fission track

fission-track dating

Method of determining the age of a mineral that utilizes the damage done by the spontaneous fission of uranium-238, the most abundant isotope of uranium. The fission results in radiation damage, or fission tracks, that can be made visible by preferential leaching (removal of material by solution) of the host substance with a suitable chemical reagent; the leaching process allows the etched fission-track pits to be viewed and counted under a microscope. The amount of uranium present can be determined by irradiation that produces thermal fission of uranium-235, which produces another population of tracks, related to the uranium concentration of the mineral. Thus, the ratio of naturally produced, spontaneous fission tracks to induced fission tracks is a measure of the age of the sample. See also dating.

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Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium bearing minerals and glasses. Uranium-238 undergoes spontaneous fission decay at a known rate. The fragments emitted by this fission process leave trails of damage in the crystal structure of the minerals enclosing the uranium. Etching of polished surfaces of these minerals reveals the spontaneous fission tracks for counting by optical microscopic means. The number of tracks correlates directly with the age of the sample and the uranium content. To determine the uranium content the sample is annealed by heating and exposed to a barrage of thermal neutrons. The neutron bombardment produces an induced fission of the uranium-235 in the sample and the resulting new induced tracks are used to determine the uranium content of the sample as the U-235:U-238 ratio is well known. Alternatively, a uranium-free piece of mica, the external detector, is attached to the sample and both sample and mica are exposed to a barrage of thermal neutrons. The resulting induced fission of the uranium-235 in the sample creates new induced track in the external detector, which are revealed by etching. The ratio of spontaneous tracks to induced tracks is proportional to the age.

Apatite, sphene, zircon, micas and volcanic glass typically contain enough uranium to be useful in dating samples of relatively young age (Mesozoic and Cenozoic) and are the materials most useful for this technique. Additionally low-uranium epidotes and garnets may be used for very old samples (Paleozoic to Precambrian). Because heating of the sample above about 70 to 120 °C (for apatite - higher temperatures for other minerals) causes the fission damage tracks to heal over or anneal, the technique is useful for dating the most recent cooling event in the history of the sample. This most recent cooling event obviously may not coincide with the actual formation age of the mineral involved. This resetting of the clock can be used to investigate the thermal history of basin sediments, kilometer-scale exhumation caused by tectonism and erosion, low temperature metamorphic events, and geothermal vein formation.

The fission track method has also been used to date archaeological sites and artifacts. It was used to confirm the potassium-argon dates for the deposits at Olduvai Gorge.

References

  • Naeser, C. W., Fission-Track Dating and Geologic Annealing of Fission Tracks, in: Jäger, E. and J. C. Hunziker, Lectures in Isotope Geology, Springer-Verlag, 1979, ISBN 3-540-09158-0
  • U. S. G. S., Fission Tracks: Technique, http://geology.cr.usgs.gov/capabilities/gronemtrac/geochron/fission/tech.html Retrieved 27 October 2005.
  • Fission Track, http://www.mnsu.edu/emuseum/archaeology/dating/dat_fission.html Retrieved 27 October 2005.

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