Argon–argon (or⁴⁰Ar/³⁹Ar)dating is aradiometric dating method invented to supersedepotassium–argon (K/Ar) dating in accuracy. The older method required splitting samples into two for separatepotassium andargon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement ofargon isotopes.⁴⁰Ar/³⁹Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (³⁹K) into the radioactive³⁹Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the⁴⁰K/⁴⁰Ar* ratio, and thus to calculate the age of the unknown sample.⁴⁰Ar* refers to theradiogenic⁴⁰Ar, i.e. the⁴⁰Ar produced from radioactive decay of⁴⁰K.⁴⁰Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the³⁶Ar (which is assumed to be of atmospheric origin) and assuming that⁴⁰Ar is found in a constant ratio to³⁶Ar in atmospheric gases.

The sample is generally crushed and single crystals of a mineral or fragments of rock are hand-selected for analysis. These are then irradiated to produce³⁹Ar from³⁹K via the(n-p) reaction³⁹K(n,p)³⁹Ar. The sample is then degassed in a high-vacuummass spectrometer via a laser or resistance furnace. Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and radiogenic gases like argon and helium, generated from regular radioactive decay over geologic time. The abundance of⁴⁰Ar* increases with the age of the sample, though the rate of increase decays exponentially with the half-life of⁴⁰K, which is 1.248 billion years.

The age of a sample is given by the age equation:

${\displaystyle t=\frac{1}{\lambda }\ln (J\times R+1)}$ 

where λ is the radioactivedecay constant of⁴⁰K (approximately 5.5 x 10⁻¹⁰ year⁻¹, corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R is the⁴⁰Ar*/³⁹Ar ratio. The J factor relates to thefluence of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of³⁹K to³⁹Ar than a less dense one.

The⁴⁰Ar/³⁹Ar method only measures relative dates. In order for an age to be calculated by the⁴⁰Ar/³⁹Ar technique, the J parameter must be determined by irradiating the unknown sample along with a sample of known age for a standard. Because this (primary) standard ultimately cannot be determined by⁴⁰Ar/³⁹Ar, it must be first determined by another dating method. The method most commonly used to date the primary standard is theconventional K/Ar technique.^[1] An alternative method of calibrating the used standard is astronomical tuning (also known asorbital tuning), which arrives at a slightly different age.^[2]

The primary use for⁴⁰Ar/³⁹Ar geochronology is dating metamorphic and igneous minerals.⁴⁰Ar/³⁹Ar is unlikely to provide the age of intrusions ofgranite as the age typically reflects the time when a mineral cooled through itsclosure temperature. However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Dating of movement onfault systems is also possible with the⁴⁰Ar/³⁹Ar method. Different minerals have different closure temperatures;biotite is ~300°C,muscovite is about 400°C andhornblende has a closure temperature of ~550°C. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock.

Dating mineralsmay provide age information on a rock, but assumptions must be made. Minerals usually only record thelast time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential.⁴⁰Ar/³⁹Ar geochronology assumes that a rock retains all of its⁴⁰Ar after cooling past theclosing temperature and that this was properly sampled during analysis.

This technique allows the errors involved in K-Ar dating to be checked. Argon–argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified.

One problem with argon-argon dating has been a slight discrepancy with other methods of dating.^[3] Work by Kuiper et al. reports that a correction of 0.65% is needed.^[4] Thus theCretaceous–Paleogene extinction (when the dinosaurs died out)—previously dated at 65.0 or 65.5 million years ago—is more accurately dated to 66.0-66.1 Ma.

• Grenville Turner, inventor of the technique
• Berkeley Geochronology Center

• WiscAr Geochronology Laboratory, University of Wisconsin-Madison
• UC Berkeley press release: "Precise dating of the destruction of Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years"
• New Mexico Geochronology Research Laboratory
• Argon Isotope FacilityArchived 2010-05-10 at theWayback Machine of the Scottish Universities Environmental Research Council
• Open University Ar/Ar and Noble Gas Laboratory
• Argon Laboratory / Australian National University