In molten rock almost all of the argon will be released into the atmosphere; so in volcanic material, when the rock cools and hardens, the argon begins to accumulate in the crystals, effectively starting the clock.
Volcanic material is the ideal subject of potassium-argon dating because it is a closed system - there is none of the daughter element present when the material is formed to contaminate measurements.
Thus all K-Ar and Ar-Ar "dates" of volcanic rocks are questionable, as well as fossil "dates" calibrated by them.
Ar Analyses of Historic Lava Flows.” Nevertheless, this is no way changes the argument in question.
Over the past 60 years, potassium-argon dating has been extremely successful, particularly in dating the ocean floor and volcanic eruptions.
Furthermore, there would be no way of knowing, because the Ar not from radioactive decay, except of course by external assumptions about the ages of the rocks.
Potassium argon (40K-40Ar) dating is a form of radiometric dating widely used because of the range of dates for which it is useful.
The technique can be used for dates ranging from earth's beginning, 4550 mya (4.55bn in US terminology) to about 100,000 years ago.
While this assumption holds true in the vast majority of cases, excess argon can occasionally be trapped in the mineral when it crystallizes, causing the K-Ar age to be a few hundred thousand to a few million years older than the actual cooling age.
Secondly, K-Ar dating assumes that very little or no argon or potassium was lost from the mineral since it formed. it does not bond to any other elements), it can readily escape from minerals if they are exposed to significant amounts of heat for a prolonged period of time.For example, in the Middle Proterozoic Musgrave Block (northern South Australia), a wide scatter of K-Ar mineral "ages" was found, ranging from 343Ma to 4493Ma due to inherited (excess) , permitting inclusion of the gas in the crystallizing minerals.