Stable isotopes of chlorine

Chlorine occurs in hydrogeological systems primarily as the Cl- ion, and has two stable isotopes: 35Cl (75.53%) and 37Cl (24.47%) (Boyd et al., 1955). The chloride ion is considered to behave conservatively during most low-temperature processes in hydrologic environments and this, combined with the large abundance of the heavier isotope, results in a small range of chlorine stable isotope ratios. The use of chlorine stable isotopes to evaluate hydrologic processes and sources of chloride in hydrologic environments is, to some extent, limited by the small range of observed isotopic ratios. Chlorine stable isotope ratios (δ37Cl) are measured in %o relative to SMOC (Standard Mean Ocean Chloride) and range from -4.25 to + 3.00%o (SMOC) in groundwater, pore waters, and aerosols and from -0.84 to +7.5%o (SMOC) in evaporites, Cl-bearing amphiboles and smectites. The past five years has seen a significant increase in the analysis and reporting of chlorine stable isotope data from a range of geologic and hydrologic environments. This is due in part to development of the positive thermal ionisation mass spectrometry technique (PTIMS) that allows chlorine stable isotopes to be measured on Cs2Cl+ from samples containing as little as 2μg chlorine (e.g., Xiao and Zhang, 1992; Magenheim et al., 1994; Xiao et al., 1995). Analysis of CH3Cl from AgCl precipitation, which is more widely used, requires significantly more chlorine for analysis (>1 mg). The methyl chloride method can be used for samples with sufficient chloride concentrations such as brines in sedimentary basins and shield environments, chloride-rich pore waters in clays, Cl-rich evaporites and chlorinated solvents. Samples with limited chloride concentrations such as aerosols and Cl-bearing silicates have been analysed primarily using the PTIMS technique. The precision of these techniques ranges from ∼0.1%o for aqueous samples to 0.12-0.3%o for rock and chlorinated solvent samples. Copyright (2000) by Kluwer Academic Publishers