Single mineral pb pb dating
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.
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 sediment as such.
Uranium can and often does substitute for the element yttrium, whereas lead cannot, making xenotime suitable for radiometric dating.
For this reason we expect zircons, when formed, to contain some uranium, but virtually no lead.
Troilite (Fe S) from iron-rich meteorites fits the bill: its present ratio of uranium to lead is so tiny that either the solar system and indeed the universe is many many times older than cosmologists think, or, given the long half-life (4.5 billion years) of U, there can hardly have been any uranium in the meteorites to start with, and so its decay can hardly have affected the lead isotope ratios of these meteorites.
You might perhaps doubt that meteorites would have the same initial lead isotope ratios as the Earth.
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.
A speleothem, more colloquially known as a cave formation, is formed when minerals dissolved in water precipitate out of the water as it drips, seeps, or flows into a cave.