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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).
These atoms decay, and from the amount of atoms remaining, an estimate of the…
U-238 is not fissile, therefore it does not produce nuclear power in a reactor, only U-235 does that.
But now imagine that some geologic event disturbs things to make the lead escape.
That would take the zircons on a straight line back to zero on the concordia diagram.
Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time.
The oldest zircon yet found dates from 4.4 billion years ago.
Dating a rock involves uranium-lead measurements on many zircons, then assessing the quality of the data.
Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge.
The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.
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).
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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).