Argon–argon dating

Argon–argon (or ⁴⁰Ar/³⁹Ar) dating is a radiometric dating method invented to supersede potassium-argon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. ⁴⁰Ar/³⁹Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (³⁹K) into the radioactive ³⁹Ar. 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 the ⁴⁰K/⁴⁰Ar* ratio, and thus to calculate the age of the unknown sample. ⁴⁰Ar* refers to the radiogenic ⁴⁰Ar, i.e. the ⁴⁰Ar produced from radioactive decay of ⁴⁰K. ⁴⁰Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the ³⁶Ar (which is assumed to be of atmospheric origin) and assuming that ⁴⁰Ar is found in a constant ratio to ³⁶Ar in atmospheric gases.

The sample is generally crushed and single crystals of a mineral or fragments of rock hand-selected for analysis. These are then irradiated to produce ³⁹Ar from ³⁹K. The sample is then degassed in a high-vacuum mass 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 argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time. The abundance of ⁴⁰Ar* increases with the age of the sample, though the rate of increase decays exponentially with the half-life of ⁴⁰K, which is 1.248 billion years.

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

where λ is the radioactive decay constant of ⁴⁰K (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 the ⁴⁰Ar*/³⁹Ar ratio. The J factor relates to the fluence of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of ⁴⁰K to ⁴⁰Ar than a less dense one.

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