Zircon is especially useful because it frequently contains uranium in substitution for zirconium, but does not incorporate lead (as shown by the absence of Lead-204).
It is now clear that with recent advances the uranium–lead method is superior in providing precise age information with the least number of assumptions. Different schemes have been developed to deal with the critical assumptions stated above.
The uranium and thorium decay systems offer a multitude of radiometric dating options.
Uranium 238 decays through a series of steps to Lead 206.
In other words, the chance that a given atom will decay is constant over time.
Decay rates are measured in half-lives the amount of time in which half of a radioactive element will decay.
Finally, with few predictable exceptions, zircon grows or regrows only in liquid rock or in solid rock reheated to approach its melting point.