Seawater-basalt interaction

Fig. 2.43. Graphical illustration of sulfur isotope values of HiS (left axis and. solid line) produced during basalt-seawater interaction at various water/rock ratios. Calculations assume that seawater sulfate is mostly removed as anhydrite, that any residual sulfate is reduced by iron oxidation in reacting basalt, and that there is quantitative leaching of basaltic sulfide and homogeneous mixing of both sulfides. Dashed line...

Thompson G. (1983a) Basalt-seawater interaction. In Hydrothermal Processes at Seafloor Spreading Centers (eds. P.A. Rona, K. Bostrom, L. Laubjer and K.L. Smith), pp. 225-278. Plenum Press, New York. 

The chemistry of hydrothermal fluids indicates that basalt-seawater interactions are a source of some elements that have been stripped from ocean crust and injected into seawater. Data from hydrothermal fluids show that both Ca2+ and dissolved silica are concentrated in the hydrothermal waters compared with seawater (Table 6.6). Calcium is probably released from calcium feldspars (anorthite) as they are converted to albite by Na+ uptake, a process called albitization. Silica can be leached from any decomposing silicate in the basalt, including the glassy matrix of the rock. Globally, basalt-seawater interaction seems to provide an additional 3 5 % to the river flux of Ca2+ and silica to the oceans. 

Field observation and analytical studies on altered rocks are consistent with those of computer simulations on basalt-seawater interaction at elevated temperatures (Mottl 1983 Shikazono et al. 1995). 

It has been found that hydrothermal solution issues from the mid-oceanic ridges and hydrothermal ore deposits are forming on the seafloor and under the subseafloor envirornnent Initial chemical composition of hydrothermal solution prior to the mixing with cold seawater is estimated by extrapolating the analytical data to zero concentration of Mg (Fig. 2.13). Experimental data and thermochemical calculations on basalt-seawater interaction at elevated temperatures indicate that the hydrothermal solution after the interaction contains almost no Mg. The precipitated amotmts of minerals during the mixing can be calculated (Fig. 2.14). At early and... 

Some elements are leached from basalt to interstitial water and seawater through basalt-seawater interaction. But some elements (Mn, Si, Ca) precipitate as veins in the weathered basalt and do not remove to seawater. 

The studies on the hydrothermal systems at midoceanic ridges during the last three decades clearly revealed that the seawater-basalt interaction at elevated temperatmes (ca. 100-400°C) affects the present-day seawater chemistry (Wolery and Sleep, 1976 Edmond et al., 1979 Humphris and Thompson, 1978). For example, a large quantity of Mg in seawater is taken from seawater interacting with midoceanic ridge basalt, whereas Ca, K, Rb, Li, Ba and Si are leached from basalt and are removed to seawater (Edmond et al., 1979 Von Dammet al., 1985a,b). 

Shanks W. C., Bischoff J. L., and Rosenbauer R. J. (1981) Seawater sulfate reduction and sulfur isotope fractionation in basaltic systems interaction of seawater with fayaUte and magnetite at 200-300°C. Geochim. Cosmochim. Acta 45, 1977-1995. 

The relatively constant chemical composition of hydrothermal solution can be also explained in terms of model calculation on flow system. For example, Kawahata (1989) calculated Sr/ Sr of hydrothermal solution passing through cells of hydrothermal system (Fig. 4.5). He divided the hydrothermal system into 100 and 50 cells. Initial seawater interacts with basalt and the chemical equilibrium between altered basalt and hydrothermal solution attains in a first cell. Then, the solution moves to the second cell and interacts with basalt and the equilibrium between basalt and solution attains. The stepwise interaction occurs in the cells with... 

Sverjensky (1984) calculated the dependency of Eu +/Eu + in hydrothermal solution on /oj (oxygen fugacity), pH and temperature. According to his calculations and assuming temperature, pH and /oj for epidote-stage alteration of basalt and Kuroko ores (Shikazono, 1976), divalent Eu is considered to be dominant in the rocks and hydrothermal solution. Thus, it is reasonable to consider that Eu in the rocks was removed to hydrothermal solution under the relatively reduced condition more easily than the other REE which are all tiivalent state in hydrothermal solution. Thus, it is hkely that Eu is enriched in epidote-rich altered volcanic rocks. Probably Eu was taken up by the rocks from Eu-enriched hydrothermal solution which was generated by seawater-volcanic rock interaction at relatively low water/rock ratio. 

A negative correlation between Mg content and Ca content of hydrothermally altered basalt and dacite from the Kuroko mine area exists. This correlation indicates that Ca in the rocks is removed to fluid by the exchange of Mg in seawater. Eu may behave in the manner similar to Ca during seawater-volcanic rock interaction because of the similarity of their ionic radii. 

During the last two decades, many experimental studies on the seawater-rock interaction at elevated temperatures (100-400°C) have been conducted. Particularly, detailed seawater-basalt interaction experiments have been done. Several experimental studies on seawater-rhyolite interaction and seawater-sedimentary rock interaction are also available (Bischoff et al., 1981). Examples of chemical compositions of modified seawater experimentally interacted with various kinds of rocks are shown in Table 1.9. 

Wolery (1978) and Reed (1982, 1983) have indicated based on a computer calculation of the change in chemistry of aqueous solution and mineralogy during seawater-rock interactions that epidote is formed under the low water/rock ratio less than ca. 50 by mass. Humphris and Thompson (1978), Stakes and O Nell (1982) and Mottl (1983) have also suggested on the basis of their chemical and oxygen isotopic data of the altered ridge basalts that epidote is formed by seawater-basalt interaction at elevated temperatures (ca. 200-350°C) under the rock-dominated conditions. If epidote can be formed preferentially under such low water/rock ratio, the composition of epidote should be influenced by compositions of the original fresh rocks. 

Bischoff, J.L. and Dickson, F.W. (1975) Seawater-basalt interaction at 200°C and 500 bars Implications for origin of seafloor heavy-metal deposits and regulation of seawater chemistry. Earth Planet. Sci. Lett., 25, 385-397. 

Gena et al. (2001) reported advanced argillic alteration of basaltic andesite from the Desmos caldera, Manus back-arc basin which was caused by interaction of hot acid hydrothermal fluid originated from a mixing of magmatic gas and seawater. It is noteworthy that the acid alteration is found in back-arc basins (Manus, Kuroko area) but not in midoceanic ridges. 

Kawahata and Shikazono (1988) summarized S S of sulfides from midoceanic ridge deposits and hydrothermally altered rocks (Fig. 2.42). They calculated the variations in 5 " S of H2S and sulfur content of hydrothermally altered basalt as a function of water/rock ratio (in wt. ratio) due to seawater-basalt interaction at hydrothermal condition (Fig. 2.43) and showed that these variations can be explained by water/rock ratio. The geologic environments such as country and host rocks may affect S S variation of sulfides. For example, it is cited that a significant component of the sulfide sulfur could... 

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 ... 

Kaiho and Saito (1994) estimated 20 x 10 km /m.y. and 2x 10 km /m.y. for present-day midoceanic ridge crustal production rate and back-arc basin crustal production rate, respectively. If their estimates are correct. Mg removal to midoceanic ridge basalt during early-middle Miocene age is estimated to be 2.6 1 x 10 g/year. Although estimates of annual Mg removal by interaction of circulating seawater with midoceanic ridge basalt are uncertain, it seems likely that Mg removal by seawater-volcanic rock interaction at back-arc basins corresponds to that of Mg removal at midoceanic ridge axis. 

Hajash, A. and Chandler, G.W. (1981) An experimental investigation of high-temperature interactions between seawater and rhyolite, andesite, basalt and peridotite. Contrib. Mineral. Petrol., 78, 240-254. 

Seyfried W.E., Jr. (1976) Seawater-basalt interaction from 25-300 and 1-500 bars Implications for the origin of submarine metal-bearing hydrothermal solutions and regulation of ocean chemistry. Ph.D. dissertation, Univ. Southern California. 

Alt J. C., Honnorez J., Laverne C., and Emmermann R. (1986) Hydrothermal Alteration of a 1 km section through the upper oceanic crust DSDP hole 504B the mineralogy, chemistry and evolution of seawater-basalt interactions. J. Geophys. Res. 91, 10309-10335. 

Seylfied W. E. and Bischoff J. L. (1981) Experimental seawater—basalt interaction at 300 °C, 500 bars chemical exchange, secondary mineral formation and imphcations for the transport of heavy metals. Geochim. Cosmochim. Acta 45, 135-147. 

---