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.

Ages can be determined typically 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, 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.

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

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.

Physics

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

See also

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).

External links

A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure and specific physical properties.
..... Click the link for more information.
A geologist is a contributor to the science of geology, studying the physical structure and processes of the Earth and planets of the solar system (see planetary geology).
..... Click the link for more information.
This article may require cleanup to meet Wikipedia's quality standards.
Please [ improve this article] if you can. <includeonly></includeonly><noinclude>
This high-risk template has been protected from editing to prevent vandalism.
..... Click the link for more information.
Eolian (or aeolian or æolian) processes pertain to the activity of the winds and more specifically, to the winds' ability to shape the surface of the Earth and other planets.
..... Click the link for more information.
loess, from the German Löss or Löß, and ultimately from Swiss German lösch (loose), pronounced in several different ways in English (IPA: [ləs, lʌs, lʌrs, lo.
..... Click the link for more information.
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. Sedimentation is the deposition by settling of a suspended material.
..... Click the link for more information.
SOiL is a five-piece Hard Rock band from Chicago, Illinois, United States. They formed in 1997 and are still active. They are signed to DRT Entertainment and have released four albums, their most recent being True Self which was released in March 27 2006.
..... Click the link for more information.
Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This decay, or loss of energy, results in an atom of one type, called the parent nuclide
..... Click the link for more information.
Isotopes are any of the several different forms of an element each having different atomic mass (mass number). Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons.
..... Click the link for more information.
Uranium (IPA: /jʊˈreɪniəm/)is a white/black metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92.
..... Click the link for more information.
Thorium (IPA: /ˈθɔːriəm/) is a chemical element in the periodic table that has the symbol Th and atomic number 90.
..... Click the link for more information.
Rubidium (IPA: /ruːˈbɪdiəm, rəˈbɪdiəm/) is a chemical element in the periodic table that has the symbol Rb and atomic number 37.
..... Click the link for more information.
Potassium (IPA: /pə(ʊ)ˈtasiəm/, /pə'tæsiəm/) is a chemical element. It has the symbol K (Arabic: al qalja
..... Click the link for more information.
Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This decay, or loss of energy, results in an atom of one type, called the parent nuclide
..... Click the link for more information.
Ionizing radiation is energetic particles or waves that have the potential to ionize an atom or molecule through atomic interactions. It is a function of the energy of the individual particles or waves, and not a function of the number of particles or waves present.
..... Click the link for more information.
Quartz (from German Quarz  [1]) is the second most common mineral in the Earth's continental crust, feldspar being the first.
..... Click the link for more information.
Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust.[1]

Feldspars crystallize from magma in both intrusive and extrusive rocks, and they can also occur as compact minerals, as veins, and are also present in
..... Click the link for more information.
Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. The name means "below red" (from the Latin infra, "below"), red being the color of visible light with the longest wavelength.
..... Click the link for more information.
Simon Fraser University (SFU) is located on Burnaby Mountain in Burnaby, British Columbia, Canada, part of the metropolitan area of Vancouver, British Columbia. It was established in 1965.
..... Click the link for more information.
Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This decay, or loss of energy, results in an atom of one type, called the parent nuclide
..... Click the link for more information.
Cosmic rays are energetic particles originating from space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons.
..... Click the link for more information.
The gray (symbol: Gy) is the SI unit of absorbed dose.

Definition


Multiple Name Symbol Multiple Name Symbol
100 gray Gy      
101 decagray daGy 10–1 decigray dGy
102

..... Click the link for more information.
Quartz (from German Quarz  [1]) is the second most common mineral in the Earth's continental crust, feldspar being the first.
..... Click the link for more information.
Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust.[1]

Feldspars crystallize from magma in both intrusive and extrusive rocks, and they can also occur as compact minerals, as veins, and are also present in
..... Click the link for more information.
Photon

Photons emitted in a coherent beam from a laser
Composition: Elementary particle
Family: Boson
Group: Gauge boson
Interaction: Electromagnetic
Theorized: Albert Einstein (1905–17)
Symbol: or
Mass: 0[1]
..... Click the link for more information.
Photoluminescence is a process in which a chemical compound absorbs photons (electromagnetic radiation), thus jumping to a higher electronic energy state, and then radiates photons back out, returning to a lower energy state.
..... Click the link for more information.
Quartz (from German Quarz  [1]) is the second most common mineral in the Earth's continental crust, feldspar being the first.
..... Click the link for more information.
Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust.[1]

Feldspars crystallize from magma in both intrusive and extrusive rocks, and they can also occur as compact minerals, as veins, and are also present in
..... Click the link for more information.
Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X-rays. It is so named because the spectrum starts with wavelengths slightly shorter than the wavelengths humans identify as the color violet
..... Click the link for more information.
Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. The name means "below red" (from the Latin infra, "below"), red being the color of visible light with the longest wavelength.
..... Click the link for more information.


This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.