Zircon age dating

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.

Since we know the ratios in which the various lead isotopes are usually found, we can then apply the same sort of correction we used to account for atmospheric argon in the K-Ar method.

While zircon has been the most popular mineral for U-Pb dating, other minerals have been employed, including apatite, monazite, titanite, allanite and, most interesting of all, xenotime.

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.

But of course for isochron dating we need more than one mineral; zircons alone would not be enough.

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.First of all, uranium will readily substitute for the zirconium (Zr) in the mineral, whereas lead is strongly rejected.For this reason we expect zircons, when formed, to contain some uranium, but virtually no lead.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.

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