A thermal event that will leave most radioactive clocks relatively unaffected may have a drastic effect on the helium radioactive clock.In the future, helium dating may be found very useful for dating rocks of the late Cenozoic and Pleistocene, because rocks and minerals of this age have not been subject to the complex history of older rocks and minerals; thus, all the helium is more likely to have been retained.Thus any of the radioactive isotopes and its lead daughter product can be used for dating, or a combination may be used.In addition, some of the longer-lived intermediate daughter products have uses in dating.With a little luck some of the points will plot close to the true age, increasing the precision of the date.U-Pb ages are accurate to within less than 1 per cent, highly resistant to metamorphism, and are considered the most precise ages now available.
Helium ages, however, tend to be too low because the gas escapes from the rock.
Also, since the lead isotopes differ by less than 2% in mass, fractionation has been insignificant. We can say, plot Pb-207/Pb-204 versus U-235/Pb-204 and derive the age from an isochron plot, exactly the way Rb-Sr dating works.
Because of this decay, the helium content of any mineral or rock capable of retaining helium will increase during the lifetime of that mineral or rock, and the ratio of helium to its radioactive progenitors then becomes a measure of geologic time.
With modern laser ablation methods, a detailed history can be derived for single zircon grains showing the ages of growth zones within the zircon. " Suppose you actually want to date uranium minerals themselves, say in a pegmatite or uranium deposit.
So a zircon formed in a granite, eroded and redeposited into sediments, which were then metamorphosed to gneiss, can often supply dates for both the gneiss and the original granite. You can't simply assume there was no initial lead in duch situations.