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.
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.
The reader will probably be familiar with stalagmites and stalagtites; more speleothems are shown in the photograph to the right.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.There is a difficulty in using radiometric dating to put an age on sedimentary rock.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.