^{40}Ar/^{39}Ar Dating Method 

Since ^{40}Ar/^{39}Ar technique of KAr dating was first established in 1960s, the technique has been gradually developed and utilized with marked success to investigate a wide range of geological problems. In particular, the knowledge obtained from ^{40}Ar/^{39}Ar dating measurements has provided a powerful approach to the study of kinetic processes in geological terranes and thermal histories of terrestrial and extraterrestrial samples.
The ^{40}Ar/^{39}Ar dating method is a variation of the conventional KAr method which is based on the decay scheme of ^{40}K to ^{40}Ar with a halflife of 1.3 billion years. It requires bombardment of samples with fast neutrons in a nuclear reactor to convert ^{39}K (the most abundant potassium isotope) to ^{39}Ar. Samples are then fused in an ultrahigh vacuum gas extraction system and ^{39}Ar/^{40}Ar ratio determined by a gassource massspectrometer. Calibration of the reactor flux with standards of known age allows the amount of ^{39}K (and consequently the ^{40}K) in the sample to be determined by analyzing ^{39}Ar. Natural decay of ^{40}K generates radiogenic ^{40}Ar (^{40}Ar*) with time. With utilizing the measurements of ^{40}K and ^{40}Ar cumulated in the sample, ^{40}Ar/^{39}Ar date (t) can be estimated with using the following equation: where l = 5.543 x 10^{10} a^{1} (1) The above equation stands as the central for KAr dating, socalled age equation. In ^{40}Ar/^{39}Ar dating, the sample has to be irradiated in reactor and ^{39}K in sample can be converted to ^{39}Ar. Through the analysis of ^{39}Ar and adequate interference corrections, the amount of ^{40}K in the sample can be calculated via the following equation: (2) where, ^{39}Ar_{k}: amount of ^{39}Ar generated from irradiation of ^{39}K in sample in reactor, D: irradiation duration, f(E): neutron flux at energy E, s(E): neutron capture cross section at E for the ^{39}K(n,p)^{39}Ar reaction, and ( ^{40}K/^{39}K)_{i}: isotopic abundance ratio of K. Combing Eqns. (1) and (2), Eqn. (1) becomes: (3) By assigning , Eqn. (3) becomes (4) Practically, J value can be obtained with analyzing a sample of accurately known K/Ar age (standard mineral or irradiation monitor), be irradiated together with the unknown sample. In this case, the age of the unknown sample can be calculated from the isotopic ratio (^{40}Ar*/^{39}Ar_{K}) obtained from the analysis.
^{40}Ar/^{39}Ar
technique has the following advantages over the conventional
K/Ar technique

