Chlorine fluid inclusions

Dubessy, J., Lhomme, T., Boiron, M.-C., Rull, F. 2002. Determination of chlorinity in aqueous fluids using Raman spectroscopy of the stretching band of water at room temperature application to fluid inclusions. Applied Spectroscopy, 56, 99-106. 

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). 

Based on the work of Philippot et al. (1998), one might expect to observe a certain proportion of chlorine-rich fluid inclusions in mantle-derived xenoliths, but inclusions in these xenoliths are overwhelmingly C02-rich, and chlorine-rich inclusions have not been reported (cf. reviews by Roedder, 1984 Pasteris, 1987 Andersen and Neumann, 2001), with the intriguing exception of the brines reported as inclusions in some diamonds (Johnson et al., 2000 Izraeli et al., 2001). The lack of direct observation of chlorine-rich fluid inclusions in mantle-derived xenoliths may be a result of the lack of examination of appropriate samples that record a previous history as subducted oceanic crust, an absence of these fluids in deeper samples because of participation of these fluids in other petrological processes, such as melt production, or because such fluids do not survive subduction below the slab dehydration limit. Conversely, the presence of chlorine in fluid inclusions in diamonds argues for the existence of chlorine-rich fluids at least in some circumstances in the mantle in the pressure range of diamond stability. 

Potential insight into the fate of a chlorinebearing fluid came from the study of Andersen et al. (1984) of xenoliths from Bullenmerri and Gnotuk maars in southwestern Australia that contained abundant C02-rich fluid inclusions and vugs up to 1.5 cm in diameter. They found the trapped fluids had reacted with the host minerals to produce secondary carbonates and amphiboles, such that the original composition of the fluid was inferred to be a chlorine- and sulfurbearing CO2-H2O fluid. The evidence for chlorine was the presence of a chlorine peak in the energy-dispersive spectmm of the amphibole unfortunately, no quantitative analyses were possible on these amphiboles. This does pose the possibility that this sort of reaction is common, and that the normal host for chlorine in the mantle is a mineral phase, such as apatite, amphibole, and mica. 

Finally, the study of Banks et al. (2000) presents data from fluid inclusions at two crystalline sites. The S Cl values in mineral inclusions of the Capitan pluton of New Mexico, USA were near Q%c. The authors concluded that an evaporitic source was responsible for the chlorine isotopic signature. Their other site was a batholith in southwestern England and the 5 Cl signature was approximately +1.8%o. They felt this was more representative of deeper magmatic sources similar to the MORB data reported by Magenheim et al. (1994). The data are also similar to water and rock data from similar young granites in Finland (Frape et al., 1998). 

Banks D. A., Green R., Cliff R. A., and Yardley B. W. D. (2000) Chlorine isotopes in fluid inclusions determination of the origins of salinity in magmatic fluids. Geochim. Cosmochim. Acta 64, 1785-1789. 

Finally, it should be noted that just as a typical " °Ar- Ar irradiation also produces argon from calcium and chlorine, a typical I-Xe irradiation also produces xenon from tellurium, barium and uranium (Turner 1965) and krypton from bromine. Conversion factors for many or all of these reactions have been measured or calculated by several authors, most recently by Irwin and co-workers for use in studies of fluid inclusions (Irwin and Reynolds 1995 Irwin and Roedder 1995). Many of these elements are not routinely determined by other techniques, so direct comparisons with other techniques are rare (but see (Garrison et al. 2000 Swindle et al. 1991b)). 

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