Abstract
The 40K/40Ar method and its variant, 40Ar/39Ar, are based on the natural radioactive decay of 40K, one of the isotopes of potassium, in 40Ar, one of the isotopes of argon. 40K decreases in 40Ar* (the * symbol indicates that this is a radiogenic isotope) with a period of 1.25 × 109 years, according to the law of radioactive decay N = N0 e−λt. In other words, if we consider a closed system, containing at an initial time (t0) N0 atoms of 40K, then N0/2 atoms of 40K will remain in the system after 1.25 × 109 years. This gives us an indication of the geochronological application. If, in a geological sample, both the number of parent atoms remaining (40K) and the number of daughter atoms formed (40Ar*) can be measured, then it is possible to calculate the age of formation of this sample.
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