updating documents - Three methods dating fossils
The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.
With 238U the Pb/U ratio grows much more slowly with age, but the idea is the same.
If you took rocks of all ages and plotted their two Pb/U ratios from their two isotope pairs against each other on a graph, the points would form a beautiful line called a concordia (see the example in the right column).
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.
However, zircon is so overwhelming a favorite that geologists often just refer to "zircon dating."But even the best geologic methods are imperfect.