Argon isotope dating
It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40) ,decays to the gas Argon as Argon-40 (Ar-40).By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.But micas, plagioclase, hornblende, clays, and other minerals can yield good data, as can whole-rock analyses.Young rocks have low levels of The rock samples are crushed, in clean equipment, to a size that preserves whole grains of the mineral to be dated, then sieved to help concentrate these grains of the target mineral.The key is to put the mineral sample in a neutron beam, which converts potassium-39 into argon-39.Because Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clear indicator of the potassium content.Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.
As the K-40 in the rock decays into Ar-40, the gas is trapped in the rock.
In this simulation, a unit of molten rock cools and crystallizes. Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C-14 graph.
A precise amount of argon-38 is added to the gas as a "spike" to help calibrate the measurement, and the gas sample is collected onto activated charcoal cooled by liquid nitrogen.
Then the gas sample is cleaned of all unwanted gasses such as H A variant of the K-Ar method gives better data by making the overall measurement process simpler.