Pb pb dating equation

In these cases, the concordia diagram is a valuable tool.Consider the concordia: as zircons age, they move outward along the curve.Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable.Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U).

If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept.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).

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