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level magma chamber 33. This drives the hydrothermal convection system and gives rise
to black smokers at the seafloor. High-resolution seismic reflection studies have indicated
that some of these magma reservoirs may occur only 1.5-3.5 km below the seafloor 34,35.
The crustal residence time of seawater in the convection system has been constrained to
be 3 years or less 36. Data from water/rock interaction experiments indicate that, with
increasing temperatures, the Mg 2+ dissolved in seawater (about 1,280 ppm) combines
with OH-groups (which originate from the dissociation of seawater at higher
temperatures) to form Mg(OH)2, which is incorporated in secondary minerals such as
smectite (200°C) 37,38,39,40. The removal of OH-groups creates an
excess of H + ions, which is the principal acid-generating reaction responsible for the drop
in pH from seawater values (pH 7.8 at 2°C) to values as low as pH 2 41. Exchange of H + for
Ca 2+ and K + in the rock releases these elements into the hydrothermal fluid.
The leaching of Ca 2+ balances the continuous removal of Mg 2+ from seawater.
Endmember hydrothermal fluids are defined as presumed deep-seated high-temperature
fluids computed by extrapolating compositions and physical parameters back to Mg=0
on the assumption of quantitative removal of Mg. At high temperatures, however, the
formation of epidote (Ca fixation) also results in an excess of H + which further contributes
to the acidity of the hydrothermal fluid. These reactions take place at water/rock ratios of
less than five and commonly close to one 42. The oxygen which is present in the circulating
seawater in the form of sulphate is removed partly by precipitation of anhydrite and
partly through conversion of igneous pyrrhotite to secondary pyrite and the oxidation of
Fe 2+ to Fe 3+ forming Fe-oxyhydroxides and secondary magnetite in the basalt 40. Partial
reduction of seawater SO4 2- contributes to the formation of H2S, but most of the reduced S
in the fluid is derived from the rock itself.
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