Optical dating


Optical dating

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.

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

Contents

Conditions and accuracy

Ages can be determined typically from 300 to 100,000 years BP, 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%.[citation needed]

The optical dating method relies on the assumption that the mineral grains were sufficiently exposed to sunlight before they were buried. This is usually, but not always, the case with Aeolian deposits, such as sand dunes and loess, and some water-laid deposits.

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, green 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.

History

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.

In 1994 the principles behind Optical dating (and thermoluminescence dating) were extended to include surfaces last seen by the sun before buried, of carved rock types from ancient monuments and artifacts, made of granite, basalt and sandstone, and this has proved possible. The initiator of ancient buildings luminescence dating Prof. Ioannis Liritzis has shown this in several cases of various monuments.

Physics

Optical dating is one of several techniques in which an age is calculated as follows: (age) = (total absorbed radiation dose) / (radiation dose rate). 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 total absorbed radiation dose is determined by exciting specific minerals (usually quartz or feldspar) extracted from 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

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 blue or 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

  • M.J.Aitken, An Introduction to Optical Dating, Oxford University Press (1998) ISBN 0-19-854092-2
  • D.J. Huntley, D.I. Godfrey-Smith and M.L.W. Thewalt. Optical Dating of Sediments. Nature v.313, 105-107 (1985).
  • A. Wintle and M. Murray. A review of quartz optically stimulated luminescence characteristics and their relevance in single-aliquot regeneration dating protocols. Radiation Measurements v.41. 369-391 (2006).
  • Greilich., S, Glasmacher, G.A, Wagner, G.A (2005) Optical dating of granitic stone surfaces. Archaeometry 47(3), 645-665.
  • Habermann, J, Schilles,T, Kalchgruber,R, Wagner, G.A, 2000. Steps towards surface dating using luminescence, Radiation Measurements 32, 847-851
  • Liritzis,Ι. 1994 A new dating method by thermoluminescence of carved megalithic stone building. Comptes Rendus (Academie des Sciences), Paris, t. 319, serie II, 603-610, ibid. Archaeometry: Dating the past. EKISTICS, t.368/364, 361-366.
  • Liritzis I., Guilbert P., Foti F., Schvoerer M. (1997) The Temple of Apollo (Delphi) strengthens new thermoluminescence dating method. Geoarchaeology International, vol. 12, no. 5, 479-496
  • Liritzis., I (2010) Strofilas (Andros Island, Greece): New evidence of Cycladic Final Neolithic dated by novel luminescence and Obsidian Hydration methods. Journal of Archaeological Science (DOI 10.1016/j.jas.2009.12.041, in press).
  • Liritzis. I, Sideris. C, Vafiadou, A and Mitsis.J (2007) Mineralogical petrological and radioactivity aspects of some building material from Egyptian Old Kingdom monuments. Journal of Cultural Heritage, 9, 1-13
  • Morgenstein, M.E, Luo, S, Ku, T.L, and Feathers J., (2003) Uranium series and luminescence dating of volcanic lithic artefacts. Archaeometry 45, 503-518,
  • Theocaris P.S., Liritzis I. and Galloway R.B. (1994). Dating of two Hellenic pyramids by a novel application of thermoluminescence. J. Archaeological Science, 24, 399-405

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