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.