Fractionation in dating techniques
Unfortunately, the breakdown of uranium creates radiation damage in the surrounding crystal, creating avenues for lead to escape.The final point where the sample plots, will be some linear combination of the true concordia age and some final composition where lead diffusion stopped, either the present or some past metamorphic event.There was probably radiogenic lead in the mineralizing fluids plus possibly lead leached out of the host rocks.Fortunately, there is Lead 204, which we can be sure has no radiogenic component.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.
This sort of plot is a discordia plot, shown above.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.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.