The accumulation of isotopes that starts when the tektite goes from being liquid to solid. When the meteor strikes it turns solid rock to liquid rock, and ejects it out of the meteor crater. When the rock is liquid, it outgases all of the argon in the molten rock. When the rock resolidifies and rains down on the Earth as a tektite, it has little to no argon. However, the tektite can have potassium, especially a radioactive isotope of potassium with an atomic weight of 40. Potassium-40 decays into Argon-40. Since the tektite starts with no argon, any argon we find had to of come from the decay of Potassium-40. By measuring the history of Potassium-40 decay in the rock, you can determine how long it has been since it solidified from liquid meteor crater ejecta.
In the rock.