Hydrothermal vent fluids phase separation

Bray A. M. and Von Damm K. L. (2003a) The role of phase separation and water—rock reactions in controlling the boron content of mid-ocean ridge hydrothermal vent fluids. Geochim. Cosmochim. Acta (in revision). 

Butterfield D. A., McDuff R. E., Mottl M. J., Lilley M. D., Lupton J. E., and Massoth G. J. (1994) Gradients in the composition of hydrothermal fluids from the endeavour segment vent field phase separation and brine loss. J. Geophys. Res. 99, 9561-9583. 

Lonsdale P, Becker K (1985) Hydrothermal plumes, hot springs, and conductive heat flow in the Southern Trough of Guaymas Basin. Earth Planet Sci Lett 73 211-225 Massoth GJ, Butterfield DA, Lupton JE, McDuff RE, Lilley MD, Jonasson IR (1989) Submarine venting of phase-separated hydrothermal fluids at Axial Volcano, Juan de Fuca Ridge. Nature 340 702-705 McKenzie DP, Davies D, Molnar P (1970) Plate tectonics of the Red Sea and east Africa. Nature 226 243-248... 

Submarine venting of phase-separated hydrothermal fluids at Axial Volcano, Juan de Fuca Ridge. Nature, 340 702-765. 

Vapor and brine from the Brandon vent of the East Pacific Rise have identical Fe isotope compositions, implying that phase separation does not produce an isotopic fractionation (Beard et al. 2003a). The role that sulfide precipitation plays in controlling the Fe isotope composition of the fluid remains unknown. The precision of the two sulfide analyses reported by Sharma et al. (2001) was not sufficient to resolve if sulfide precipitation would produce Fe isotope fractionation in the vent fluid. In a detailed study of sulfldes from the Lucky Strike hydrothermal field from the mid Atlantic Ridge, however, Rouxel et al. (2004) found that sulfldes span a range in 5 Fe values from -2.0 to +0.2%o, and that pyrite/marcasite has lower 5 Fe values ( l%o) as compared to chalcopyrite. The variations in mineralogy and isotope composition are inferred to represent open-system equilibrium fractionation of Fe whereby... 

The pressure conditions at any hydrothermal field are largely controlled by the depth of the overlying water column. Pressure is most critical in terms of phase separation and vent fluids are particularly sensitive to small changes in pressure when close to the critical point. It is in this region, close to the critical point, when fluids are very expanded (i.e., at very low density) that small changes in pressure can cause significant changes in vent-fluid composition. 

DouviUe E., Charlou J. L., OeUcers E. H., Bienvenu P., Colon C. E. J., Donval J. P., Fouquet Y., Prieur D., and Appriou P. (2002) The rainbow vent fluids (36 degrees 14 N, MAR) the influence of ultramafic rocks and phase separation on trace metal content in Mid-Atlantic Ridge hydrothermal fluids. Chem. Geol. 184, 37-48. 

Significant hydrothermal sites are known from a number of on- and off-axis seamounts. These include the Axial Volcano site on the JFR, a large sulfide deposit on a near-axis volcano at 13°N EPR, Loihi seamount in the Hawaiian-Emperor chain, the Lucky Strike hot-spot-related seamount site on the MAR, and a number of other localities. Axial Volcano and Lucky Strike have been studied most thoroughly, and have high-temperature hydrothermal systems. The Ashes vent field on the summit of Axial Volcano was the first to show effects of boiling at the reduced pressures encountered on the seamount relative to a normal ridge crest (Massoth et al. 1989). Many ridge-crest vent fields have been discovered in the last decade that show the effects of phase-separation into low-salinity vapor and more saline fluid (Butterfield 2000). 

The salinities of vent fluids have been shown to range from values of about 30% (176 mM/kg Von Damm and Bischoff 1987) to 200% (1090 mM/kg Massoth et al. 1989) seawater values (546 mM/kg). These variations are important because Cl is the major complexing anion in the hydrothermal fluids. The observed salinities cannot be accounted for by hydration of the oceanic crust (Cathles 1983) or by precipitation and dissolution of Cl-bearing mineral phases (Edmond et al. 1979b Seyfried et al. 1986) but are interpreted to be a result of phase separation at the top of the magma chamber followed by mixing of the brines and more dilute... 

Compositions of hydrothermal fluids not only vary widely but also are almost always very different from those of sea water (Table 1). While some of the low temperature diffusely venting fluids may be close to sea water in their major element compositions, they will often have very different compositions of dissolved gases (e.g., H2S, CO2, CH4, H2, and He) and will usually be highly enriched in iron and manganese compared to local ambient sea water. Compared to sea water, hydrothermal fluids have lost essentially all of their magnesium and sulfate, and are highly enriched in H2S, CH4, H2, He, Si, Li, Fe, and Mn. As hydrothermal fluids are very acid, they also have no alkalinity, and with the loss of sulfate, chloride becomes the major, and almost only anion (bromide is present in much lower concentrations). The behavior of the cations is more variable. As the amount of chloride present is a result of the phase separation history of the fluids, and the fluids must maintain electroneutrality, to determine whether a particular cation has been added to or removed from the fluid. 

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