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So we can apply the same technique to speleothems as we do to zircons.
We can exploit our double system of Pb ratios on another (similar, though not identical, to what we did when constructing isochron diagrams).
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.
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.
Now lead and uranium are particularly susceptible to such shuffling in the event of even mild metamorphism.
The other problem is that uranium is particularly susceptible to weathering.
The reader will probably be familiar with stalagmites and stalagtites; more speleothems are shown in the photograph to the right.
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.
Although it is not abundant in igneous rocks, it is sufficiently common to be used for the purposes of radiometric dating.
It has two properties which make it useful for this purpose.
Second, zircons are durable and chemically inert, able to resist chemical weathering and even high-grade metamorphism up to about 900 °C.