Seawater mixing with hydrothermal fluid

Figure 12. Depth profile of Li isotopic composition (a) and concentration (b) in drilled oceanic crust at ODP Sites 504B (open symbols) and 896A (filled symbols) off Costa Rica (Chan et al. 2002a). The transition zone exhibits mixing between hydrothermal fluids and seawater. Average oxygen isotopic (5 0) composition of bulk samples decreases with depth upper volcanic zone = +7.8, lower volcanic zone = +6.4, transition zone = +5.4, sheeted dikes = +4.3. However, despite many sheeted dike samples having 5 Li less than unaltered MORB, there is no simple relationship between concentration and Li isotopes.

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). 

The behavior of silica and barite precipitation from the hydrothermal solution which mixes with cold seawater above and below the seafloor based on the thermochemical equilibrium model and coupled fluid flow-precipitation kinetics model is described below. 

Isotopic compositions of minerals and fluid inclusions can be used to estimate those of Kuroko ore fluids. Estimated isotopic compositions of Kuroko ore fluids are given in Table 1.10. All these data indicate that the isotopic compositions lie between seawater value and igneous value. For instance, Sr/ Sr of ore fluids responsible for barite and anhydrite precipitations is 0.7069-0.7087, and 0.7082-0.7087, respectively which are between present-day. seawater value (0.7091) and igneous value (0.704-0.705). From these data, Shikazono et al. (1983), Farrell and Holland (1983) and Kusakabe and Chiba (1983) thought that barite and anhydrite precipitated by the mixing of hydrothermal solution with low Sr/ Sr and seawater with high Sr/ Sr. 

The chemical processes occurring within a black smoker are certain to be complex because the hot, reducing hydrothermal fluid mixes quickly with cool, oxidizing seawater, allowing the mixture little chance to approach equilibrium. Despite this obstacle, or perhaps because of it, we bravely attempt to construct a chemical model of the mixing process. Table 22.3 shows chemical analyses of fluid from the NGS hot spring, a black smoker along the East Pacific Rise near 21 °N, as well as ambient seawater from the area. 

To model the mixing of the hydrothermal fluid with seawater, we begin by equilibrating seawater at 4 °C, picking up this fluid as a reactant, and then reacting it into the hot hydrothermal fluid. In react, we start by suppressing several minerals ... 

As fluid from the hydrothermal vent mixes with seawater, chemolithotrophic microbes by this process harvest energy from the chemical disequilibrium among redox reactions, forming the base of the ecosystem s food chain. Microbes can... 

Table 22.5. Electron donating and accepting redox couples and limiting reactant species for various anaerobic and aerobic microbial metabolisms favored in fluid from a subsea hydrothermal vent, as it mixes with seawater...

Fig. 22.7. Thermodynamic driving forces for various anaerobic (top) and aerobic (bottom) microbial metabolisms during mixing of a subsea hydrothermal fluid with seawater, as a function of temperature. Since the driving force is the negative free energy change of reaction, metabolisms with positive drives are favored thermodynamically those with negative drives cannot proceed. The drive for sulfide oxidation is the mirror image of that for hydrogentrophic sulfate reduction, since in the calculation 02(aq) and H2(aq) are in equilibrium.

The energetics depicted in this way are in accord with the microbial ecology observed at deep sea hydrothermal systems (e.g., Kelley el al., 2002 Huber el al., 2003 Schrenk et al, 2003). Sediments and black smoker walls invaded by hydrothermal fluids there contain sparse microbial populations of mostly thermophilic methanogens and sulfate reducers. Abundant populations of mesophilic aerobes dominated by sulfide reducers, in contrast, are found in the open ocean where hydrothermal fluids mix freely with seawater. 

In the case of iron and manganese, most of these metals are removed from the hydrothermal fluids and converted to particulate form close to their point of entry. Some of these removals are in the form of sulfides, which fc>rm as the fluids emerge into the deep sea. The rest occurs as the fluids mix with cold, oxic, alkaline seawater, which promotes the oxidation of reduced metals. Thus, Fe (aq) and Mn (aq) are transformed into insoluble iron and manganese oxides, forming colloids and particles, the latter of which eventually settle onto the sediments. As described in the next chapter, at least some of these oxidation reactions are biologically mediated. Some of... 

The formation of these anhydrite walls prevents the hydrothermal fluids flowing through the chimney from mixing with seawater and provides a framework to enable precipitation of sulfide minerals. In some cases, the discharge of fluids is so rapid that the sulfide precipitates are emitted as clouds of black particles moving at a speed of... 

Hydrothermal venting injects fluids into seawater as buoyant, jetlike pliunes. These turbulent flows mix rapidly with seawater becoming diluted by factors of lO" to 10. This mixing eventually makes the plumes neutrally buoyant, after which they are transported laterally through the ocean basins as part of the intermediate and deepwater currents. Hydrothermal plumes have the potential to greatly affect seawater chemistry. From global estimates of hydrothermal fluid emissions and dilution ratios, a volume of seawater equivalent to the entire ocean can be entrained in the hydrothermal plumes every few thousand years. 

Thus, turbulent mixing of the hydrothermal fluids with oxic seawater is important to these microbes. 

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).

The minerals produced in these metalliferous deposits reflect the mixing of the two end-member solutions (hydrothermal solution and seawater). The mixing process involves cooling of the hydrothermal fluids and heating of seawater, changes in pH and oxidation state, reaction with previously formed precipitates or sedimentary components, and nonequilibrium kinetic effects... 

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