- Uranium-thorium dating
Uranium-thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a
radiometric datingtechnique commonly used to determine the age of carbonate materials such as speleothemor coral[ [http://www.geo.arizona.edu/Antevs/ecol438/uthdating.html Uranium-Thorium Dating] ] . Unlike other commonly used radiometric datingtechniques such as rubidium-strontium or uranium-lead dating, the uranium-thorium technique does not measure accumulation of a stable end-member decay product, instead calculating an age from the degree to which equilibrium has been restored between the radioactive isotope thorium-230 and its radioactive parent uranium-234 within a sample.
uraniumis soluble to some extent in all natural waters, any material that precipitatesor is grown from such waters also contains trace uranium, typically at levels of between a few parts per billion and few parts per millionby weight. In contrast, thoriumis not soluble in natural waters under conditions found at or near the surface of the earth and so materials grown in or from these waters do not usually contain thorium. As time passes after the formation of such a material, the uranium-234 in the sample decays to thorium-230, with a half-lifeof 245,000 years. The thorium-230 is itself radioactive with a half-lifeof 75,000 years and so instead of accumulating indefinitely (as for instance is the case for the uranium-lead system) it instead approaches secular equilibriumwith its parent isotope. At equilibrium, the number of thorium-230 decays per year within a sample is equal to the number of uranium-234 decays per year in the same sample.
Uranium-thorium dating has an upper age limit of somewhat over 500,000 years, defined by the half-life of thorium-230, the
accuracywith which we can measure the thorium-230/uranium-234 ratio in a sample, and the precisionto which we know the half-lives of thorium-230 and uranium-234. Note that to calculate an age using this technique the ratio of uranium-234 to its parent isotope uranium-238 must also be measured.
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