Hydrothermal vent fluids chemical composition

The Chemical Composition of Hydrothermal Vent Fluids and Precipitates... 

Table 1 Range of physical parameters and chemical compositions for hydrothermal vent fluids ...

Origin of ore fluids is constrained by (1) chemical compositions of ore fluids estimated by thermochemical calculations (section 1.3.2) and by fluid inclusion analyses, (2) isotopic compositions of ore fluids estimated by the analyses of minerals and fluid inclusions (section 1.3.3), (3) seawater-rock interaction experiments, (4) computer calculations on the seawater-rock interaction, and (5) comparison of chemical features of Kuroko ore fluids with those of present-day hydrothermal solutions venting from seafloor (section 2.3). 

The above argument on the calculation of chemical composition of ore fluids, seawater-rock interaction experiments, and isotopic compositions of ore fluids clearly demonstrates that Kuroko ore fluids were generated by seawater-rock interaction at elevated temperatures. The chemistry of present-day hydrothermal solution venting from back-arc basins and midoceanic ridges (sections 2.3 and 2.4) also support this view. 

Hydrothermal vents are another source of water entering the ocean. These vents are submarine hot-water geysers that are part of seafloor spreading centers. The hydrothermal fluids contain some major ions, such as magnesium and sulfete, in significantly different ratios than foimd in seawater. The importance of hydrothermal venting in determining the chemical composition of seawater is described in Chapters 19 and 21. 

Chemical composition of hydrothermal fluids collected from the Galapagos vents (a) Dissolved silicon concentrations versus temperature and (b) magnesium versus dissolved silicon concentrations. Source-. From Edmond, J. M., et al. (1979). Earth and Planetary Sciences Letters 46, 1-18. 

The rest of the chapter is organized as follows. In Section 6.07.2 we discuss the chemical composition of hydrothermal fluids, why they are important, what factors control their compositions, and how these compositions vary, both in space, from one location to another, and in time. Next (Section 6.07.3) we identify that the fluxes established thus far represent gross fluxes into and out of the ocean crust associated with high-temperature venting. We then examine the other source and sink terms associated with hydrothermal circulation, including alteration of the oceanic crust, formation of hydrothermal mineral deposits, interactions/uptake within hydrothermal plumes and settling into deep-sea sediments. Each of these fates for hydrothermal material is then considered in more detail. Section 6.07.4 provides a detailed discussion of near-vent deposits, including the formation of polymetallic sulfides and... 

The compositions of vent fluids found on the global MOR system are of interest for several reasons how and why those compositions vary has important implications. The overarching question, as mentioned in Section 6.07.1.3, is to determine how the fluids emitted from these systems influence and control ocean chemistry, on both short and long timescales. This question is very difhcult to address in a quantitative manner because, in addition to all the heat flux and related water flux uncertainties discussed in Section 6.07.1, it also requires an understanding of the range of chemical variation in these systems and an understanding of the mechanisms and variables that control vent-fluid chemistries and temperatures. Essentially every hydrothermal vent that is discovered has a different composition (e.g.. Von Damm, 1995) and we now know that these compositions often vary profoundly on short... 

To advance our understanding of the chemical variability of vent fluids, it will be important to continue to find new sites that may be at evolutionary stages not previously observed. Equally, it wUl be important to continue studies of temporal variabihty at known sites both those that have varied in the past and those that have appeared to be stable over the time intervals at which they have been sampled. Understanding the mechanisms and physical processes that control these vent-fluid compositions are key to calculating hydrothermal fluxes. 

Figure 13 Schematic representation of an MOR hydrothermal system and its effects on the overlying water column. Circulation of seawater occurs within the oceanic crust, and so far three types of fluids have been identified and are illustrated here high-temperature vent fluids that have likely reacted at >400 °C high-temperature fluids that have then mixed with seawater close to the seafloor fluids that have reacted at intermediate temperatures, perhaps 150 °C. When the fluids exit the seafloor, either as diffuse flow (where animal communities may live) or as black smokers, the water they emit rises and the hydrothermal plume then spreads out at its appropriate density level. Within the plume, sorption of aqueous oxyanions may occur onto the vent-derived particles (e.g., phosphate, vanadium, arsenic) making the plumes a sink for these elements biogeochemical transformations also occur. These particles eventually rain-out, forming metalliferous sediments on the seafloor. While hydrothermal circulation is known to occur far out onto the flanks of the ridges, little is known about the depth to which it extends or its overall chemical composition because few sites of active ridge-flank venting have yet been identified and sampled (Von Damm, unpublished).

Bowers TS (1989) Stable isotope signatures of water-rock interaction in mid-ocean ridge hydrothermal systems sulfur, oxygen, and hydrogen. J Geophys Res 94(B5) 5775-5786 Bowers TS, Campbell AC, Measures Cl, Spivack AJ, Khadem M, Edmond JM (1988) Chemical controls on the composition of vent fluids at 13°N-11°N and 21°N, East Pacific Rise. J Geophys Res 93 4522-... 

Adiabatic-mixing pathways, where seawater (2°C) mixes into hydrothermal fluid (350°C), have been successfully used to model formation of sulfide minerals associated with venting hydrothermal solutions at mid-ocean ridges (9). Sulfate reduction can be quantitatively and isotopically important in such reactions (. Combinations of three types of isotopic path constraints discussed above have been examined, using the mixing reaction pathways calculated by Janecky and Seyfried ( ) for chemical equilibrium and initial sulfur isotopic compositions of 1 per mil for the hydrothermal solution and 21 per mil for seawater (Figure 1). 

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