Ar/Ar dating is based on the natural decay of 40Ar from 40K, and is a development of the potassium-argon (K/Ar) dating method. In a mineral that contains potassium, 40Ar* is produced (termed ‘radiogenic argon’) by the decay of the radioactive isotope 40K (which has a half-life of 1250 million years). Since we know the rate of decay, the ratio of parent 40K to daughter 40Ar* will be proportional to the age of the mineral.
For K/Ar dating, the 40K is measured by flame photometry on one half of the sample, and the 40Ar* is measured by mass spectrometry on the other half of the sample. The Ar/Ar dating method is based on the same decay system, but the samples are irradiated in a nuclear reactor, transforming the 39K in the mineral in part to 39ArK. 39K is also naturally occurring and as the 39K/40K ratio is constant the reactor-produced 39ArK is measured to determine the 40K in the mineral. In practice the Ar/Ar dating method is actually the measurement of the 40Ar*/39ArK ratio. Both isotopes are measured by mass spectrometry, performed on the same piece of sample. This means that we can use far smaller samples than for K/Ar dating.
Here at the OU we use lasers for all our Ar/Ar and Noble Gas research, rather than the furnace heating methods that can be used as an alternative. We have 2 main types of laser, Infra-red wavelength lasers used for melting and Ultraviolet wavelengths for ablation. Our research methods can be grouped into the following main techniques:
Infrared laserprobe stepped heating – single or multiple grains are heated in multiple steps with increasing laser power
Infrared laserprobe spot dating
nfrared single grain fusion – dating individual mineral grains
ltraviolet intra-grain ablation [insitu dating] - Our high spatial resolution tool, used to ablate ultra small areas within individual grains, can be <50µm* diameter
* achievable resolution dependant on several factors, particularly the age and K2O content of the mineral, young or low K2O phases need much larger spots to get usable precision on the age.