Hydrothermal circulation

The other most likely explanation for the mass imbalance of major ions in Fig. 2.4 is that there are substantial fluxes of seawater into hydrothermal areas where the chemistries of dissolved constituents are amended by contact with basalt at high temperatures and pressures. This phenomenon is described here by first reviewing the most important chemical changes in hydrothermal waters, and then discussing what these changes mean in terms of fluxes to and from the ocean. We shall see that the chemical aspects of this question are pretty well understood. However, as is usual in marine chemistry, estimation of fluxes has proven to be more difficult. 

Schematic map of the global ridge crest showing active hydrothermal vents that have been discovered (dark circles) and those that are known to exist because of characteristic signals in the overlying water column (dark triangles). From German and Von Damm (2003). 

While Ca is removed from solution by precipitation of anhydrite (Eq. (2.2)), the subsequent reactions above (Eqs. (2.3) and (2.4)) and others alter the Ca/Mg quotient of these silicates to become more Mg-rich at the expense of Ca, resulting in a net increase in Ca + in solution. The release of hydrogen ion in the above reactions titrates 

Schematic diagram of hydrothermal convection. The left half is a cross section of a spreading center with a shallow heat source indicating the water flow. The right half is a schematic description of the relative locations of water-rock reactions (W-R Rxn) and phase separation along the flow of hydrothermal circulation. Adapted from German and Von Damm (2003). 

HCO3 and CO3 to form CO2 in hydrothermal solutions, resulting in a much lower pH than in normal seawater. H4Si04 is released to solution hy high-temperatme silicate reactions, and SO4 is reduced to hy oxidation of Fe + to Fe within iron-containing silicates. 

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Croup lb (Mg, SO4, probably K). The key property of this group is removal during seafloor hydrothermal circulation. This fits in with Broecker s original group I, tectonically controlled elements, but enlarged by two (Mg, K). 

There is some debate about what controls the magnesium concentration in seawater. The main input is rivers. The main removal is by hydrothermal processes (the concentration of Mg in hot vent solutions is essentially zero). First, calculate the residence time of water in the ocean due to (1) river input and (2) hydro-thermal circulation. Second, calculate the residence time of magnesium in seawater with respect to these two processes. Third, draw a sketch to show this box model calculation schematically. You can assume that uncertainties in river input and hydrothermal circulation are 5% and 10%, respectively. What does this tell you about controls on the magnesium concentration Do these calculations support the input/removal balance proposed above Do any questions come to mind Volume of ocean = 1.4 x 10 L River input = 3.2 x lO L/yr Hydrothermal circulation = 1.0 x 10 L/yr Mg concentration in river water = 1.7 X 10 M Mg concentration in seawater = 0.053 M. 

A solution, still controversial, has been recently proposed. This is the loss of sulfate from seawater during hydrothermal circulation through mid-ocean ridges (Edmond et al., 1979). The flow of water through these systems is estimated to be about 1.4 x 10 L/yr, about 0.4% of the flow of rivers. However, sulfate is quantitatively removed, yielding a flux of 125 Tg S/yr, capable of balancing the river flux. The controversy is whether the chemistry involved in removing sulfate is the formation of... 

Uyeda, S. (1983) Hydrothermal circulation in the Mariana trough, Kuroko and the mode of subduction. Mining Geology Special Issue, 11, 37-53 (in Japanese with English abst.). 

Delaney, M. and Boyle, E.A. (1986) Lithium in foraminiferal shells Implications for high-temperature hydrothermal circulation fluxes and oceanic crustal generation rates. Earth Planet. Sci. Lett., 80, 91-105. 

Wolery, T.J. and Sleep, N.H. (1976) Hydrothermal circulation and geochemical flux at midocean ridges. J. Geol., 84, 249-275. 

Alteration is always a cause for concern in geochemical investigations and the best approach will always be to avoid samples with visual or chemical evidence for alteration. The differential fluid mobility of U, Th, Pa and Ra undoubtedly provides the potential for weathering or hydrothermal circulation to disturb the U-series signatures of arc lavas. In a study of lavas from Mt. Pelee on Martinique, Villemant et al. (1996) found that domeforming lavas were in U-Th equilibrium whereas plinian deposits from the same eruptions had small U-excesses which they interpreted to reflect hydrothermal alteration. However, whilst the addition of U could be due to hydrothermal alteration, the plinian deposits were also displaced to lower °Th/ Th ratios which cannot. Instead, the two rock types may just be from separate magma batches. 

The ridge crest is a dynamic setting in which volcanic activity creates new vents while old ones die. On fast-spreading centers, hydrothermal circulation supports focused discharges through chimneys that have an average life span of a few decades. 

Off-axis hydrothermal circulation is responsible for 70% of the convectively driven heat flow from hydrothermal systems. This circulation is thought to occur on the flanks of the mid-ocean ridges and rises at temperatures on the order of 20 to 54°C. The... 

The footwall units below the ore horizon include the Watson Lake Rhyolite and the Bell River Complex intrusion. Both have U-Pb dates of 2724 Ma (Mortensen 1993), and the Bell River Complex is considered to the heat source which drove the hydrothermal circulation that created the ore deposits. 

An extremely fine localization of primordial 3He injection, on a 10-m scale, has also been observed. It has been suggested that lower-than-expected conductive heat flow at oceanic ridges could be due to significant heat transport by hydrothermal circulation (e.g., Talwani, Windisch Langseth, 1971), in which recently emplaced hot rock drives convection of local sea water. On the basis of temperature-salinity relationships, Weiss et al. (1977) made the first identification of hydrothermal circulation in the open ocean, observing several plumes (temperature differential <0.2°C)... 

More detailed examination and sampling allows association of hydrothermal circulation with specific vent fields. In such waters samples in the Galapagos Rift by the Alvin deep submersible, Jenkins, Edmond, and Corliss (1978) report juvenile He enrichments which dwarf the normal saturation concentrations by factors up to 11 for 4He and 60 for 3He (Figure 4.6). A particularly significant feature of this report is that added He occurs roughly in proportion to added heat AT up to 12°C in sampled water), corresponding to about 7.6 x 10 xcal/atom of 3He (Figure 4.7). Jenkins et al. note that if this value is representative, hydrothermal circulation may indeed account for the depression of conductive heat flow relative to models for total heat flux. As... 

If this continued over geologic time, the ocean would consist of nothing but sulphate. You must have some sink in the ocean. There is a possible sink that is the hydrothermal circulation in the bottom of the ocean. The entire volume of ocean water apparently passes through the top one or two kilometers of the crust every ten million years, and the sulphate is probably taken out at that time. 

Ogata Y, Imai E, Honda H, Hatori K, Matsuno K. Hydrothermal circulation of seawater through hot vents and contribution of interface chemistry to prebiotic synthesis. Orig. Life Evol. Biosph. 2000 30 527-537. 

Phipps Morgan J. and Chen Y. J. (1993) The genesis of oceanic crust magma injection, hydrothermal circulation, and cmstal flow. J. Geophy. Res. 98, 6283-6297. 

Alternatively, the duration of hydrothermal convection in the oceanic crust can be estimated by mapping the distribution of nonlinear temperature profiles taken during heat-flow measurements as a function of oceanic-crustal age. Purely conductive heat loss (i.e., no hydrothermal circulation) results in linear temperature profiles in sediments, while convective heat loss results in concave or convex profiles, depending on whether the water penetrates into or comes out of the sediments. 

Langseth et al., 1988). Crust of site 332B is even closer to the mid-ocean ridge and is expected to be experiencing active hydrothermal circulation. Unfortunately, sites 332B and 504B have very low recovery rates, and for this reason, some effort is needed to determine the true differences in time-variant alteration behavior. 

Gregory R. T. and Taylor H. P., Jr. (1981) An oxygen isotope profile in a section of Cretaceous oceanic crast, Samail ophiolite, Oman evidence for buffering of the oceans by deep (> 5 km) seawater-hydrothermal circulation at midocean ridges. J. Geophys. Res. 86, 2737-2755. 

LangsethM. G., Mottl M. J., Hobart M. A., and Fisher A. (1988) The distribution of geothermal and geochemical gradients near site 501/504 implications for hydrothermal circulation in the oceanic emst. Ocean Drill. Initial Rep. Ill, 23 —32. 

Sleep N. H. (1991) Hydrothermal circulation, anhydrite precipitation, and thermal structure at ridge axes. J. Geophys. Res. 96, 2375-2387. 

Stein C. A., Stein S., and Pelayo A. (1995) Heat flow and hydrothermal circulation. In Seafloor Hydrothermal Processes, Geophysical Monograph (eds. S. E. Humphris, R. A. Zierenberg, L. S. Mullineaux, and R. E. Thomson). American Geophysical Union, Washington, DC, vol. 91, pp. 425-445. 

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