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The use of stable-isotopic ratios of a number of elements (e.g., H, Li, B, C, N, O, and S) is well established in the study of a broad spectrum of geological and environmental problems such as alteration of the oceanic crust, magmatic-crustal interactions, global chemical fluxes, the nature of the Precambrian crust and identifying the source and fate of pollutants (e.g., Taylor 1968; Muelenbachs and Clayton 1976; Muelenbachs 1980; Gregroy and Taylor 1981; Poreda et al. 1986; Spivack and Edmond 1986; Tanaka and Rye 1991; Mojzsis et al. 2001; Wilde et al. 2001; Numata et al. 2002). The fractionation of these light stable isotopes is a function of the relative mass differences between isotopes (e.g., Richet et al. 1977; Schauble, et al. 2003; Schauble 2004). The two stable isotopes of chlorine are 35Cl and 37Cl with a natural relative abundances of approximately 76% and 24%, respectively, and a relative mass difference of 5.7%, similar in magnitude to the relative mass differences between the isotopes of C and N. Hence, by analogy with these elements it is expected that stable isotopes of chlorine significantly fractionate and can be similarly exploited to understand and solve geological and environmental problems.
Early attempts at reproducibly determining 37Cl/35Cl ratios in natural samples were largely unsuccessful (e.g., Curie 1921; Owen and Schaeffer 1955; Hoering and Parker 1961), primarily, because of the limited effectiveness of the extraction and sample preparation techniques, the relatively poor precision of mass spectrometers at the time, and possibly the 37Cl/35Cl of the samples selected for analyses were inappropriate for the precision possible (e.g., Eggenkamp and Schuiling 1995). Taylor and Grimsrud (1969) developed a method for precisely measuring chlorine isotope ratios by mass spectrometry of methyl …