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The reasoning is as follows: the atmosphere does not only contain Ar as being atmospheric argon.However, this only works if all the excess argon did indeed come from the atmosphere.(However, see the section below on the limitations of the method.) This suggests an obvious method of dating igneous rocks.If we are right in thinking that there was no argon in the rock originally, then all the argon in it now must have been produced by the decay of Ar in them will be so small that it is below the ability of our instruments to measure, and a rock formed yesterday will look no different from a rock formed fifty thousand years ago.Argon, on the other hand, is an inert gas; it cannot combine chemically with anything.As a result under most circumstances we don't expect to find much argon in igneous rocks just after they've formed.Another concern with K-Ar dating is that it relies on there being no Ar in the rock when it was originally formed, or added to it between its formation and our application of the K-Ar method.
A second problem is that for technical reasons, the measurement of argon and the measurement of potassium have to be made on two different samples, because each measurement requires the destruction of the sample.If the mineral composition of the two sample is different, so that the sample for measuring the potassium is richer or poorer in potassium than the sample used for measuring the argon, then this will be a source of error.K has a half-life of 1.248 billion years, which makes it eminently suitable for dating rocks.Potassium is chemically incorporated into common minerals, notably hornblende, biotite and potassium feldspar, which are component minerals of igneous rocks.
The severity of this problem decreases as the accuracy of our instruments increases.
Still, as a general rule, the proportional error in K-Ar dating will be greatest in the youngest rocks.