Seawater-basalt reaction

Reed, M.H. (1983) Seawater-basalt reaction and the origin of greenstones and related ore deposits. Econ. Geol, 7, 446-485. 

Redox Disequilibrium in the Aqueous Phase. Aqueous redox disequilibrium modeling is permitted in the 3245 version of EQ3NR, but not the corresponding version of EQ6. It will be permitted in the 3270 version of EQ6. This will allow, for example, calculation of seawater/basalt reaction models in which total sulfate is conserved instead of partially reduced to sulfide. 

A O and AD values of vent fluids (Table 1) can best be understood in terms of water-rock interaction within the ocean crust. Field studies, experimental studies, and isotopic exchange computations (Muehlenbachs 1972 Stakes and O Neil 1982 Bowers and Taylor 1985 Cole et al. 1987 Bowers 1989 Bohlke and Shanks 1994 Shanks et al. 1995) have clearly shown that both A 0 and AD increase due to water-rock interaction with igneous crust. Oxygen and hydrogen isotope values of end-member vent fluids (Fig. 6) follow a calculated seawater-basalt reaction vector (within AD error of 1.5%o ), due to fluid evolution to decreasing water/rock mass ratios (Shanks et al. 1995). 

Fig. 2.1 The change in alteration minerals formed during seawater-basalt reaction (Wolery 1978)...

The Mg content of hydrothermally altered volcanic rocks is reflected by the extent of seawater-volcanic rock interaction at elevated temperatures, because it has been experimentally and thermodynamically determined that nearly all of the Mg in seawater transfer to volcanic rocks, owing to the reaction of the cycled seawater with volcanic rocks at elevated temperatures (Bischoff and Dickson, 1975 Mottl and Holland, 1978 Wolery, 1979 Hajash and Chandler, 1981 Reed, 1983 Seyfried, 1987). It has been shown that the CaO content of hydrothermally altered midoceanic ridge basalt is inversely correlated with the MgO content with a slope of approximately — 1 on a molar basis (Mottl, 1983). This indicates that Ca of basalt is removed to seawater and Mg is taken up from seawater by the formation of chlorite and smectite during the seawater-basalt interaction. This type of reaction is simply written as ... 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

Figure 17. Phase diagram showing mineral reactions that control oxidation-reduction am) and total sulfur (flH2S ) conditions during seawater-basalt interaction in the deep subseafloor reaction zone. From Seyfried and Ding (1995) used with permission of the American Geophysical Union.

Figure 25. Geochemical reaction models for seawater-basalt and seawater-mud reactions at 220°C and 300°C. FromBohlke and Shanks (1994).

Wolery, T.J. (1978) Some chemical aspects of hydrothermal processes at midoceanic ridges — A theoretical study I, Basalt-seawater reaction and chemical cycling between the oceanic crust and the oceans. II, Calculation of chemical equilibrium between aqueous solutions and minerals. Ph.D. Thesis, Northwestern U. 

Grustal reservoirs are also variable in Gl-isotope compositions (Figs. 1-6) due to fractionation of the Gl-isotope compositions inherited from their mantle source through fluid-mineral reactions, incorporation of G1 derived from the oceans and fractionation within fluid reservoirs by diffusion (see below). For example, the oceanic crust is enriched in Gl (and pore fluids depleted in Gl) through reaction of seawater with basaltic crust derived from the depleted mantle (Fig. 1 Magenheim et al. 1995). Undoubtedly, future investigations of Gl-isotopes in whole rocks and mineral separates will address the Gl-isotope compositions of these reservoirs and their evolution. 

These generalizations regarding high-temperature fluid-rock reactions are based on field observations and laboratory simulations. Geochemists have attempted to run high-temperature reactions in the lab by reacting basalt with seawater at pressures up to 1000 bar and temperatures of 70 to 500°C at rock-to-water ratios from 1 to 62 for periods as long as 20 months. Their reaction products support the conclusions made from field observations. 

To begin the discussion, we will present briefly a view of the modern carbon cycle, with emphasis on processes, fluxes, reservoirs, and the "CO2 problem". In Chapter 4 we introduced this "problem" here it is developed further. We will then investigate the rock cycle and the sedimentary cycles of those elements most intimately involved with carbon. Weathering processes and source minerals, basalt-seawater reactions, and present-day sinks and oceanic balances of Ca, Mg, and C will be emphasized. The modern cycles of organic carbon, phosphorus, nitrogen, sulfur, and strontium are presented, and in Chapter 10 linked to those of Ca, Mg, and inorganic C. In conclusion in Chapter 10, aspects of the historical geochemistry of the carbon cycle are discussed, and tied to the evolution of Earth s surface environment. 

One set of schematic major equations describing the reactions between seawater and basalt is (Drever et al., 1987) ... 

Table 9.11. Fluxes involving submarine basalt-seawater reactions (units of 10 2 moles yl). (After Wolery and Sleep, 1988.)...

Figure 9.20. Schematic diagram of fluxes and processes evaluated for the global cycle of an element. Rj, Rp, Sp, Dp, H

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