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If, for example, we apply U-Pb dating to a grain of zircon found in sandstone, we aren't dating the formation of the sandstone, we're dating the formation of the granite that the zircon came from; all we could say about the sandstone is that it must be younger than that.
However, it is possible to put a date on some sedimentary rocks using the mineral xenotime (YPO).
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.
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.
The problem is that sediment is made up of clasts of some parent rock, and when we date these clasts, we are in effect dating the parent rock rather than the the sediment as such.
Pb (lead-204), which is neither unstable nor radiogenic.
We can always try U-Pb dating using the isochron method, but this often doesn't work: the compositions of the minerals involved, when plotted on an isochron diagram, fail to lie on a straight line. First of all, the straight-line property of the isochron diagram is destroyed when the isotopes involved get shuffled between minerals.
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.