Radioactive dating uranium 238
Uranium easily substitutes for zirconium while lead is strongly excluded.This means the clock is truly set at zero when zircon forms.Third, zircon is widespread in igneous rocks as a primary mineral.This makes it especially valuable for dating these rocks, which have no fossils to indicate their age.However, zircon is so overwhelming a favorite that geologists often just refer to "zircon dating."But even the best geologic methods are imperfect.Dating a rock involves uranium-lead measurements on many zircons, then assessing the quality of the data.With 238U the Pb/U ratio grows much more slowly with age, but the idea is the same.
It may sound straight-forward, but there are many variables that have to be considered.
Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge.
In these cases, the concordia diagram is a valuable tool.
The oldest zircon yet found dates from 4.4 billion years ago.
With this background in the uranium-lead method, you may have a deeper appreciation of the research presented on the University of Wisconsin's "Earliest Piece of the Earth" page, including the 2001 paper in Nature that announced the record-setting date.