Radiometric Dating: Uranium and Thorium

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The uranium and thorium decay systems offer a multitude of radiometric dating options. Uranium 238 decays through a series of steps to Lead 206. Uranium 235 decays to lead 207, and Thorium 232 decays to lead 208. In addition there is another stable isotope, lead 204, that is entirely primordial and does not form via radioactive decay at all. 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.

Uranium minerals themselves are too uncommon to be very useful in dating. The most common dating method involves the use of minerals like zircon and monazite that are relatively common in granitic rocks. Zircon is especially useful because it frequently contains uranium in substitution for zirconium, but does not incorporate lead (as shown by the absence of Lead-204). Thus all the lead in the zircon can be assumed to be radiogenic.

U-235 decays much faster than U-238 (700 my and 4500 my, respectively). Also, Lead 206 forms only from the decay of U-238 and Lead 207 only from the decay of U-235. A plot of the two ratios (Pb-206/U-238) and (Pb-207/U-235) follows a curve as shown above. The curve depends only on the two decay rates, so if a rock sample plots on the curve, its age can be read off directly.

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. If we measure ages for a number of zircons in the sample, and we can now use laser ablation to determine ages for zones within a single crystal, they should plot on a line between those two ages. 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. 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.

"But Wait!!!" says the late-night pitchman, "There's More!!!!" Suppose you actually want to date uranium minerals themselves, say in a pegmatite or uranium deposit. You can't simply assume there was no initial lead in duch situations. 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. Also, since the lead isotopes differ by less than 2% in mass, fractionation has been insignificant.

For cases like this, we can plot isochrons. 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.

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Created xxxxxxxxxxxxxxxx, Last Update 02 May 2013

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