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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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.