Dating of zircons
The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay).Consider the concordia: as zircons age, they move outward along the curve.
Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead (Pb).
The results from these zircons therefore plot along that straight line, establishing what is called a discordia. If a 1500-million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the concordia, always pointing to the age of the disturbance.
This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life.
Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time.
If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept.