Carbon potassium argon dating
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.
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.
This rare, unstable isotope is produced from ordinary nitrogen 14.
In earth's upper atmosphere, on the edge of what is commonly called outer space, light atomic nuclei from unknown sources outside of our solar system traveling at speeds approaching the speed of light called rain down continuously.
A secondary cosmic ray neutron of sufficient energy striking a common nitrogen 14 nucleus can force it to eject a proton.
Finally, we must consider the possibility of argon loss.
This activity reached its peak in the early 1960s when an atmospheric blast occurred somewhere on earth every two to three days.
Nuclear bombs generate large numbers of high energy neutrons, which can in turn transmute nitrogen 14 into carbon 14 in exactly the same way as naturally occurring secondary cosmic rays.
(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.