Fission track dating method and its applications.

How the clock works

Uranium-238 occasionally undergoes spontaneous fission. Each fission leaves a tiny track in the crystal. The sample is polished and etched so the tracks can be counted under a microscope. The uranium content is measured, often by inducing extra tracks in a reactor. Age is then a function of track density and uranium concentration.

Useful materials include zircon, apatite and volcanic glass (obsidian or tephra). The method works over a wide geological span, from relatively young volcanic glasses to much older crustal minerals. The range depends on the mineral and on whether tracks have been reset by heat.

Heat can reset as well as date. Heating can anneal old tracks and restart the clock, which is why the method also records thermal history, not only “the age of the rock”.

Applications

• Hominin and Palaeolithic sites: Volcanic ash and tuff above or below fossils can be dated, alongside potassium–argon and argon–argon where those apply. East African rift volcanics are a standard setting.
• Obsidian and burnt flint: Provenance and last-heating events can be approached where tracks and chemistry allow.
• Siwaliks and mountain belts: Fission-track work on apatite and zircon is widely used in Himalayan cooling and uplift histories. That geological clock matters when we place Siwalik apes such as Sivapithecus in a landscape that was itself rising.
• Thermochronology: Even when the “age” is not an occupation date, the thermal path of a basin or a tool-stone source is anthropological context.

Limitation

Tracks fade if the mineral is heated above a closure temperature. Mixed ages and partial annealing need specialist interpretation. Fission-track dates should be cross-checked with stratigraphy and, where possible, with another radiometric method.