PREVIOUS WORK ON CHLORINE-ISOTOPIC VARIATIONS IN GEOFLUIDS

Chlorine isotopes of ODP or Deep Sea Drilling Project (DSDP) interstitial water samples have rarely been studied because chlorine is generally considered a relatively nonreactive element in earth surface environments, not participating in most mineral reactions because of its large ionic radius, 0.181 nm, and negative charge. Most geologic fluids in sedimentary basins on land and in evaporites show only minor chlorine isotopic variations (<±2; Kaufmann et al., 1984; Long et al., 1993); variations of connate waters often remain within the analytical uncertainty for SMOC (0 ± 0.2, Eastoe and Guilbert, 1992). Eggenkamp and Coleman (1998), however, have recently reported ³⁷Cl values from North Sea oilfield waters covering a range from near 0 to almost -5 and ascribed the extremely low values to reactions between Cl in solution and the surrounding rocks. From active-margin environments, even stronger ³⁷Cl depletions, down to almost -8, have been reported and attributed to isotopic fractionation related to mineral precipitation (Ransom et al., 1995). Diffusion-related Cl-isotopic fractionation in pore waters has been inferred for upper Pleistocene glacial and estuarine/marginal-marine settings, respectively (Desaulniers et al., 1986; Eggenkamp et al., 1994), where faster diffusion of ³⁵Cl along a chloride concentration gradient would have caused negative ³⁷Cl values. Membrane filtration has been proposed as another mechanism for chlorine isotope fractionation, but experimental evidence (Campbell, 1985) seems to be in conflict with theoretical results (Phillips and Bentley, 1987).