Techniques

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 ³⁹Ar_K. ³⁹K is also naturally occurring and as the ³⁹K/⁴⁰K ratio is constant the reactor-produced ³⁹Ar_K is measured to determine the ⁴⁰K in the mineral. In practice the Ar/Ar dating method is actually the measurement of the ⁴⁰Ar*/³⁹Ar_K 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

1. For samples such as fine-grained igneous rocks (basalts, rhyolite glasses) and meteorite impact melts.
2. For young metamorphic or volcanic mineral separates.
3. Ultra-small pieces of strain-fringe

Infrared laserprobe spot dating

1. For fine-grained metamorphic rocks and pseudotachylites (friction melts)
2. For low-potassium, or young, and heterogeneous materials such as meteorites or coarse-grained metamorphic rocks.

Infrared single grain fusion – dating individual mineral grains

1. For melting individual detrital minerals for sediment provenance
2. For melting individual old or highly potassic metamorphic or igneous grains

Ultraviolet intra-grain ablation [insitu dating] - Our high spatial resolution tool, used to ablate ultra small areas within individual grains, can be <50µm* diameter

1. Authigenic K-feldspar overgrowths for diagenesis and metallogenesis
2. Muscovite strain-fringes for deformation ages and excess argon studies
3. K-bearing and K-free metamorphic minerals and microtextures for excess argon studies
4. Detrital muscovites for thermochronology
5. Pseudotachylites and their host rock minerals

_{* 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.}