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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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Loss (leakage) of lead from the sample will result in a discrepancy in the ages determined by each decay scheme.
This effect is referred to as discordance and is demonstrated in Figure 1.
The method relies on two separate decay chains, the uranium series from Pb) leads to multiple dating techniques within the overall U–Pb system.
The term U–Pb dating normally implies the coupled use of both decay schemes in the 'concordia diagram' (see below).
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, use of a single decay scheme (usually Pb) leads to the U–Pb isochron dating method, analogous to the rubidium–strontium dating method.
Finally, ages can also be determined from the U–Pb system by analysis of Pb isotope ratios alone. Clair Cameron Patterson, an American geochemist who pioneered studies of uranium–lead radiometric dating methods, used it to obtain one of the earliest estimates of the age of the Earth.
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).