Uranium-thorium dating

Uranium-thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique commonly used to determine the age of calcium carbonate materials such as speleothem or coral. Unlike other commonly used radiometric dating techniques 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.

Because uranium is soluble to some extent in all natural waters, any material that precipitates or is grown from such waters also contains trace uranium, typically at levels of between a few parts per billion and few parts per million by weight. In contrast, thorium is 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-life of 245,000 years. The thorium-230 is itself radioactive with a half-life of 75,000 years and so instead of accumulating indefinitely (as for instance is the case for the uranium-lead system) it instead approaches secular equilibrium with its parent isotope. At...
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