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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.
However, these facts about zircons, combined with what we know about uranium, suggest an alternative method of dating.Uranium can and often does substitute for the element yttrium, whereas lead cannot, making xenotime suitable for radiometric dating.The key fact about xenotime is that since it has the same crystal structure as zircon, it can grow on zircon crystals, forming a crust; and this process, of course, cannot begin to take place while the zircon crystal is still locked inside its parent rock.It can be shown mathematically that if the rock has been undisturbed, so that the isotope ratios reflect nothing but the passage of time, then just as with the isochron diagrams we've already discussed (though for a different reason) the minerals so plotted will lie on a straight line on the graph; and the age of the rock can be calculated from the slope of the line.Unlike the ordinary isochron methods such as Rb-Sr, the Pb-Pb method does not allow us to deduce the original proportions of the various lead isotopes from the data acquired from the sample. We can do this by finding minerals that contain lead but never contained any uranium, or only ever contained it in negligible quantities.