More or less Argon
It's not that simple Johannes. The Argon that they are measuring comes from natural radioactive decay of potassium. You measure the amounts and isotopes of K and Ar and see how much is missing. If all the Ar is still there, matching the residual K, then none was lost. Any extra Argon in the atmosphere is excluded by the minerals, unless it gets trapped in bubbles, and these can be identified and excluded...