Optical dating is a method of determining how long ago minerals were last exposed to daylight. It is useful to geologists and archaeologists who want to know when such an event occurred.

Ages can be determined from a few hundred years to 100,000 years, and can be reliable when suitable methods are used and proper checks are done. Ages can be obtained outside this range, but they should be regarded with caution. The accuracy obtainable under optimum circumstances is about 5%.

Crucial to the optical dating method is that there was adequate daylight exposure to the mineral grains before they were buried. Eolian deposits, such as sand dunes and loess, usually (but not always) satisfy this criterion. Some water-laid deposits do too.

All sediments and soils contain trace amounts of radioactive isotopes including uranium, thorium, rubidium and potassium. These slowly decay over time and the ionizing radiation they produce is absorbed by other constituents of the soil sediments such as quartz and feldspar. The resulting radiation damage within these minerals remains as structurally unstable electron traps within the mineral grains. Stimulating samples using either blue or infrared light causes a luminescence signal to be emitted as the stored unstable electron energy is released, the intensity of which varies depending on the amount of radiation absorbed during burial. The radiation damage accumulates at a rate over time determined by the amount of radioactive elements in the sample. Exposure to sunlight resets the luminescence signal and so the time period since the soil was buried can be calculated.

Alternate names sometimes used are optically-stimulated-emission dating (OSL dating) and photoluminescence dating (PL dating).

History

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Optical dating was invented in 1984 in the physics department at Simon Fraser University, British Columbia, Canada, by David Huntley and colleagues. It was quickly used by Martin Aitken’s laboratory in Oxford, England, but it was many years before it was adopted elsewhere. Now there are numerous laboratories around the world, though most are in Europe.

Physics

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Optical dating is one of several techniques in which an age is calculated from:

(radiation dose) = (radiation dose rate) x (age)

The radiation dose rate is calculated from measurements of the radioactive elements (K, U, Th and Rb) within the sample and its surroundings and the radiation dose rate from cosmic rays. The dose rate is usually in the range 0.5 - 5 grays/1000 years. The radiation dose is determined by exciting specific minerals in the sample with light and measuring the light emitted as a result. The photons of the emitted light must have higher energies than the excitation photons in order to avoid measurement of ordinary photoluminescence. A sample in which the mineral grains have all been exposed to at least a few seconds of daylight can be said to be of zero age; when excited it will not emit any such photons. The older the sample is, the more light it emits.

Minerals

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The minerals that are measured are usually either quartz or feldspar sand-sized grains, or unseparated silt-sized grains. There are advantages and disadvantages to using each. For quartz one normally uses green excitation and measures the near ultra-violet emission. For feldspar or silt-sized grains one normally uses near infra-red excitation and measures the violet emission.

References

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• M.J.Aitken, An Introduction to Optical Dating, Oxford University Press (1998) ISBN 0198540922
• D.J. Huntley, D.I. Godfrey-Smith and M.L.W. Thewalt. Optical Dating of Sediments. Nature v.313, 105-107 (1985).

External links

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• links to many optical dating laboratories are at ^*

Geochronology | Methods and principles in archaeology