The driveway was poured in 1950, and the coins are all dated 1920. Radiometric dating generally requires that a system be closed - in other words, has not had material added or removed.Crystallization of a mineral is a good way to close a system. Any disturbance of the system effectively resets the clock to zero by allowing decay products to escape or reshuffling the abundances of elements.A maximum age is the oldest the object can possibly be. But they obviously have to have been made first, so 1920 is the maximum age of the burial.Suppose, in repaving your driveway, you find a stash of old coins buried in the ground. Of course there are more outlandish explanations, like somebody counterfeiting 1920 coins in 1900 (and successfully anticipating any changes in design in the meantime), or secretly tearing up part of the driveway after 1950, but unless someone comes up with really persuasive evidence, we're justified in ignoring these hypotheses.Weathering and metamorphism are the two most common ways to disturb a system.Potassium-argon dating is very susceptible to resetting because the argon decay products are merely held in place mechanically by surrounding atoms.Let t stand for time and N(t) stand for the number of atoms at time t .In calculus terms, we write: d N(t)/dt = -K * N(t) or d N(t)/N(t) = -K dt The minus sign means that each decay decreases the total number of atoms.
So accurate determinations require very pure samples, very accurate and selective detectors, or both.If there were such a pair of isotopes, radiometric dating would be very simple.We could be sure that a mineral containing parentium originally had no daughterium.The most common dating methods for rocks are based on radioactive isotopes of potassium, rubidium, uranium, and thorium.
If you don't have minerals with those elements, you can't date the rock.
If only there were long-lived isotopes of silicon, calcium, and magnesium!