How does potassium argon dating work
In a separate article (Radiometric dating), we sketched in some technical detail how these dates are calculated using radiometric dating techniques.
As we pointed out in these two articles, radiometric dates are based on known rates of radioactivity, a phenomenon that is rooted in fundamental laws of physics and follows simple mathematical formulas.
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).
To understand argon-argon dating, you need to understand potassium-argon dating.
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.
The isotope potassium-39 makes up about 93% of natural potassium.
Potassium-40 is radioactive but has such a long half-life that it is primordial - it has been around since the earth was being formed. About 10.7% decays to Argon-40 by emitting a positron.
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Technical details on how these dates are calculated are given in Radiometric dating. As with any experimental procedure in any field of science, these measurements are subject to certain "glitches" and "anomalies," as noted in the literature.