axis. In the last case, 3D–histograms are introduced as an alternative to histograms of c–axis projected track lengths. Thermal history modeling of samples is not necessary for track age distribution calculation. In an example the age equations are applied to apatites with pre–depositional (inherited) tracks, to extract the post–depositional track length histogram. Fission tracks generated before deposition in detrital apatite crystals are mixed with post–depositional tracks. This complicates the calculation of the post– sedimentary thermal history as the grains have experienced different thermal histories until deposition. Thereafter the grains share a common thermal history. The extracted post–depositional histogram without inherited tracks may be used for thermal history calculation.

Peter Klint Jensen and Kirsten Hansen

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Peter Klint Jensen and Kirsten Hansen

Peter Klint Jensen and Kirsten Hansen

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Short summary
Fission tracks are generated in minerals due to spontaneous fission of U-238. The initial track length decreases with time due to recrystallization. The age of a track can be calculated by counting the number per volume of the shorter tracks and include the decay constant. However, the theoretical order of track length versus time is disrupted by uncertainties. We have shown how the order can be reestablished. It is then possible to determine the age of tectonic events such as uplift or burial.