Radiogenic isotopes in geochronology

The foundations of modern geochronology were laid at the turn of the century in the work of Rutherford and Soddy (1903) on natural radioactivity. They showed that the process of radioactive decay is exponential and independent of chemical or physical conditions. Thus rates of radioactive decay may be used for measuring geological time. Isotopic systems used in age calculations are listed in Table 6.1 and Box 6.1. In this section we discuss two of the most common techniques used in geochronological calculations — isochron diagrams and model age calculations. This is followed by a discussion of the significance of the calculated ages. 

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This problem, in which a radiogenic element is allowed to leak out of its host mineral as it forms, has found important applications in geochronology, particularly for the K-Ar method (Wasserburg, 1954) and the U-Pb method (Tilton, 1960 Wasserburg, 1963) with the so-called continuous loss model. The equation for radial diffusion of a radiogenic element in a sphere with radius a and uniform parent isotope concentration P = P0 at t = 0 can be written... 

Geochronological (radiogenic isotope and palynological) results are used here to understand the kimberlite-host rock relationships in the Buffalo Head Hills kimberlite field, and to provide new philosophy on the timing and distribution... 

Mother isotope Abundance of mother isotope a, % Half life 1/2/ a Radioactive decay Stable radiogene daughter nuclide Abundance of daughter isotope a, % Preferable application fields in geochronology... 

The rubidium-strontium geochronometer used in the Rb-Sr geochronological method is based on the radioactive 3 -decay of Rb to Sr. The growth of radiogenic Sr in a Rb rich mineral can be described by the following Equation (9.6). In the rubidium-strontium age dating method, the radioactive Rb isotope with a natural isotope abundance of 27.85 % and a half-hfe of 4.88 X 10 ° years is fundamental to the 3 decay to the isobar Sr. The equation for the Rb-Sr method can be derived from the general equation of radioactive decay (Equation 8.8 in Section 8.8) ... 

Shirey S. B. (1991) The Rb-Sr, Sm-Nd and Re-Os isotopic systems a summary and comparison of their applications to the cosmochronology and geochronology of igneous rocks. In Applications of Radiogenic Isotope Systems to Problems in Geology, Short Course Handbook (eds. L. Heaman and J. Ludden). Mineral. Assoc. Can., Nepean, vol. 19, pp. 109-166. 

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. 

Bevier, M.L Whalen, J.B. 1990a. U-Pb geochronology of Silurian granites, Miramichi terrane, New Brunswick. In Radiogenic Age and Isotopic Studies Report 3. Geological Survey of Canada Paper, 89-2, 93-100. 

Pb / U vs. Pb / U plot, where denotes the radiogenic portion of Pb isotopes. This diagram also finds wide applications in zircon U-Pb geochronology. For U-Pb zircon dating, the daughter-parent ratios are given by... 

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