U pb isotope dating
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.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.
However, it is possible to put a date on some sedimentary rocks using the mineral xenotime (YPO).
Now since all rocks are somewhat porous, and since we are pretty much obliged to date rocks from near the surface, it's hard to find instances in which uranium has not been lost.
; as you can see from its chemical formula, it is one of the silicate minerals.
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).