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If there is no lead in the zircon originally, and if no lead or uranium has been added or subtracted to the zircon since its formation, then the following formula will hold: , in which case the two t values are said to be concordant; whereas if lead and/or uranium has been added or subtracted, then it would require some sort of statistical fluke for the two t values to end up identical.
So analysis of both the U ratio acts as a check on the correctness of the date we come up with in the same way that step heating does in the Ar-Ar method and the plotting of several minerals on an isochron diagram does for the Rb-Sr and related methods: it allows us to find out if the isotope ratios have been affected by something other than the passage of time, and to reject any "dates" calculated from the isotope ratios if this turns out to be the case. If we suspect that the zircon, despite its chemical properties, still managed to incorporate a little lead at or after its formation, then since all lead isotopes are chemically the same, we can measure the amount of Pb the zircon contains.
But of course for isochron dating we need more than one mineral; zircons alone would not be enough.
However, these facts about zircons, combined with what we know about uranium, suggest an alternative method of dating.
It has two properties which make it useful for this purpose.
First of all, uranium will readily substitute for the zirconium (Zr) in the mineral, whereas lead is strongly rejected.
For this reason we expect zircons, when formed, to contain some uranium, but virtually no lead.
Since we know the ratios in which the various lead isotopes are usually found, we can then apply the same sort of correction we used to account for atmospheric argon in the K-Ar method.
While zircon has been the most popular mineral for U-Pb dating, other minerals have been employed, including apatite, monazite, titanite, allanite and, most interesting of all, xenotime.