Elements with Radiogenic Nuclides

Extraterrestrial materials In some extraterrestrial material such as meteorites, elements may show isotopic compositions that are distinct from all terrestrial material investigated. This is related to decay of radionuclides that may already be extinct, due to half-lives which are very short compared with the age of the solar system of 4.6 x 10 years. Such variations are rare for terrestrial materials, in large part due to preferential sampling of the crust, whereas some extraterrestrial material, such as iron meteorites, resemble the Earth s core, in which parent to daughter element ratios may be much higher than in the cmst. 

Interaction between cosmic rays and terrestrial matter The Earth s atmosphere and, to a lesser extent, its surface are constantly bombarded with cosmic radiation which interacts with terrestrial material, resulting in isotopic variations in some elements. The best known example is the production of C from by (n,p) reaction in the atmosphere, with the neutron involved created by cosmic ray-induced spallation. a radionuclide with a half-life of 5730 years, is oxidized to CO2 and enters the food chain via photosynthesis, thus affecting the isotopic composition of C in all living organisms. 

Mass-independent fiactionation Most cases of isotope fractionation are characterized by a hnear relationship between the magnitude of the effect estabhshed and the difference in mass between the isotopes considered. For an increasing number of elements, however, an apparently aberrant behavior is estabhshed for some of their isotopes. This is currently a hot topic of research 

Various processes that result in isotopic variations will be discussed in more detail in the following sections. The way in which the quantification of such variations can be used in the context of various real-life applications will be demonstrated in later chapters. 

---

Other Parent-Daughter Pairs A summary of elements that contain one or more radiogenic nuclides and for which the isotopic composition is studied by (multicollector) ICP-MS is provided in Table 1.2. Elements with radiogenic nuclides not included in Table 1.2 either are not amenable to ICP-MS analysis (e.g., Ar) or have such long half-lives that the variation in their isotopic compositions is too limited to be quantified using present-day ICP-MS instrumentation. 

Table 1.2 Elements with radiogenic nuclides that can be measured via (multi-collector) ICP-MS [9, 16],...

A pattern that is consistent with radiogenic production can be calculated by assuming that the noble gases have been degassed without substantial elemental fractionation that is, the radiogenic nuclides of the different elements are present in the mantle at relative abundances that are equal to their production ratios. In this case. 

In the early days of mass spectrometry, research was focused on gas source isotope ratio mass spectrometry (IRMS) or thermal ionization mass spectrometry (TIMS), with as the main aim the determination of the isotopic composition and molar masses of the elements. Since the 1940s, isotope ratio measurements have also been used for the determination of isotope ratios involving a radiogenic nuclide and for quantitative element determination via isotope dilution (see also Chapter 8). The age of the solar system and the Earth were also of particular interest within isotope ratio science. With the establishment and improvement of TIMS instrumentation, the awareness of the importance of precision and accuracy and the need to be able to reproduce a result in another laboratory grew, while... 

Because the short-lived nuclides are extinct, a different approach must be taken to use them as chronometers. Equation (8.9) cannot be used to calculate a date because the number of parent nuclides, N, is zero and the equation is undefined. However, if a short-lived nuclide was homogeneously distributed throughout a system, then one can determine the order in which objects formed within that system based on the amount of radionuclide that was present when each object formed. The oldest object would form with the highest amount of the radionuclide relative to a stable isotope of the same element, and the youngest will have the lowest amount. Obviously, no chronological information can be obtained about objects that formed after the radionuclide has reached a level too small to detect the radiogenic daughter isotope. 

The long-lived radioactive decay systems commonly used to characterize mantle compositions, their half-lives, and the isotope ratios of the respective radiogenic daughter elements are given in Table 1. The half-lives of Sm, Sr, Hf, Re, and Th are several times greater than the age of the Earth, so that the accumulation of the radiogenic daughter nuclide is nearly linear with time. This is not the case for the shorter-lived... 

---