Uranium/lead dating

U238 —> —> Pb206 4.47 x 109y Uranium-lead radiometric dating (1 of 2)... 

River Station Date Discharge Cond. pH SPM Vanadium Chromium Manganese Cobalt Nickel CoQper Zinc Arsenic Rubidium Strontium Cesium Barium Lead Uranium... 

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. ... 

The best sealed-in minerals are zircons, zirconium silicate minerals which are formed when melted lava on the flanks of volcanoes solidifies. When the zircons crystallize out, they incorporate radioactive uranium (in particular 238U), which decays in several steps, leading Anally to the lead isotope 208Pb. The rate of decay is very low, as the half-life of uranium-238 is 4.5 x 109 years. Thus, the U-Pb-zircon method for age determination of Precambrian rock is very important. The fossils studied by Schopf were sandwiched between two lava layers (Schopf, 1999). The volcanic layers were dated to 3.458 0.0019 x 109 years and 3.471 0.005 x 109 years the age of the fossil layer (Apex chert) was thus determined to be about 3.465xlO9 years. 

Uranium-238 has a half-life of 4.468 billion years over which time it decays into stable lead-206. This process can be used to date ancient rocks by comparing the ratio of the isotope lead-206, the last isotope in the uranium decay series, to the level of uranium-238 in the sample of rock to determine its age. This system has been used to date the oldest rocks on Earth as being about 4.5 billion years old, which is about the time of the formation of our planet. 

The decay of °Th leads to radioisotopes of other elements, ultimately concluding with the stable isotope lead-206. Happily, some of the oldest rocks on Earth, called zircons, contain no lead when they are formed. This means that the amount of lead they accumulate over time from uranium decay reflects their age. Until the rocks crystallized, uranium atoms could move freely through the molten magma from which they formed, and decayed uranium could be replenished. Solidification of a zircon does for uranium what an organism s death does for radiocarbon it stops the influx of fresh radioactive material, and the decay clock starts ticking. Because of U s long half-life, zircons can be dated back to the Earth s earliest days. 

Our planet was probably transformed to a ball of magma 4.45 billion years ago by an impact with a small planet-like body the resulting debris formed the Moon. Yet uranium-lead dating shows us how quickly this magma ocean must have cooled, since it reveals that the oldest zircons, found in Western Australia, crystallized about 4.4 billion years ago. What is more, these ancient zircons show signs of having been formed in contact with water, implying that even in that distant era the world had... 

A small proportion of natural uranium consists of the isotope This decays not to lead-206 but to lead-207. By measuring the amounts of all these isotopes of uranium and lead in rocks, geologists can date all manner of minerals, and can even reconstruct the history of our planet s formation. Some meteorites are thought to be left-over remnants of the rocky material that aggregated to produce the Earth, and they show us the mixture of elements this material contained. If they contain no uranium, then... 

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. 

A significant problem in using the uranium-lead system for age dating is that the system is relatively easily disturbed. However, George Wetherill showed that by using both the... 

SOURCE C. Lewis, The Dating Game (Cambridge Cambridge University Press, 2000) A. Holmes, The Association of Lead with Uranium in Rock-Minerals, and Its Application to the Measurement of Geological Time," Proc. Royal Soc. Lond. A 1911, 85, 248. 

Scientists use radioactive minerals to date very old nonliving things. The naturally occurring mineral isotopes uranium-238 and uranium-235 decay very slowly and ultimately become lead—but not the common isotope lead-208. Instead, as... 

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