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Selenium (masses 74, 76, 77, 78, 80, and 82; Table 1⇓) and chromium (masses 50, 52, 53 54; Table 1⇓) are treated together in this chapter because of their geochemical similarities and similar isotope systematics. Both of these elements are important contaminants in surface and ground water. They are redox-active and their mobility and environmental impact depend strongly on valence state and redox transformations. Isotope ratio shifts occur primarily during oxyanion reduction reactions, and the isotope ratios should serve as indicators of those reactions. In addition to environmental applications, we expect that there will be geological applications for Se and Cr isotope measurements. The redox properties of Se and Cr make them promising candidates as recorders of marine chemistry and paleoredox conditions.
There are only about a dozen published studies on Se isotopes and only two on Cr isotopes. This chapter summarizes what has been learned thus far, and almost all of this work concentrates on aqueous reactions at earth surface temperatures. It also attempts to provide some geochemical background and reviews some relevant points from the sulfur isotope literature, which provides insight into the isotopic systematics of Se and Cr.
Se is chemically similar to sulfur, which is immediately above it in the periodic table. Its concentration in the earth’s crust is small, with most rocks containing less than 0.1 ppm Se except for shales, which span a wide range of concentration and average roughly 1ppm (Faure 1991). Coal is also relatively rich in Se, averaging 3 ppm (Cooper et al. 1974). Se can substitute extensively for S in pyrite (Coleman and Delevaux 1957). However, Se concentrations of many sulfide minerals are quite small, suggesting strong decoupling of Se from S in some systems. …