What is argon dating
Because it is present within the atmosphere, every rock and mineral will have some quantity of Argon.Argon can mobilized into or out of a rock or mineral through alteration Ar and potassium, there is not a reliable way to determine if the assumptions are valid.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. As noted in the comments the wikipedia articles (at the time this question was submitted) are contradictory.There are quite a few steps to the logic of how argon-argon dating works but none are too complicated, although I won't go into all of the possible interferences.Because of this, we can assume that the potassium-40:potassium-39 ratio in minerals at any time is a constant. If the argon-40 stays trapped in the crystal and you can measure the ratio of potassium-40 to argon-40, then you know how long it has been since the mineral formed.This also assumes that there is no other source of argon like trapped air.One thing to keep in mind is that high-precision isotope measurements always measure ratios between isotopes, not absolute concentrations.
The standards of known age are used to account for differences in the neutron flux during irradiation.
The trick is to irradiate the sample with neutrons along with samples of known age.
Some of the potassium-39 forms argon-39 by an n,p reaction.
Due to the relatively heavy atomic weight of potassium, insignificant fractionation of the different potassium isotopes occurs.
However, the Argon, a noble gas, constitutes approximately 0.1-5% of the Earth's present day atmosphere.
The monitor flux can then be extrapolated to the samples, thereby determining their flux.