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	Argon-Argon Dating	Potassium-Argon Dating
Fagområde	Arkæologi	Arkæologi
Familie	Regression model	Regression model
Oprindelsesår	1999	1999
Ophavsperson	Craig Merrihue and Grenville Turner (method, 1960s); synthesized by McDougall and Harrison	Developed from 1940s-1950s radiometric work; codified for the 40Ar/39Ar successor by McDougall and Harrison
Type	Radiometric dating clock measuring argon isotope ratios after neutron irradiation	Radiometric dating clock based on radioactive decay of potassium-40 to argon-40
Oprindelig kilde	McDougall, I., & Harrison, T. M. (1999). Geochronology and Thermochronology by the 40Ar/39Ar Method (2nd ed.). Oxford University Press. ISBN: 9780195109207	McDougall, I., & Harrison, T. M. (1999). Geochronology and Thermochronology by the 40Ar/39Ar Method (2nd ed.). Oxford University Press. ISBN: 9780195109207
Aliasser	40Ar/39Ar Dating, Argon-Argon Geochronology, Single-Crystal Laser-Fusion Dating, Ar-Ar Step-Heating Dating	K-Ar Dating, Potassium-Argon Geochronology, K-Ar Radiometric Dating, Potassium-Argon Method
Relaterede	3	3
Resumé	Argon-argon (40Ar/39Ar) dating is the modern, high-precision successor to conventional potassium-argon dating, in which the parent potassium is measured indirectly by converting it to a measurable argon isotope inside a nuclear reactor. A potassium-bearing sample is irradiated so that potassium-39 transmutes into argon-39, which then stands as a proxy for the parent potassium; both the radiogenic argon-40 daughter and this argon-39 proxy can be measured on the same aliquot by a single mass spectrometer, eliminating the sample-heterogeneity problem of the older method. Crucially, the gas can be released either by fusing a single crystal with a laser or by heating the sample in incremental steps, the latter producing an age spectrum that reveals argon loss, excess argon, and disturbance. Synthesized in McDougall and Harrison's monograph, the technique delivers the precise, internally checkable ages now standard for dating volcanic deposits at hominin and Palaeolithic sites.	Potassium-argon (K-Ar) dating is a radiometric technique that determines the age of volcanic rocks and minerals from the slow radioactive decay of potassium-40 to argon-40. Potassium is abundant in many rock-forming minerals, and a fixed fraction of its naturally radioactive isotope decays to argon gas at a precisely known rate, so the amount of argon trapped inside a crystal is a clock that starts when the mineral cools below its argon-retention temperature. By measuring how much radiogenic argon has accumulated relative to the remaining potassium, the analyst inverts the decay equation to obtain the time elapsed since crystallization. Because potassium-40 has a half-life of about 1.25 billion years, the method reaches far beyond the radiocarbon range and became the workhorse for dating the volcanic deposits that bracket Plio-Pleistocene hominin fossils at sites such as Olduvai Gorge.