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.And we know that there's a generalized way to describe that.
The amount of Argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors.
These factors introduce error limits on the upper and lower bounds of dating, so that final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure and/or open-air.
(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.