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.

It refers to techniques including electron spin resonance (ESR), thermoluminescence (TL) and optically stimulated luminescence (OSL).

Conditions and accuracy

Ages can be determined typically from 100 to 200,000 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 optical dating method relies on the assumption that the mineral grains were sufficiently exposed to sunlight ("bleached") 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 trapped charge 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 and specific properties of the mineral. The trapped charge accumulates at a rate over time determined by the amount of background radiation at the location where the sample was buried. Exposure to sunlight resets the luminescence signal and so the time elapsed since the sample 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.[1] It was quickly used by Martin Aitken's laboratory in Oxford, England, many years before it was adopted elsewhere. Now there are numerous laboratories around the world.

In 1994, the principles behind optical and thermoluminescence dating were extended to include surfaces made of granite, basalt and sandstone, such as carved rock from ancient monuments and artifacts. The initiator of ancient buildings luminescence dating Prof. Ioannis Liritzis has shown this in several cases of various monuments. [2][3][4]

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 amount of 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 sufficient daylight (seconds for quartz; hundreds of seconds for feldspar) 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, up to a saturation limit.

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.

Notes

  1. ↑ Huntley, D. J., Godfrey-Smith, D. I., & Thewalt, M. L. W. Optical dating of sediments" Nature 313, 105 - 107 (10 January 1985); doi:10.1038/313105a0
  2. ↑ Liritzis, I. (2011). "Surface Dating by Luminescence: An Overview". Geochronometria (Silesian University of Technology, Poland) 38 (3): 292–302. doi:10.2478/s13386-011-0032-7.
  3. ↑ Liritzis, I., Polymeris, S.G., and Zacharias, N. (2010). "Surface Luminescence Dating of 'Dragon Houses' and Armena Gate at Styra (Euboea, Greece)". Mediterranean Archaeology and Archaeometry 10 (3): 65–81.
  4. ↑ Liritzis, I. (2010). "Strofilas (Andros Island, Greece): new evidence for the cycladic final neolithic period through novel dating methods using luminescence and obsidian hydration". Journal of Archaeological Science (Elsevier) 37: 1367–1377. doi:10.1016/j.jas.2009.12.041.

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