Chlorinity hydrothermal fluids

Seawater and marine pore fluids. As discussed above, the chlorine isotopic composition of modem seawater does not vary measurably. This is not surprising in light of its long residence hme (approximately 90 million years) and its conservative behavior in the water column. In contrast, marine pore fluids have been demonstrated to vary considerably. There is also the likelihood that hydrothermal fluids may be fractionated as a result of exchange with mineral phases, as phase separation under marine hydrothermal conditions does not appear to lead to measurable fractionation (e.g., Magenheim et al. 1995). However, to date no stable-chlorine isotopic compositions of marine hydrothermal fluids have been reported in the literature. 

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). 

An element that is relatively conservative through water-rock reaction is chlorine in the form of the anion chloride. Chloride is key in hydrothermal fluids, because with the precipitation and/or reduction of SO4 and the titration of HC03"/C03, chloride becomes the overwhelming and almost only anion (Br is usually present in the seawater proportion to chloride). Chloride becomes a key component, therefore, because almost all of the cations in hydrothermal fluids are present as chloro-complexes thus, the levels of chloride in a fluid efiectively determine the total concentration of cationic species that can be present. A fundamental aspect of seawater is that the major ions are present in relatively constant ratios—this forms the basis of the definition of salinity (see Volume Editor s Introduction). Because these constant proportions are not maintained in vent fluids and because chloride is the predominant anion, discussions of vent fluids are best discussed in terms of their chlorinity, not their salinity. 

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