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Reheating events and diffusion of argon from the boundaries of the grain can result in lower Ar date.
Sometimes, when a large amount of argon has been lost, this is not possible.
The other important advantage of Ar-Ar dating is the extra data gained from step heating: instead of heating the irradiated sample to the highest possible temperature all at once, and so releasing all the argon all at once, we can increase the temperature in steps starting at a low temperature. Well, different minerals within the rock will give up their argon at different temperatures, so each step will give us a ratio of K from which these are derived must have appeared in the same ratio in each mineral, because both isotopes of potassium have the same chemical properties.
This means that if the rock cooled rapidly enough that all the minerals in it have the same date, and if there has been no argon loss, and if there is no excess argon added to the system, then the dates we calculate at each step of the heating will be the same date.
Or if we consistently get one date for the steps below (for example) 400°C, and consistently get another date in the steps above 400°C, then it seems as though argon loss occurred as a result of metamorphism at a temperature of about 400°C, with the younger date representing the date of the metamorphism, and the older date representing the formation of the rock; and we can investigate this clue further by looking for other evidence of the metamorphic event.
And if the dates we get are all over the place, then we are probably looking at excess argon.
There are several steps that one must take to obtain an argon-argon date: First, the desired mineral phase(s) must be separated from the others.
Common phases to be used for argon-argon dating are white micas, biotite, varieties of potassium feldspar (especially sanidine because it is potassium-rich), and varieties of amphibole. This can be used to solve equation 2 for the sample.
However, in Ar-Ar dating the two isotopes of argon are both measured from the same sample, and so at least one potential source of error is eliminated.However, if you put it near the core of a nuclear reactor, so that it is bombarded by neutrons, then this will convert it into Ar.This isotope of argon is quite unstable, having a half-life of only 269 years.For these reasons Ar-Ar dating has largely superseded K-Ar dating, although the simpler method is still employed in some cases where it is known to be unproblematic or where Ar-Ar is unsuitable for some technical reason.Ar dating is a major method that researchers have used to understand the structural evolution of the Maria Fold and Thrust Belt.
So far, all we seem to have done is taken the K-Ar dating method and made it much more complicated for no apparent reason.