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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.
The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth.
Dating a rock involves uranium-lead measurements on many zircons, then assessing the quality of the data.
Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.
In these cases, the concordia diagram is a valuable tool.
Consider the concordia: as zircons age, they move outward along the curve.
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.