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Now, compounds of uranium are often highly soluble in water (this, indeed, is one of the major problems with U-Pb isochron dating) whereas compounds of lead are stubbornly insoluble.As a result, we expect speleothems when they are first formed to contain some uranium but little or no lead — just like zircons.Now since all rocks are somewhat porous, and since we are pretty much obliged to date rocks from near the surface, it's hard to find instances in which uranium has not been lost.; as you can see from its chemical formula, it is one of the silicate minerals.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.
You might perhaps doubt that meteorites would have the same initial lead isotope ratios as the Earth.Planetary scientists maintain that they should, for reasons which are somewhat beyond the scope of this textbook.Another reason for believing it is that if we calculate Pb-Pb dates on this basis, the dates we get are in agreement with dates produced by other methods where they can be applied: this would hardly be possible if we were using the wrong figures for the initial lead isotope ratios.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.