Potassium is chemically incorporated into common minerals, notably hornblende, biotite and potassium feldspar, which are component minerals of igneous rocks.
Argon, on the other hand, is an inert gas; it cannot combine chemically with anything.
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.
That is, a fresh mineral grain has its K-Ar "clock" set at zero.
If the mineral composition of the two sample is different, so that the sample for measuring the potassium is richer or poorer in potassium than the sample used for measuring the argon, then this will be a source of error.
Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials.
If we are right in thinking that there was no argon in the rock originally, then all the argon in it now must have been produced by the decay of Ar in them will be so small that it is below the ability of our instruments to measure, and a rock formed yesterday will look no different from a rock formed fifty thousand years ago.
The severity of this problem decreases as the accuracy of our instruments increases.
A second problem is that for technical reasons, the measurement of argon and the measurement of potassium have to be made on two different samples, because each measurement requires the destruction of the sample.
Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.
Ar) dating is a radiometric dating method invented to supersede potassium-argon (K/Ar) dating in accuracy.
Potassium–Argon dating or K–Ar dating is a radiometric dating method used in geochronology and archaeology.
It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar).
For every 100 K-40 atoms that decay, 11 become Ar-40.