Seawater equilibrium models

Several workers have intended to estimate the chemical compositions of Kuroko ore fluids based on the chemical equilibrium model (Sato, 1973 Kajiwara, 1973 Ichikuni, 1975 Shikazono, 1976 Ohmoto et al., 1983) and computer simulation of the changes in mineralogy and chemical composition of hydrothermal solution during seawater-rock interaction. Although the calculated results (Tables 1.5 and 1.6) are different, they all show that the Kuroko ore fluids have the chemical features (1 )-(4) mentioned above. 

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. 

Seawater has high concentrations of solutes and, hence, does not exhibit ideal solution behavior. Most of this nonideal behavior is a consequence of the major and minor ions in seawater exerting forces on each other, on water, and on the reactants and products in the chemical reaction of interest. Since most of the nonideal behavior is caused by electrostatic interactions, it is largely a function of the total charge concentration, or ionic strength of the solution. Thus, the effect of nonideal behavior can be accoimted for in the equilibrium model by adding terms that reflect the ionic strength of seawater as described later. 

Although the details of the equilibrium model are still uncertain, the general trends are likely reliable. As shown in Figme 5.16, most of the Fe(III) in seawater is predicted to be in the form of the FeL complex. The equilibrium model also predicts that this degree of complexation should enhance iron solubility such that 10 to 50% of the iron delivered to the ocean as dust will eventually become dissolved if equilibrimn is attained. If this model is a reasonable representation for iron speciation in seawater, uptake of [Fe(III)]jQjgj by phytoplankton should induce a spontaneous dissolution of additional particulate iron so as to drive the dissolved iron concentrations back toward their equilibrium values. 

Before discussing the chemical dynamics of estuarine systems it is important to briefly review some of the basic principles of thermodynamic or equilibrium models and kinetics that are relevant to upcoming discussions in aquatic chemistry. Similarly, the fundamental properties of freshwater and seawater are discussed because of the importance of salinity gradients and their effects on estuarine chemistry. 

In this sense we have already described in Chapters 4 and 7 the equilibrium of the CaC03(s)-H20-C02 system. Specifically, we have used equilibrium models to characterize the concentrations of the carbonate species as a function of pco2 and of pH. We have already shown (Example 7.8) that CaCOj in surface seawater is oversaturated and we calculated in Example 4.10 how the composition of seawater changes as a result of increasing the CO2 concentration in the atmosphere. 

The thermodynamic data compilations of Sillen and Martell catalyzed rapid advances in equilibrium models of seawater speciation. These works were followed by additional compilations that were critically important to modern sea-water speciation assessments. In view of these developments, and additional extensive experimental analyses appropriate to seawater. Principal Species assessments ten to fifteen years after the pioneering work of Sillen demonstrated a much improved awareness of the importance of hydrolysis in elemental speciation. 

Chemical changes during the hydrothermal solution-seawater mixing associated with sekko ore in Kuroko deposits are considered below based on partial equilibrium model. 

Chimney (Fig. 4.2) is mainly composed of sulfides, sulfates and silica which precipitated by the mixing of hydrothermal solutirai and cold ambient seawater. Formation mechanism of chimney has been explained in terms of several models. Partial equilibrium model was applied to the precipitatirais of minerals during the mixing of hydrothermal solution and cold seawater (Ohmoto et al. 1983). However, this model cannot explain the following features of chimney and submarine hydro-thermal ore deposits. 

The ratio of sulfide content/sulfate content of ore cannot be explained by equilibrium model of seawater-hydrothermal solution mixing. 

The failure to identify the necessary authigenic silicate phases in sufficient quantities in marine sediments has led oceanographers to consider different approaches. The current models for seawater composition emphasize the dominant role played by the balance between the various inputs and outputs from the ocean. Mass balance calculations have become more important than solubility relationships in explaining oceanic chemistry. The difference between the equilibrium and mass balance points of view is not just a matter of mathematical and chemical formalism. In the equilibrium case, one would expect a very constant composition of the ocean and its sediments over geological time. In the other case, historical variations in the rates of input and removal should be reflected by changes in ocean composition and may be preserved in the sedimentary record. Models that emphasize the role of kinetic and material balance considerations are called kinetic models of seawater. This reasoning was pulled together by Broecker (1971) in a paper called "A kinetic model for the chemical composition of sea water."... 

In a second example of a flow-through path, we model the evaporation of seawater (Fig. 2.6). The equilibrium system in this case is a unit mass of seawater. Water is titrated out of the system over the course of the path, concentrating the seawater and causing minerals to precipitate. The minerals sink to the sea floor as they... 

For a first chemical model, we calculate the distribution of species in surface seawater, a problem first undertaken by Garrels and Thompson (1962 see also Thompson, 1992). We base our calculation on the major element composition of seawater (Table 6.2), as determined by chemical analysis. To set pH, we assume equilibrium with CO2 in the atmosphere (Table 6.3). Since the program will determine the HCOJ and water activities, setting the CO2 fugacity (about equal to partial pressure) fixes pH according to the reaction,... 

Fig. 6.1. pH of surface seawater from the western Pacific Ocean (Skirrow, 1965), as measured in situ during oceanographic cruises (various symbols). Line shows pH predicted by the model for seawater in equilibrium with atmospheric CO2 at a fugacity of 10-3-5. Dashed lines show pH values that result from assuming larger fugacities of 10-33 and... 

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. 

Fig. 22.6. Redox potentials (mV) of various half-cell reactions during mixing of fluid from a subsea hydrothermal vent with seawater, as a function of the temperature of the mixture. Since the model is calculated assuming 02(aq) and H2(aq) remain in equilibrium, the potential for electron acceptance by dioxygen is the same as that for donation by dihydrogen. Dotted line shows currently recognized upper temperature limit (121 °C) for microbial life in hydrothermal systems. A redox reaction is favored thermodynamically when the redox potential for the electron-donating half-cell reaction falls below that of the accepting half-reaction.

To reproduce their results, we trace the reaction path taken by seawater at 25 °C as it evaporates to desiccation. Our calculations follow those of Harvie et al. (1980) and Eugster et al. (1980), except that we employ the more recent Harvie-Mpller-Weare activity model (Harvie et al., 1984), which accounts for bicarbonate. We include an HCO3 component in our calculations, assuming that the fluid as it evaporates remains in equilibrium with the CO2 in the atmosphere. 

In a first calculation, we specify that the fluid maintains equilibrium with whatever minerals precipitate. Minerals that form, therefore, can redissolve into the brine as evaporation proceeds. In react, we set the Harvie-Moilcr-Weare model and specify that our initial system contains seawater... 

Fig. 24.7. Volumes of minerals precipitated during a reaction model simulating the evaporation of seawater as an equilibrium system at 25 °C, calculated using the Harvie-Mpller-Weare activity model. Abbreviations Ep = Epsomite, Hx = Hexahydrite.

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