Geochronology

Theory for geochronology

Concordia diagram and discordant line in U-Pb dating (credit: https://www.ucl.ac.uk/~ucfbpve/geotopes/indexch5.html)

Why discordant line is straight?

A question bothers me many years is that why the discordant line is a straight line. \(T_1\) is the time when Pb-loss occurred.

In the figure presented above, the leakage event occurred in \(T_1\). If all previous lead (\(Pb^{206}\) and \(Pb^{207}\)) lossed during this event, the current lead contents inside the zircon sample are all decayed results since \(T_1\). For a zircon sample which suffered no lead-leakage during the event in \(T_1\), then its location will be in \(T_0\), the time when these samples formed blocked environment.

The leakage scenario assumes that the \(Pb^{206}\) and the \(Pb^{207}\) present no difference. Therefore, they have the same leakage percentage, and the zircon samples with different leakage degree will be in a straight line.