Argon isotope dating
The rock sample to be dated must be chosen very carefully.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
I can't exactly follow the logic, but I'm asking here about the dating process itself. I do not think that Argon-40 decays into Argon-39 as the article states, at least not all by itself. Based on the atmospheric 40-Ar/36-Ar ratio and the 36-concentration, the 40-Ar concentration at the time of formation is calculated.
And when I look at the Wikipedia article, the discussion is so technical and defensive that I can't actually picture what is going on. You are right: there is no decay of 40-Ar into 39-Ar. The difference between measured 40-Ar and 40-Ar at formation is used in the procedure.
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.
But then, different passages in the Wikipedia article contradict each other (first section: 39-K is converted into 39-Ar by neutron bombardment; but "age equation" section: 40-K is bombarded; I think it should be 39-Ar).
There's a more basic explanation here: books.google.com/…
The method relies on satisfying some important assumptions: Given careful work in the field and in the lab, these assumptions can be met.The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present.