Argon argon dating range
is a radiometric dating method invented to supersede potassium-argon (K-Ar) dating in accuracy.
The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.
Argon-argon dating gets around many of the issues by measuring only multiple isotopes of argon.
The isotope potassium-39 makes up about 93% of natural potassium.
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.
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.
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.
On the other hand, the abundance of argon in the Earth is relatively small because of its escape to the atmosphere during processes associated with volcanism.