3. U-Pb dating of zircons
The results are plotted on
what is known as a Concordia
diagram (see Box 1.1 Fig. 3), a
graph of 206Pb/238U and
207Pb/235U ratios, showing
the locus of agreement of U-Pb
ages obtained in the two
U-Pb decay systems (238U
and 235U). Analyses which
plot on the Concordia curve
indicate the time of
crystallization of the grain.
Analyses which plot off the
Concordia are interpreted in
terms of late Pb-loss from the
grain, most normally as the
result of a subsequent
thermal event which affected
the rock.
