Mantle fluid inclusions

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). 

Other geochemical characteristics of Italian volcanism are also not easily explained by the plume hypotheses. For example, deep mantle plumes are commonly associated with high 3He/4He ratios (e.g. Farley and Neroda 1998). However, measurements carried out on fluid inclusions in olivine phenocrysts from mafic Italian rocks have yielded low He isotopic ratios with R/Ra < 7.5 (e.g. Sano et al. 1989 Graham et al. 1993 Marti et al. 1994 Di Liberto 2003 Martelli et al. 2004), which are much lower than compositions found for plume-related magmas. 

Izraeli E. S., Harris J. W., and Navon O. (2001) Brine inclusions in diamonds a new upper mantle fluid. Earth Planet. Sci. Lett. 187, 323-332. 

Rosenbaum J. M., Zindler A., and Rubenstone J. L. (1996) Mantle fluids evidence from fluid inclusions. Geochim. Cosmochim. Acta 60, 3229—3252. 

Although shallow-mantle xenoliths, hosted in alkali basalts, commonly contain C02-rich fluid inclusions (see below), there have been no reports, to the author s knowledge, of H20-rich fluid inclusions in these samples. The C02-rich fluid inclusions are commonly attributed to late, possibly magma-derived, metasomatism of the samples. If such metasomatism was produced by silicate- or carbonate-rich melts, ascent of such a melt could produce saturation in a C02-rich vapor, but H2O would partition strongly into either residual melt or hydrous phases such as phlogopite or amphibole. Thus, the absence of H2O in the fluid inclusions in these samples cannot be taken as evidence that the metasomatic agent was anhydrous. 

Shallow-mantle xenoliths, hosted in alkali basalts, commonly contain C02-rich fluid inclusions (e.g., Roedder, 1965, 1984 Frey and Prinz, 1978 Murck eta/., 1978 Miller and Richter, 1982 Pasteris, 1987 Frezzotti et al., 1994 Bumard et al., 1998 Ertan and Leeman, 1999 Andersen and Neumann, 2001). In most cases, these fluid inclusions are related to metasomatic processes to which the samples were subjected. As outlined previously, the C02-rich nature of these inclusions may be a natural consequence of degassing from an ascending melt that contains both H2O and CO2, because the greater solubility of H2O in sihcate melts would allow it to remain in solution in the residual melt. Alternatively, as proposed by Andersen and Neumann (2001), the high CO2 content in these inclusions could be the result of removal of water via reactions between the original fluid and the host mineral surrounding the inclusion. 

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. 

Andersen T. and Neumann E.-R. (2001) Fluid inclusions in mantle xenoliths. Uthos 55, 301 -320. 

Ertan 1. E. and Leeman W. P. (1999) Fluid inclusions in mantle and lower crustal xenoliths from the Simcoe volcanic field, Washington. Chem. Geol. 154, 83-95. 

Frezzotti M. L., Touret J. L. R., Lustenhouwer W. J., and Neumann E. R. (1994) Melt and fluid inclusions in dunite xenoliths from La Gomera, Canary Islands tracking the mantle metasomatic fluids. Euro. J. Mineral 6, 805 - 817. 

Pasteris J. D. (1987) Fluid inclusions in mantle xenoliths. In Mantle Xenoliths (ed. P. H. Nixon). Wiley, pp. 691—707. 

Schiano P., Clocchiatti R., and Joron J. L. (1992) Melt and fluid inclusions in basalts and xenoliths from Tahaa Island, Society archipelago evidence for a metasomatized upper mantle. Earth Planet. Sci. Lett. Ill, 69—82. 

Turner G., Burgess R., and Bannon M. (1990) Volatile-rich mantle fluids inferred from inclusions in diamond and mantle xenolith. Nature 344, 653-655. 

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