Zircon and radiometric dating
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.This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life.The oldest zircon yet found dates from 4.4 billion years ago.Other minerals sometimes used for uranium-lead dating include monazite, titanite and two other zirconium minerals, baddeleyite and zirconolite.
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).
Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable.
Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.
The straight line takes the zircons off the concordia. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched.
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.