Rocks, uranium/lead dating

McNaughton, N.J., Rasmussen B. Fletcher I.R. (1999) SHRIMP uranium-lead dating of diagenetic xenotime in siliciclastic sedimentary rocks. Science 285, 78-80. 

EXAMPLE 19.6 Using Uranium/Lead Dating to Estimate the Age of a Rock... 

The kinetics of radioactive decay is used to date objects and artifacts. The age of materials that were once part of living organisms is measured by carbon-14 dating. The age of ancient rocks and even Earth itself is determined by uranium/lead dating. 

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... 

Isochron age calculations are commonly made for the Rb-Sr (rubidium-strontium], Sm-Nd (samarium-neodymium], and U-Pb (uranium-lead] radioactive systems. They are most commonly applied to whole-rock systems, that is, a suite of samples thought to have formed at the same time, such as an igneous plu-ton or a suite of lavas. Isochron age calculations may also be made for a suite of minerals in a rock, in which case they date the time at which the minerals lost isotopic contact with each other, that is, became closed systems. This approach can be useful in dating metamorphism. 

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. 

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. 

If you know the half-lives of uranium isotopes and the percentage of lead isotopes in some uranium-bearing rock, you can calculate the date the rock was formed. Rocks dated in this way have been found to be as much as 3.7 billion years old. Samples from the moon have been dated at 4.2 billion years, which is close to the estimated age of our solar system 4.6 billion years. 

For dating rocks, geologists have used the uranium-238 isotope, which decays to lead-206 via a number of intermediate products. The ratio of... 

The decay process of a different radioisotope, uranium-238 to lead-206, is commonly used to date objects such as rocks. Because the half-life of uranium-238 is 4.5 X 10 years, it can be used to estimate the age of objects that are too old to be dated using carbon-14. By radiochemical dating of meteorites, the age of the solar system has been estimated at 4.6 X 10 years of age. 

Holmes A (1911) The association of lead with uranium in rock-minerals and its application to the measurement of geological time. Proc Roy Soc London A 85 248-256 Holmes A (1954) The oldest dated minerals of the Rhodesian Shield. Nature 173 612 Holmes A (1955) Dating the Precambrian of peninsular India and Ceylon. Proc Geol Soc Canada 7 81-106 Holmes A, Cahen L (1955) African geochronology. Col Geol Min Res 5 3-38... 

Other isotopes can be similarly used to date other types of objects. For example, it takes 4.5 X 10 yr for half of a sample of uranium-238 to decay to lead-206. The age of rocks containing uranium can therefore be determined by measuring the ratio of lead-206 to uranium-238. If the lead-206 had somehow become incorporated into the rock by normal chemical processes instead of by radioactive decay, the rock would also contain large amounts of the more abundant isotope lead-208. In the absence of large amounts of this geonormal isotope of lead, it is assumed that all of the lead-206 was at one time uranium-238. 

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 ... 

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