Dating radiometric, isotopes used

Figure 4. The decay schemes of 23SU and 235 U, showing the longer-lived isotopes used in radiometric dating of Pleistocene and Holocene deposits.

One can apply the formalism discussed above to a wide variety of systems to produce a radiometric date. In this book, we will use the word date to mean the time calculated from the ratio of a radioactive isotope and its daughter isotope using the equation for radioactive decay. An age is the time between a natural event and the present. A date becomes a valid age when the conditions described in the previous paragraph are met. This terminology, suggested by Faure (1986), is not always used in the literature, where age and date are often used interchangeably. But there is value to the distinction because it helps a reader understand which numbers are significant. 

Radiometric dating As radioisotopes have a given half-life they can be used to calculate or date a sample. Referred to as radiometric dating (including carbon dating), it is possible on the basis of a measure of half-life of an isotope in a sample to determine its age or how long it has existed. Radiocarbon dating was discovered in 1949 by the American chemist, Willard Frank Libby, who was awarded the Nobel Prize in Chemistry in 1960 /or his method to use carbon-14 for age determination . 

Radiometric dating is a method that scientists use to measure the age of things. The most widely known form of radiometric dating is that based on carbon-14. It works like this. Carbon-12 is the most common form of carbon, and carbon-14 is what is called an isotope of it. (An isotope is a variation of the normal atom of an element, in that it has more or less neutrons than the standard atom.) Carbon-12 has six protons and six neutrons at its nucleus, and is therefore said to have an atomic weight of 12. The component of an atom that determines its character is the number of electrons in orbit around its nucleus. In carbon s case there are six. 

The major application of TIMS is to the geochronology and tracer studies using terrestrial radiometric systems, for example, U-Th-Pb, Rb-Sr, Sm-Nd, and Lu-Hf. Geochronology exploits the radioactive decay in closed systems to obtain the date of a specific geological event. Tracer studies use the growth of daughter nuclides from radioactive decay to evaluate the interaction between geochemical systems and/or reservoirs. The application of TIMS in cosmochemical analysis is limited however, it is used to measure the isotopic compositions as tracers of nucleosynthesis processes. This includes the measurement of radionuclides observed mainly in meteorites, for example, Mn-Cr, Al-Mg, Fe-Ni, and Ca-K system in addition to the above-mentioned systems. 

A second radiometric dating technique, used to measure much longer periods of time, involves U-238. U-238 decays with a half-life of 4.5 X 10 yr through a number of intermediates, and eventually ends as lead. As a result, all rocks on Earth that contain uranium also contain lead. If a rock is assumed to have been only uranium when it was formed (which can be determined by the relative amounts of different lead isotopes in the rock), the ratio of uranium to lead can be used to date the rock. For example, if one half-life has passed, the rock would contain 50% uranium atoms and 50% lead atoms. After two half-lives, the rock would be 25% uranium atoms and 75% lead atoms. 

The P-decay of Rb to Sr, with a half-life of 48.8 billion years (Table 10.2), is one of the dassic radiometric dating systems and has been used in cosmo- and geochronology for almost half a century. Strontium has four stable isotopes and as more than two stable non-radiogenic isotopes are thus available, instrumental mass bias can be corrected by internal normalization (see also Chapter 5). For the correction of the radiogenic Sr/ Sr ratios, Sr/ Sr is most suitable, as this blankets the isotope of interest ( Sr) and features two isotopes with relatively high abundances, which minimizes the analytical uncertainty of the measurements. 

The radioactive isotopes in our environment and their predictable decay with time can therefore be used to estimate the age of rocks or artifacts containing those isotopes. The technique is known as radiometric dating, and we examine two different types individually. 

Radiometric dating relies on the accumulation with time of daughter isotopes created by the radioactive decay of a certain amoimt of parent in a closed system. This relationship is expressed by either Eqs. (6) or (7) depending on the absence or presence of an initial quantity of daughter isotope. In practice, the presence of initial daughter isotope generally precludes the use of simple accrrmtrlation methods. 

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