USGS-GD-Scientific Capabilities - FISSION TRACKS Technique

 

FISSION TRACKS

Technique


 

Based on radiation damage (tracks) due to spontaneous fission of 238U. When combined with temperature required for annealing of the tracks in specific minerals can yield cooling ages (e.g. age of most recent uplift, pluton emplacement and cooling, etc.). Requires high-U mineral, normally apatite or zircon. Low temperature (<200°C) thermal history. This capability exists in Reston only.

Fission-track analysis is useful in determining the thermal history of a sample or region. Fission-tracks are preserved in minerals that contain small amounts of uranium, such as apatite, sphene, and zircon. These tracks come from the spontaneous fission of 238U present in the mineral. By determining the number of tracks present on a polished surface of a grain and the amount of uranium present in the grain, it is possible to calculate how long it took to produce the number of tracks preserved. As long as the mineral has remained cool, near the earth surface, the tracks will accumulate. If the rock containing these minerals is heated, the tracks will begin to disappear. If the rock is heated high enough, >120°C for apatite, all tracks will disappear. Zircon and sphene loose their tracks at higher temperatures =ca.200° and =ca.300°C, respectively. The tracks will then begin to accumulate when the rock begins to cool. If a rock cools quickly as in the case of a volcanic rock or a shallow igneous intrusion, the fission-track ages will date this initial cooling. If the mineral formed at depth or was deeply buried after formation, the fission-track age will reflect this later heating and cooling. Fission-track analysis has been successfully applied to many diverse areas of the earth sciences: volcanology, mineral deposits, stratigraphy, basin analysis, tectonics, and impact of extraterrestrial bodies.

 


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URL http://geology.cr.usgs.gov/capabilities/gronemtrac/geochron/fission/tech.html