Radiogenic nuclides

In this section we will briefly examine the various methods adopted in geochronology, all based essentially on the constancy of the decay rates of radiogenic nuclides, which, within reasonable limits, are unaffected by the physicochemical properties of the system. For more detailed treatment, specialized textbooks such as Rankama (1954), Dalrymple and Lanphere (1969), Faure and Powell (1972), Jager and Hunziker (1979), Faure (1986), and McDougall and Harrison (1988) are recommended. 

If solid and melt are in equilibrium during partial melting, the conservation law which is concerned with the concentration of a radioactive and radiogenic nuclide in the solid and melt is given by McKenzie (1985) as the following differential equation... 

Impact ages of meteorites and lunar samples can be determined from radioisotope chronometers that are reset by shock. These involve radiogenic nuclides that are gases... 

Terrestrial mother/daughter nuclide pairs suitable for dating are listed in Table 16.2. Dating by means of these nuclide pairs requires evaluation of eq. (16.1). In doing this, it has to be taken into account that, in general, at time t = 0 stable nuclides identical with the radiogenic nuclides are already present. This leads to the equation... 

Applications of the Lu/ Hf method and of the Re/ Os method have no advantages over the methods mentioned previously. For simultaneous determination of the mother and daughter nuclide by MS, suitable non-radiogenic nuclides have to be selected as reference nuclides. In the case of Hf, the use of Hf is preferred for this purpose. However, the main drawbacks of the Lu/ Hf method are the low concentration of Lu in minerals (< 1 mg/kg) and the difficulties in measuring Hf by MS due to the low ionization yield. In the Re/ Os method, Os is used as reference nuclide. As in the case of dating by the Lu/ Hf method, the low concentration of Re in minerals (on the average 1 ng/kg) is a basic drawback with respect to broad application. The method has been used for dating of meteorites and... 

Note that storage of helium in the core remains only one component of a noble gas model that can describe the range of noble gas observations. The core has only been evaluated as a possible storage of He. The incorporation in the core of other noble gases, and their relative fractionations, cannot be clearly evaluated without more data. Also, the distribution of radiogenic nuclides such as "" Ar, Xe, and Xe that are produced within the mantle must be explained with a model that fully describes the mantle reservoirs. While these issues may be tractable, a comprehensive model that incorporates a core reservoir remains to be formulated. It should be emphasized that the core does not completely explain the distribution of helium isotopes, since the issue of the " He-heat imbalance is not addressed at all by this model. It appears that even if high He/ He ratios are the signature of involvement of core material in the source of mantle plumes, several mantle reservoirs are still required. 

The total fraction of a species that has been degassed to the atmosphere can only be calculated for radiogenic nuclides, which have total planetary abundances that are constrained by the parent element abundances. Most attention has focused on " °Ar, although as discussed below Xe also provides valuable constraints. Similar calculations cannot be done for " He, which does not accumulate in the atmosphere, nor for neon isotopes, since the production rate of Ne, as well as the amount of nonradiogenic Ne in the atmosphere, are too uncertain. Also, the amount of radiogenic Xe in the bulk Earth is not well constrained due to early losses from the planet. 

Examples of reactions proceeding during stellar nucleosynthesis are shown in Table 1. To illustrate the sequence of events, the decay series of uranium-238 is depicted in this table. Radiogenic nuclides decay by the emission of alpha, beta and gamma radiation or by electron capture into so called daughter nuclides at their half-lives. This half-life ranges from parts of seconds to billions of years. 

Radiogenic nuclides. The measured ratio is 1.11 0.02, substantially less... 

Therefore, there is potential for considerable production of radiogenic nuclides both in the kimberlite source region, and in the matrix surrounding diamonds after eruption. 

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. 

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],...

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... 

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