Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.
What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.
The key is to put the mineral sample in a neutron beam, which converts potassium-39 into argon-39.
Young rocks have low levels of Ar, so as much as several kilograms may be needed.
Rock samples are recorded, marked, sealed and kept free of contamination and excessive heat on the way to the lab.
Then the gas sample is cleaned of all unwanted gasses such as H, nitrogen and so on until all that remains are the inert gasses, argon among them.
Finally, the argon atoms are counted in a mass spectrometer, a machine with its own complexities.
That is, a fresh mineral grain has its K-Ar "clock" set at zero.
The method relies on satisfying some important assumptions: Given careful work in the field and in the lab, these assumptions can be met.Ar-Ar analyses cost around 00 per sample and take several weeks.The Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method.Of the naturally occurring isotopes of potassium, 40K is radioactive and decays into 40Ar at a precisely known rate, so that the ratio of 40K to 40Ar in minerals is always proportional to the time elapsed since the mineral formed [ 40K is a potassium atom with an atomic mass of 40 units; 40Ar is an argon atom with an atomic mass of 40 units].This relationship is useful to geochronologists, because quite a few minerals in the Earth’s crust contain measurable quantities of potassium (e.g. In theory, therefore, we can estimate the age of the mineral simply by measuring the relative abundances of each isotope.Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems.