Argon 39 argon 40 dating
is a radiometric dating method invented to supersede potassium-argon (K-Ar) dating in accuracy.Ar dating is a major method that researchers have used to understand the structural evolution of the Maria Fold and Thrust Belt.
The standards of known age are used to account for differences in the neutron flux during irradiation.
Argon-36 in the sample accounts for any air contamination.
This plateau must be used with caution, however, because it could have been lowered by later events and therefore may represent a minimum constraint on the age of crystallization.
Hypothetical profiles of argon concentrations through the grain, and associated step-heating results.
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.
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.
To understand argon-argon dating, you need to understand potassium-argon dating.
The isotope potassium-39 makes up about 93% of natural potassium.
Some of the potassium-39 forms argon-39 by an n,p reaction.