Other meanings of Argon–argon dating
Geochronology
Argon–argon dating (often written 40Ar/39Ar dating) is a radiometric dating method that compares the ratio of two argon isotopes, 40Ar and 39Ar, to determine the age of rocks and minerals. It is a refinement of potassium–argon (K–Ar) dating, which measures the accumulation of radiogenic 40Ar from the decay of 40K. In Ar–Ar dating, a sample is irradiated with neutrons to convert a portion of its 39K to 39Ar, and the 40Ar/39Ar ratio is measured by mass spectrometry. Because both isotopes are measured in the same aliquot, the method avoids the need for separate potassium and argon measurements, reducing analytical uncertainty and allowing more precise age determinations. Ar–Ar dating is widely applied to volcanic rocks, tektites, and impact melts, and has been used to date the K–Pg boundary and the timing of hominin evolution.
The method relies on the radioactive decay of 40K to 40Ar (half-life ~1.25 billion years) and the production of 39Ar from 39K by fast-neutron irradiation in a nuclear reactor.1 After irradiation, the sample is heated in a furnace or with a laser, releasing argon gas that is purified and analyzed in a mass spectrometer. The measured 40Ar/39Ar ratio is compared to that of a standard mineral of known age (e.g., Fish Canyon sanidine) to calculate the age. The technique allows step-heating, where the gas is released in temperature increments, yielding a spectrum of apparent ages that can reveal argon loss or excess argon.2
Ar–Ar dating eliminates the need to measure potassium and argon in separate aliquots, which is a major source of error in conventional K–Ar dating. Because only a single sample is used, heterogeneity in mineral composition is less problematic, and the method is more sensitive for young or low-potassium samples. Another advantage is the ability to analyze individual mineral grains or even parts of grains using laser ablation. This permits the dating of detrital minerals and microtektites, and the technique can be applied to very small samples, such as those returned by the Apollo missions.
Ar–Ar dating has been used to date volcanic ash layers (tephrochronology), which are key markers in stratigraphy and paleontology. It provided some of the most precise ages for the Cretaceous–Paleogene boundary at 66.0 Ma, linking the impact at Chicxulub to the mass extinction.3 It has also dated hominin fossil sites in the East African Rift, such as the Hadar and Olduvai Gorge sequences, refining the timeline of human evolution. In addition, the method has been applied to impact glasses, meteorites, and the lunar regolith, offering insights into Solar System history.
The technique was first described by Craig Merrihue and Grenville Turner in 1966, building on earlier work by others.4 A lesser-known variant, laser step-heating, allows in situ analysis of thin sections, enabling the dating of multiple zones in a single crystal. Another subtlety is that the method requires a correction for argon isotopes produced from calcium, chlorine, and potassium during irradiation; these interference corrections are critical for dating carbonates or evaporites. Ar–Ar dating has also been used to date the eruption of the Deccan Traps, which may have contributed to the K–Pg extinction, and to constrain the age of the Earth's oldest rocks in the Acasta Gneiss.
The method is also known as 40Ar/39Ar dating; it is the preferred technique for dating young volcanic rocks and is often used to calibrate the geological timescale.
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