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Encyclopedia > 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. Minerals are natural compounds formed through geological processes. ... A geologist is a contributor to the science of geology, studying the physical structure and processes of the Earth. ... Archaeology or sometimes in American English archeology (from the Greek words αρχαίος = ancient and λόγος = word/speech) is the study of human cultures through the recovery, documentation and analysis of material remains, including architecture, artefacts, biofacts, human remains, and landscapes. ...


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. Eolian (or aeolian) processes pertain to the activity of the winds. ... Loess in Hungary has travelled by wind from Asia This article is about the geologic material, for the statistical technique see Loess curve. ...


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. Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. ... Soil is the material on the surface of a lithosphere subject to weathering, and especially the earthy portion of that material. ... Radioactive decay is the set of various processes by which unstable atomic nuclei (nuclides) emit subatomic particles. ... Isotopes are forms of an element, therefore their nuclei have the same atomic number — the number of protons in the nucleus — but different mass numbers because they contain different numbers of neutrons. ... General Name, Symbol, Number uranium, U, 92 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Atomic mass 238. ... General Name, Symbol, Number thorium, Th, 90 Chemical series Actinides Group, Period, Block n/a, 7, f Appearance silvery white Atomic mass 232. ... General Name, Symbol, Number rubidium, Rb, 37 Chemical series alkali metals Group, Period, Block 1, 5, s Appearance grey white Atomic mass 85. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... Radioactive decay is the set of various processes by which unstable atomic nuclei (nuclides) emit subatomic particles (radiation). ... Ionizing radiation is either particle radiation or Electromagnetic radiation in which an individual particle/photon carries enough energy to ionize an atom or molecule by completely removing an electron from its orbit. ... Quartz is amongst one of the most common minerals in the Earths continental crust. ... Feldspar is the name of an important group of rock-forming minerals which make up perhaps as much as 60% of the Earths crust. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of microwave radiation. ...


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

Contents


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. Simon Fraser University (SFU) is located in Burnaby, British Columbia, Canada, a suburb of Vancouver, British Columbia. ...


Physics

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. Radioactive decay is the set of various processes by which unstable atomic nuclei (nuclides) emit subatomic particles. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... The gray (symbol: Gy) is the SI unit of absorbed dose. ... In physics, the photon (from Greek φως, phōs, meaning light) is the quantum of the electromagnetic field; for instance, light. ... Photoluminescence is a process in which a chemical compound absorbs a photon wich wave length is in the range of visible electromagnetic radiation, thus transitioning to a higher electronic energy state, and then radiates a photon back out, returning to a lower energy state. ...


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 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. Quartz is amongst one of the most common minerals in the Earths continental crust. ... Feldspar is the name of an important group of rock-forming minerals which make up perhaps as much as 60% of the Earths crust. ... Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ...


References

  • 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

  • links to many optical dating laboratories are at [1]

 
 

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