Potassium argon isotope dating
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.
Potassium (K) is one of the most abundant elements in the Earth's crust (2.4% by mass).
One out of every 10,000 Potassium atoms is radioactive Potassium-40 (K-40).
This could even be the case when the K-Ar and Ar-Ar analyses yield "dates" compatible with other radioisotopic "dating" systems and/or with fossil "dating" based on evolutionary assumptions.
Furthermore, there would be no way of knowing, because the Ar not from radioactive decay, except of course by external assumptions about the ages of the rocks.
The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium.
Second, the resulting argon atoms are merely mechanically trapped in the crystal lattice, not bonded in any way.This is perhaps the closest approach to an ideal dating system. Furthermore, since argon is an inert gas and doesn't combine naturally with other elements, it has no business being in a crystal lattice at all.It can only be there if it formed by radioactive decay.It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40) ,decays to the gas Argon as Argon-40 (Ar-40).By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.