The Chemical Composition of Hydrothermal Vent Fluids and Precipitates

5 The Chemical Composition of Hydrothermal Vent Fluids and Precipitates 

Since the discovery of high-temperature hydro-thermal vents at the East Pacific Rise 21°N in 1979, hydrothermal fluids have been sampled at numerous sites at mid-ocean ridges and back-arc spreading centers. As noted above, in most high-temperature vent fluids, both Mg and SO show a negative correlation with temperature, and an extrapolation to zero Mg and zero SO intersects the temperature axis at a point corresponding to the end-member temperature (Fig. 13.9). Controls on the major element compositions of these fluids 

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 

Concentrations of trace elements such as Ag, As, Cd, Co, Se, and Au have been measured in some vent fluids (Von Damm 1990 Campbell et al. 1988a,b Fouquet et al. 1993a Trefry et al. 1994  

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