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).
Using the argon-argon dating technique, by which scientists measure the decay of an isotope called Argon-40 into Argon-39 in order to find the age of crystals, they came up with a rough approximation of the footprints' age: 19,000 years at the oldest, 10,000 or 12,000 years at the youngest.Instead of measuring 40K/40Ar, you assume a fixed 39K/40K ratio, convert a fraction $f$ of 39K to 39Ar by (n,p), then measure 39Ar/40Ar.If you simultaneously irradiate and measure a 2nd sample of known age, you can infer the fraction $f$.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.
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The isotope potassium-39 makes up about 93% of natural potassium.