Where just a single flowstone layer is present, only a minimum or maximum age can be given for the archaeological horizons above or below it, but these ages are still invaluable. This is the case for some of the Neanderthal bearing layers at the cave site of La Chaise de Vouthon in France, which are sandwiched between flowstones, allowing a date to be measured to between 98 and 122 ky. In the best scenario, cultural horizons will be interstratified by flowstones or stalagmites. However, dating carbonate deposits only indirectly dates the archaeology. Generally, calcium carbonate deposits remain a closed system and provide the most accurate uranium-series chronologies. Hence, both 234U/ 238U and 230Th/ 238U are required to calculate a date.Ĭonventionally, alpha spectrometry was used to determine the isotopic ratios, but more recently thermal ionization and plasma mass spectrometric methods have been employed, greatly increasing precision, decreasing sample size requirements, and pushing back the older limit of datable samples.Īn additional requirement of the method is that the sample remains a ‘closed system’, that is, there is no further input or loss of uranium after initial formation of the sample.
Fission uranium 238 series#
An additional complication for the 238U decay series is that 234U, intermediate to 238U and 230Th, is fractionated by geochemical processes, and is rarely in secular equilibrium with 238U. The daughters in-grow at a rate defined by their half-life, and the measurement of 230Th/ 238U and 231Pa/ 235U can be used to calculate the time since precipitation of the material. Hence, materials such as calcium carbonate precipitates (e.g., stalagmites or flowstones in caves) will initially contain no daughter isotopes. Uranium is relatively soluble, so in aqueous systems, the parent uranium isotopes are separated from their insoluble 230Th and 231Pa daughters.
Of particular interest to archaeology are the long-lived daughters of 238U ( 234U and 230Th, with half-lives of 245 and 75.7 ky, respectively) and the long-lived 231Pa daughter (with a half-life of 32.7 ky) of 235U. Natural processes can disrupt this equilibrium, and the subsequent in-growth of the long-lived daughter isotopes, as equilibrium is re-established, provides the chronological information. In an old system (≫500 ky) a radioactive secular equilibrium is established between the parent 238U or 235U and their daughter radioisotopes. Uranium-238 and Uranium-235 are the parent isotopes of decay chains that can be used to provide a chronology back to ∼500 ky. Pike, in Encyclopedia of Analytical Science (Second Edition), 2005 Uranium-Series Dating
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