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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