Fission tracks are zones of intense damage formed when fission fragments of ²³⁸U pass through a solid. Spontaneous fission of ²³⁸U takes place at a constant rate. Therefore, the age of a mineral can be calculated by determining the number of tracks and the amount of uranium the mineral contains. Once formed, fission tracks are stable in most minerals at temperatures below about 80°C. However, if a mineral is heated to a high enough temperature, the tracks fade and disappear, resulting in an anomalously young fission-track age. The temperature at which this "annealing" occurs depends on the mineral and the duration of heating.

The two minerals most commonly used in fission-track annealing studies are apatite and zircon. Fission tracks in apatite are totally annealed at temperatures of about 150°C to 105°C over periods of 10⁵ to 10⁸ yr, respectively. Annealing temperatures of fission tracks in zircon are not as well known, but are probably in the range of 200°C ± 25°C for heating lasting longer than 10⁶ yr. The annealing temperatures of apatite and zircon span the main temperature range of oil generation, and both minerals are present in the heavy-mineral suites of many sediments. Consequently, fission-track dating is a valuable method for studying the time-temperature history of sedimentary basins.

In the southern San Joaquin Valley, California, fission-track ages of apatite from drill-hole samples of Eocene to Miocene sandstones suggest that sediments in the rapidly subsiding basin northwest of the active White Wolf fault have been near their present temperature for about 10⁶ yr, whereas those in the Tejon platform southeast of the fault have been near their present temperature for about 10⁷ yr. These estimates agree with estimates based on other thermal history indicators in these rocks.

End_of_Article - Last_Page 291------------