instrumental techniques applied to mineralogy and geochemistry

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114 Kjell Bilström ratios ( 204 Pb is unradiogenic and is used as a reference isotope) are typically determined. These ratios increase with geological time and with increasing levels of U and Th in a studied system, but in opposite to the behaviour of Sr in the crust/mantle, the obtained Pb isotope ratios of galena (with a U/Pb ratio virtually being zero) do not change with time once Pb has been incorporated into its lattice. Hence, galena-bearing ores formed at different times (and in different geological environments) are characterized by different sets of Pb isotope ratios, and occasionally the Pb isotopic signature of a specific ore is unique providing a direct link between a specimen and a known ore. The way the Pb isotope ratios evolve with time in different environments (e.g. upper crust, lower crust and mantle) has been modelled by different authors, and one of the most widely used model is known as the plumbotectonics model (Doe & Zartman, 1979). Figure 2 displays galena data from the Gangdese copper belt in Tibet (Qu et al., 2004) forming a linear array in consistency with a mixing process involving lead from different environments. FIGURE 1. Principal sketch-map showing the differences in Sr isotope ratios that develop once a crustal segment is separated from the mantle at some 3 billion years ago (after Wilson, 1989).
Radiogenic isotopes and their applications within a range of scientific fields 115 FIGURE 2. Plumbotectonics diagram showing data of galena from the Gangdes copper belt, Tibet along with the evolution curves for the upper crust, lower crust, mantle and orogene (Qu et al., 2004). Also shown is the two-stage model of crustal lead of Stacey and Kramers (1975). A wide range of applications Applications where radiogenic isotopes are used will here be demonstrated within the fields of bedrock geology, archaeology, forensics, food and beverage industry, and environmental sciences. Given the space limitation, only a few, illustrative case studies can be presented. Essentially, many isotope studies make use of the fact that isotope ratios could act as finger-prints that constrain the origin of the studied object. Sr and Pb isotopes are two isotopic systems which were developed for geological applications, but which are also useful in other disciplines. Compared to many lighter elements which are presently explored (e.g. Si, Sn, Mo, Fe, etc.), Sr and Pb have some advantages. Their isotope systematics is well characterized, their isotopic variations are quite large and easily analyzed, and proven methods for their chemical isolation before analysis exist. In addition, there are several sample types (e.g. carbonates) exhibiting very low Rb/Sr element ratios which make them particularly relevant to analyze. Remembering that carbonates are rich in Ca, and that Sr is geochemically similar to Ca, there will also be substantial amounts of Sr present in carbonates, but basically no Rb. From this follows, that the Sr isotopic ratios in carbonates do not change with time as a result of radioactive decay, and such data have been used to monitor the temporal changes in Sr isotope ratios of the oceans (see e.g. Faure and Mensing, 2005).
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INDEX Electron back-scattered diffr
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