Pb pb dating method
In a 704-million-year-old rock, 235U is at its half-life and there will be an equal number of 235U and 207Pb atoms (the Pb/U ratio is 1).
In a rock twice as old there will be one 235U atom left for every three 207Pb atoms (Pb/U = 3), and so forth.
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).
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.
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.
So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero.
Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U).
Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead (Pb).
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.