Submarine hydrothermal systems are an integral component of crustal
construction along the global system of mid-ocean ridges. Thermally-induced
circulation of seawater through the permeable parts of the crust and upper
mantle has been estimated to account for 34% of the predicted global oceanic
heat flux, which in turn comprises close to 25% of the total heat flux of
the Earth [ Stein and Stein, 1994, and references therein]. Discharge
of hydrothermal fluids is manifest along mid-ocean ridges as high temperature
(
200-400 C) focused and lower temperature (<200 C) diffuse fluid flow.
Off-axis fluid flow may extend out to the crustal sealing age of 65
10
million years, and may be responsible for more than 70% of the hydrothermal
heat flux [ Stein and Stein, 1994].
Circulation of seawater through the oceanic crust and upper mantle gives rise to a complex series of physical and chemical reactions that lead to the formation of seafloor mineral deposits and to the existence of unique biological communities. In addition, alteration of the basement rocks and mineral precipitation within fractures dramatically influence the thermal structure and physical properties of the oceanic lithosphere, and change the chemistry of the crustal material that is returned to the mantle by subduction. On longer time scales, these water-rock interactions play a role in regulating the chemistry of seawater, although the magnitude of the associated elemental fluxes are not well constrained.
The distribution and characteristics of hydrothermal circulation within the oceanic crust are controlled by the thermal regime and the permeability structure, both of which are strongly influenced by magmatic and tectonic processes. Understanding the spatial and temporal relations between these variables is key to the development of quantitative models of hydrothermal systems and assessment of their global scope and importance. New and improved remote sensing technologies, such as multibeam and sidescan sonar systems, real-time acoustic monitoring, and sensors to detect physical and chemical anomalies in the water column, have enhanced our ability to locate hydrothermal fields, thereby expanding our knowledge of their tectonic and volcanic settings, their characteristics, and the temporal variability of active hydrothermal systems. Studies of hydrothermally altered material from the sea floor and from ophiolites have documented the heterogeneity of water-rock interactions and the variations in the mineralogical and geochemical changes, which often reflect more than one hydrothermal event. Recent experimental work has examined the mobilities of elements under different physical and chemical conditions, provided thermodynamic data for important mineral phases and aqueous complexes, and begun to investigate the impact of fluid phase separation within the reaction zone. Mathematical models have been used to constrain the conductive heat transfer from a magma chamber and the temporal evolution of high temperature vents, as well as to examine the relations between fluid flow and permeability within axial and off-axis hydrothermal systems.
This review attempts to highlight some of the research published during
the last four years that has significantly advanced our understanding of
mid-ocean ridge hydrothermal systems. The focus is on the geological
aspects of these systems; a complete discussion of the impact of
hydrothermal circulation on the water column and on biological production
is beyond the scope of this review. I have selected specific topics under
three broad categories: (i) distribution and temporal variability of
submarine hydrothermal systems; (ii) rock-water interactions within the
crust; and (iii) the evolution of permeability and its relation to off-axis
hydrothermal systems. Portions of the mid-ocean ridge that are discussed
in these sections are illustrated in
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