Characteristic Features of Cl Isotope Geochemistry

Chlorine is the major anion in surface- and mantle-derived fluids. It is the most abundant anion in hydrothermal solutions and is the dominant metal complexing agent in ore forming environments (Banks et al. 2000). Despite its variable occurrence, chlorine isotope variations in natural waters conunonly are small and close to the chlorine isotope composition of the ocean. This is also true for chlorine from fluid inclusions in hydrothermal minerals which indicate no significant differences between different types of ore deposits such as Mississippi-Valley and Porphyry Copper type deposits (Eastoe et al. 1989 Eastoe and Guilbert 1992). 

Relatively large isotopic differences have been found in slow flowing groundwater, where Cl-isotope fractionation is attributed to a diffusion process (Kaufmann et al. 1984,1986 Desaulniers et al. 1986). Cl depletions detected in some pore waters have been attributed to processes such as ion filtration, alteration and dehydration 

Recently Sharp et al. (2007) have questioned the findings of Magenheim et al. (1995). Sharp et al. (2007) found that the large differences between mantle and crustal material do not exist and that the mantle and the crust have very similar isotopic composition. A possible explanation for this apparent discrepancy might be related to analytical artifacts of the TIMS technique (Sharp et al. 2007). Bonifacie et al. (2008) also observed small Cl-isotope variations only in mantle derived rocks. They demonstrated that 5 Cl values correlate with chlorine concentrations Cl-poor basalts have low S Cl values and represent the composition of uncontaminated mantle derived magmas, whereas Cl-rich basalts are enriched in Cl and are contaminated by Cl-rich material such as ocean water. 

Volcanic gases and associated hydrothermal waters have a large range in 8 C1-values from —2 to +12%o (Bames et al. 2006). To evaluate chlorine isotope fractionations in volcanic systems, HCl liquid-vapor experiments performed by Sharp (2006) yield large isotope fractionations of dilute HCl at 100°C. These results are in contrast to liquid-vapor experiments by Liebscher et al. (2006) observing very little fractionation at 400 - 450° C. Clearly more data are needed to resolve these discrepancies. 

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