Fission-Track Ages and Track-Annealing Behavior of Some Micas.

The density of tracks from spontaneously fissioning uranium atoms in mica has been used to make quantitative measurements of geologic ages. In comparison with methods in which ages are derived from measurements of radioactive decay products, the fission track method is much simpler and is usually accurate for micas up to a few hundred million years old. Annealing experiments suggest that high ambient temperatures are responsible for track fading in the older samples.