Age of rocks

Other isotopes can be used to determine the age of samples. The age of rocks, for example, has been determined from the ratio of the number of radioactive atoms to the number of stable gfPb atoms produced by radioactive decay. For rocks that do not contain uranium, dating is accomplished by comparing the ratio of radioactive fgK to the stable fgAr. Another example is the dating of sediments collected from lakes by measuring the amount of g Pb present. 

Thus, the ratios of lead isotopes 204,206,207 and 208 can vary markedly depending on the source of the lead. One use of these ratios lies in determination of the ages of rocks from the abundances of the various isotopes and the half-lives of their precursor radioactive isotopes. 

Isotopes are also used to determine properties of the environment. Just as carbon-14 is used to date organic materials, geologists can determine the age of very old substances such as rocks by measuring the abundance in rocks of radioisotopes with longer half-lives. Uranium-238 (t1/2 = 4.5 Ga, 1 Ga = 10y years) and potassium-40 (t,/2 = 1.26 Ga) are used to date very old rocks. For example, potassium-40 decays by electron capture to form argon-40. The rock is placed under vacuum and crushed, and a mass spectrometer is used to measure the amount of argon gas that escapes. This technique was used to determine the age of rocks collected on the surface of the Moon they were found to be 3.5-4.0 billion years old, about the same age as the Earth. 

Radioisotopes are used as long-lasting power sources, to study the environment, and to track movement. They are used in biology to trace metabolic pathways, in chemistry to trace reaction mechanisms, and in geology to determine the ages of rocks. 

The determination of the potassium-argon age of rock involves the following experimental stages ... 

A major attraction is the ability to perform isotope ratio measurements, e.g. in many geological applications to determine the age of rocks, and isotope dilution analysis. The latter in particular is gaining-popularity as a highly accurate, precise and hence traceable, method of analysis, so it is worthwhile describing these techniques in more detail. 

Age of Rocks. In the table of nuclides given under Chemical Elements, there are listed a number of naturally occurring radionuclides with long half-lives. From these known half-lives, the geological age of a rock may be calculated. One method of making this estimate is based upon the amount of radionuclide and its daughter nuclide contained in the rock. This method is based upon various assumptions, which may be stated as follows ... 

THOR1ANITE. This mineral of thorium oxide, ThO , is isomorphous with uraninite and occurs in black, nearly opaque cubic crystals in Ceylon and in Madagascar. Often containing rare-earth metals and uranium, the ore is strongly radioactive. Because of its radioactivity, it is valuable in helping to date the relative ages of rocks in which it occurs. 

The steady decay of radioactive elements has some other important uses. One radioactive form of cesium releases radiation at such a steady rate that the worlds most accurate clocks rely on this rate to keep time. Another timekeeper is the precise decay of radioactive potassium into the more stable element argon. Scientists use potassium-argon dating to determine the age of rocks and stone tools at ancient human sites that are more than a million years old. The researchers know how long it takes all the potassium in a rock... 

Also, laboratory experiments will be run with rock cores containing naturally-aged fissures. This will be an important step in testing the appropriateness of laboratory aging of rock surfaces to simulate natural aging processes. 

Staudacher, T., Jessberger, E. K., Dominik, B., Kirsten, T., Shaffer, O. A. (1982) 40Ar-39Ar ages of rocks and glasses from the Nordlinger Ries Crater and the temperature history of impact breccias. J. Geophys., 51, 11. 

The isotopes composition of Pb (and thus its atomic mass) varies detectably according to the source, and such variations have been used to estimate the age of rocks and of the Earth. 

Radioactive ages of rocks Variations of argon isotopic ratios are found within solids because of radioactive decay of 4°K, which also enables the ages of minerals to be determined. This works as follows. Ifrocks solidify with all three I< isotopes present... 

Gislason S. R., Arnorsson S., and Armannsson H. (1996) Chemical weathering of basalt in southwest Iceland effects of runoff, age of rocks and vegetative/glacial cover. Am. J. Sci. 296(8), 837-907. 

To determine the age of a rock, then, scientists measure the amount of strontium-87 compared to the amount of strontium-86. The higher the ratio of strontium-87 to strontium-86, the longer the rock has been in existence. This method for measuring the age of rocks has been used to measure the age of Earth and the age of meteorites. 

The decay of radioactive nuclides with known half-lives enables geochemists to measure the ages of rocks from their isotopic compositions. Suppose that a uranium-bearing mineral was deposited some 2 billion (2.00 X 10 ) years ago and has remained geologically unaltered to the present time. The in the mineral decays with a half-life of 4.51 X 10 years to form a series of short-lived intermediates, ending in the stable lead isotope ° Pb (Fig. 19.6). The fraction of uranium remaining after 2.00 X 10 years should be... 

IDMS is a definitive analytical method not commonly used by chemists, except in Standards Laboratories (e.g., National Institute of Standards and Technology, Institute of Reference Materials and Measurements) where concentrations must be certified. The Earth Sciences also use IDMS when high accuracy is required, for instance where element ratios are needed to calculate the age of rocks. 

Staley, J.T., Adams, J.B., Palmer, F., Long, A., Donahue, D.J. Jull, A.J.T. (1991) Young 14Carbon Ages of Rock Varnish Coatings from the Sonoran Desert. Unpublished Manuscript. 

Used to determine the ages of rocks and of archeological artifacts... 

Potassium is also famous for one of its isotopes, radioactive potassium-40, which has a long half-life of 1.25 billion years. (Half-life refers to the amount of time it takes for half of the elements atoms to disintegrate.) Potassium-40 occurs naturally and is used by researchers to determine the age of rocks. As potassium-40 decays, it becomes a noble gas called argon. By determining how much argon is present in a rock, researchers can estimate the rock s age. Using this technique, scientists have estimated some rocks on Earth to be as old as 3.8 billion years. 

Strontium isotopes can be used to determine the ages of rocks. Radioactive rubidium 87 decays into strontium 87, with a half-life of 4.9 billion years. Geologists can use the ratio of Sr-87 to naturally occurring... 

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