Ar-Ar dating: principles

Ar-Ar dating is the workhorse in geochronology and allows dating of samples that range in age from the origin of the solar system up to a few hundred thousand years.

The basic principle of this dating method is accumulation of radiogenic 40Ar from 40K by an electron-capture decay. The method is thus a modified K-Ar dating method and allows dating of all types of samples that contain reasonable amounts of potassium. Particularly usefull are K-rich minerals such as K-feldspar, micas and hornblende. The half-life of 40K is 1.25 Ga, it's present day isotope abundance is 0.01167%. 10.5% of 40K decays to 40Ar by an electron capture process (half-life 11.9 Ga), while 89.5% decay to stable 40Ca by electron emission (half life 1.4 Ga).

Age determinations require the knowledge of parent and daughter isotope abundances within a sample, i.e. of the K and Ar content with respect to the K-Ar dating method. To circumvent the necessity to measure K in a sample, rocks or minerals to be dated by the Ar-Ar method were irradiated by fast neutrons within a nuclear reactor. The neutron flux in the order of 1012 n/cm2s produces 39Ar from 39K by a (n,p) nuclear reaction. The produced 39Ar is then a measure of the K content in a sample (at a given neutron flux).

After irradiation, the Argon is thermally extracted from the samples within an ultra-high vacuum (UHV) system by using either an IR laser or a furnace system. After cleaning, the isotope abundances of Ar were measured using a sector field mass spectrometer. From the measured 40Ar/39Ar ratio, the age of a sample can be calculated.


References:
I. McDougall & T.M. Harrison: Geochronology and Thermochronology by the 40Ar/39Ar Method. Oxford University Press, 1999.

J. Pfaender - 2007