Optical dating
Encyclopedia
Optical dating is a method of determining how long ago mineral
s were last exposed to daylight. It is useful to geologist
s 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).
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 sediment
s and soil
s contain trace amounts of radioactive isotope
s 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.
, 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.
(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 ray
s. 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 photon
s 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.
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.
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s were last exposed to daylight. It is useful to geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
s 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).
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%.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
Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...
, and some water-laid deposits.
All sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s and soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
s contain trace amounts of radioactive isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s including uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
, thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
, rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
and potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
. These slowly decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
over time and the ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
they produce is absorbed by other constituents of the soil sediments such as quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. 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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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 UniversitySimon Fraser University
Simon Fraser University is a Canadian public research university in British Columbia with its main campus on Burnaby Mountain in Burnaby, and satellite campuses in Vancouver and Surrey. The main campus in Burnaby, located from downtown Vancouver, was established in 1965 and has more than 34,000...
, 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
Thermoluminescence dating
Thermoluminescence dating is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated or exposed to sunlight...
) 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
Ioannis Liritzis
Ioannis Liritzis is professor of physics in archaeology and his field of specialization is the application of natural sciences to archaeology and cultural heritage. He studied physics at the University of Patras and continued at the University of Edinburgh, where he obtained his Ph.D. in 1980...
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 ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s. The dose rate is usually in the range 0.5 - 5 grays
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....
/1000 years. The total absorbed radiation dose is determined by exciting specific minerals (usually quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
or feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
) extracted from the sample with light and measuring the amount of light emitted as a result. The photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s of the emitted light must have higher energies than the excitation photons in order to avoid measurement of ordinary photoluminescence
Photoluminescence
Photoluminescence is a process in which a substance absorbs photons and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon...
. 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 quartzQuartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
or feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
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.
External links
- Aberystwyth Luminescence Research Laboratory has links to many optical dating laboratories