Uranium/thorium/lead dating

Although uranium and thorium are found in many minerals, only a few of these minerals are suitable for dating using the uranium, thorium, lead method. These comprise those that can retain these elements adequately and probably the most retentive mineral is zircon. In sedimentary rocks, zircon derived from sources underlain by igneous or metamorphic complexes embodies a record both of the orogenic and magmatic histories of these sources. In... 

In contrast to refractory uranium and thorium, lead is a moderately volatile element. Uranium and thorium are lithophile, while lead can exhibit lithophile, siderophile, or chalcophile behavior. This means that in many cosmochemical situations, it is possible to strongly fractionate the daughter lead from parent uranium and thorium, a favorable situation for radiochronology. On the other hand, lead tends to be mobile at relatively low temperatures and can be either lost from a system or introduced at a later time. As already mentioned, uranium can also become mobile under oxidizing conditions. This means that the U-Th-Pb system is more susceptible to open-system behavior than several other commonly used dating techniques. However, as we discuss below, there are ways to recognize and account for the open-system behavior in many cases. 

U-Th-Pb Dating of Monazite Using Only Uranium, Thorium, and Lead Concentrations... 

Uranium and thorium each have radioactive isotopes with half-lives of billions of years, or roughly the same order of magnitude as the age of Earth itself (see tables 13.4 and 13.5). These elements go through a series of radioactive decay that involve several radioactive isotopes with very diiferent half-lives. The net result is the same, though. Both elements end their respective series with stable isotopes of lead. Both uranium and thorium are common enough in rocks that the ratios of uranium to lead or of thorium to lead in a rock can be used to date rocks that are billions of years old. 

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

Actually, all elements heavier than lead and bismuth are radioactive and are constantly disintegrating. Eventually, lead and bismuth will be the heaviest natural elements on earth, for the heavier elements—polonium, radon, radium, actinium, thorium, protactinium, and uranium—will have disappeared at some date in the distant future. 

Another common method of dating U-minerals is by considering its content of lead isotopes. Lead has four stable isotopes of which three are end products of radioactive decay series. The fourth lead isotope, Pb, is foimd in lead minerals in about 1.4% isotopic abundance and has no radio-genetic origin. At the time of formation of the earth, all the Pb in nature must have been mixed with unknown amounts of the other lead isotopes. If a lead-containing mineral lacks Pb, it can be assumed that presence of the other lead isotopes together with uranium and/or thorium must be due to their formation in the decay... 

The U,Th-Pb methods of dating igneous and metamor-phic rocks is based on the decay of the isotopes of uranium and thorium to stable isotopes of lead ... 

After radioactivity was identified in uranium and thorium minerals, it was used to date them, and methods included Pb/U, Pb/alpha, U/He, U/Th/Pb, and common lead of which the last... 

The decay of uranium (U) and thorium (Th) to stable isotopes of lead (Pb) is the basis for several important dating methods. Uranium has three naturally occurring isotopes, 238u 235 and all of which are radioactive. Thorium exists primarily as one radioactive isotope Th. Thorium also has rive short-lived isotopes that are intermediate daughter products in the decays of and... 

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