32 ridge segments and calculated an average spacing
between sulfide deposits of 98 km. The spacing is greater
on the slow-spreading ridges (167 km) than on the fastspreading ridges (46 km), but the individual sulfide occurrences, and therefore the cumulative heat output, on the
slow-spreading ridges are larger on average (Bach and
Frueh-Green, 2010; Hannington et al., 2011). The spacing
of known mineral deposits is similar to the global incidence of venting detected remotely as hydrothermal
plumes (Baker and German, 2004; Baker, 2007).
Ultramafic environments
At some slow-spreading ridges, tectonic processes locally
expose gabbroic and ultramafic rocks. Near large faults,
vertical tectonics and especially low-angle detachment
faulting expose large sections of the underlying oceanic
lithosphere, referred to as oceanic core complexes. At several locations on the Mid-Atlantic Ridge, hydrothermal
systems have been located on top of oceanic core complexes (e.g., Rainbow, Logatchev, Ashadze, Nibelungen,
Semyenov, Irinovskoe, Von Damm fields). Although it
has been suggested that some hydrothermal circulation at
these sites is driven by the heat of serpentinization of the
exposed ultramafic rocks, high-temperature black smoker
activity in these settings is considered to reflect a deep
magmatic heat source (Kelley et al., 2002; Mevel, 2003;
Melchert et al., 2008). Gabbro has been shown to intrude
the lithospheric mantle prior to uplift and exposure
(Cannat, 1996), which may drive high-temperature
hydrothermal circulation in the ultramafic rocks, and the
hydrothermal fluids are interpreted as products of hightemperature reactions with both gabbroic subvolcanic
intrusions and peridotite.
The Lost City vent field, located at the intersection of
the Mid-Atlantic Ridge and the Atlantis Fracture Zone,
is an example of a rare hydrothermal system that may be
driven primarily by the heat of serpentinization. Here,
peridotite exposed at the seafloor is reacting directly with
cold seawater, releasing significant quantities of a
Mg-poor, CH 4 - and H 2 -rich fluid but no metals (Kelley
et al., 2001). The fluids are buffered by equilibrium with
diopside, brucite, and serpentine produced by the hydration of peridotite, which results in pH values of ~10–11.
As the warm, up to 91
C, high-pH (thus OH-rich) fluids
mix with Mg-rich seawater, brucite is formed directly on
the seafloor and in subseafloor fractures (Kelley et al.,
2005). The formation of serpentine and other
Mg-silicates also buffers Si to very low concentrations,
and only traces of silica are deposited at the seafloor.
The high pH of the vent fluids causes the conversion of
HCO 3
À in the seawater to carbonate ions and triggers carbonate precipitation. Towering carbonate (aragonite)
chimneys, 10–30 m in height (up to 60 m), and
travertine-like deposits form (Kelley et al., 2001; Palandri
and Reed, 2004). Polymetallic sulfides are absent because
of a lack of metals and H 2 S in the vent fluids, and the
low temperatures prevent anhydrite saturation. The
serpentinization of olivine is exothermic and generates
enough heat to raise the subseafloor temperatures to as
much as 150
C. However, theoretical calculations based
on major cation concentrations in the fluids and heat balance suggest that exothermic reactions cannot be the only
source of heat (Allen and Seyfried, 2004).
Suprasubduction zones
Hydrothermal systems in subduction-related environments are broadly similar to those at the mid-ocean ridges,
and features typical of mid-ocean ridge hydrothermal systems (e.g., black smokers, hydrothermal plumes) are also
observed in arc and back-arc settings. In contrast to the
mid-ocean ridges, however, convergent margins are characterized by a range of different crustal thicknesses, heat
flow regimes, and magma compositions. The compositions of the volcanic rocks vary from mid-ocean ridge
basalt (MORB) to more felsic lavas (andesite, dacite, and
rhyolite) with geochemical affinities to island arcs. This
variability leads to major differences in the composition
of the hydrothermal fluids and the mineralogy and bulk
composition of the sulfide deposits. For instance, many
of the arc-related systems are directly affected by influx
of magmatic SO 2 , which disproportionates to native sulfur
and sulfuric acid. These seafloor solfataras are characterized by large accumulations of sulfur and fields of white
smokers venting fluids with pH <1.5. About one third of
the known high-temperature seafloor hydrothermal systems occur at submarine volcanic arcs or in back-arc
basins. Although there are no estimates of the total hydrothermal flux, distributions along axis, as determined from
overlying hydrothermal plume incidence, appears directly
proportional to the lengths of the arc and back-arc segments, just as seen along mid-ocean ridges (Beaulieu
et al., 2013) hence, accounting for about 20 % of the
mid-ocean ridge flux. The spacing of hydrothermal systems along the volcanic fronts of island arcs remain poorly
known, as these volcanoes are still being discovered and
explored. On the Mariana and Kermadec arcs, which are
the most completely surveyed, hydrothermal plumes have
been found above one third of the submarine volcanoes,
which occur at intervals of ~70 km along the lengths of
the arcs (de Ronde et al., 2003). However, the incidence
and intensity of hydrothermal activity associated with
these volcanoes are highly variable and not yet well characterized. Some plumes detected within or above the summit calderas of arc volcanoes are related to hightemperature hydrothermal venting and black smoker
activity, but many of the plume signals appear to be due
to passive degassing or diffuse venting rather than hightemperature hydrothermal vents (Embley et al., 2006).
The major controls on vent fluid compositions in arc
and back-arc settings are the same as those at mid-ocean
ridges, although there is abundant evidence that magmas
directly supply a number of components, including CO 2
and metals, to the hydrothermal fluids and boiling affects
many of the hydrothermal systems at the volcanic fronts
352
HYDROTHERMALISM
between sulfide deposits of 98 km. The spacing is greater
on the slow-spreading ridges (167 km) than on the fastspreading ridges (46 km), but the individual sulfide occurrences, and therefore the cumulative heat output, on the
slow-spreading ridges are larger on average (Bach and
Frueh-Green, 2010; Hannington et al., 2011). The spacing
of known mineral deposits is similar to the global incidence of venting detected remotely as hydrothermal
plumes (Baker and German, 2004; Baker, 2007).
Ultramafic environments
At some slow-spreading ridges, tectonic processes locally
expose gabbroic and ultramafic rocks. Near large faults,
vertical tectonics and especially low-angle detachment
faulting expose large sections of the underlying oceanic
lithosphere, referred to as oceanic core complexes. At several locations on the Mid-Atlantic Ridge, hydrothermal
systems have been located on top of oceanic core complexes (e.g., Rainbow, Logatchev, Ashadze, Nibelungen,
Semyenov, Irinovskoe, Von Damm fields). Although it
has been suggested that some hydrothermal circulation at
these sites is driven by the heat of serpentinization of the
exposed ultramafic rocks, high-temperature black smoker
activity in these settings is considered to reflect a deep
magmatic heat source (Kelley et al., 2002; Mevel, 2003;
Melchert et al., 2008). Gabbro has been shown to intrude
the lithospheric mantle prior to uplift and exposure
(Cannat, 1996), which may drive high-temperature
hydrothermal circulation in the ultramafic rocks, and the
hydrothermal fluids are interpreted as products of hightemperature reactions with both gabbroic subvolcanic
intrusions and peridotite.
The Lost City vent field, located at the intersection of
the Mid-Atlantic Ridge and the Atlantis Fracture Zone,
is an example of a rare hydrothermal system that may be
driven primarily by the heat of serpentinization. Here,
peridotite exposed at the seafloor is reacting directly with
cold seawater, releasing significant quantities of a
Mg-poor, CH 4 - and H 2 -rich fluid but no metals (Kelley
et al., 2001). The fluids are buffered by equilibrium with
diopside, brucite, and serpentine produced by the hydration of peridotite, which results in pH values of ~10–11.
As the warm, up to 91
C, high-pH (thus OH-rich) fluids
mix with Mg-rich seawater, brucite is formed directly on
the seafloor and in subseafloor fractures (Kelley et al.,
2005). The formation of serpentine and other
Mg-silicates also buffers Si to very low concentrations,
and only traces of silica are deposited at the seafloor.
The high pH of the vent fluids causes the conversion of
HCO 3
À in the seawater to carbonate ions and triggers carbonate precipitation. Towering carbonate (aragonite)
chimneys, 10–30 m in height (up to 60 m), and
travertine-like deposits form (Kelley et al., 2001; Palandri
and Reed, 2004). Polymetallic sulfides are absent because
of a lack of metals and H 2 S in the vent fluids, and the
low temperatures prevent anhydrite saturation. The
serpentinization of olivine is exothermic and generates
enough heat to raise the subseafloor temperatures to as
much as 150
C. However, theoretical calculations based
on major cation concentrations in the fluids and heat balance suggest that exothermic reactions cannot be the only
source of heat (Allen and Seyfried, 2004).
Suprasubduction zones
Hydrothermal systems in subduction-related environments are broadly similar to those at the mid-ocean ridges,
and features typical of mid-ocean ridge hydrothermal systems (e.g., black smokers, hydrothermal plumes) are also
observed in arc and back-arc settings. In contrast to the
mid-ocean ridges, however, convergent margins are characterized by a range of different crustal thicknesses, heat
flow regimes, and magma compositions. The compositions of the volcanic rocks vary from mid-ocean ridge
basalt (MORB) to more felsic lavas (andesite, dacite, and
rhyolite) with geochemical affinities to island arcs. This
variability leads to major differences in the composition
of the hydrothermal fluids and the mineralogy and bulk
composition of the sulfide deposits. For instance, many
of the arc-related systems are directly affected by influx
of magmatic SO 2 , which disproportionates to native sulfur
and sulfuric acid. These seafloor solfataras are characterized by large accumulations of sulfur and fields of white
smokers venting fluids with pH <1.5. About one third of
the known high-temperature seafloor hydrothermal systems occur at submarine volcanic arcs or in back-arc
basins. Although there are no estimates of the total hydrothermal flux, distributions along axis, as determined from
overlying hydrothermal plume incidence, appears directly
proportional to the lengths of the arc and back-arc segments, just as seen along mid-ocean ridges (Beaulieu
et al., 2013) hence, accounting for about 20 % of the
mid-ocean ridge flux. The spacing of hydrothermal systems along the volcanic fronts of island arcs remain poorly
known, as these volcanoes are still being discovered and
explored. On the Mariana and Kermadec arcs, which are
the most completely surveyed, hydrothermal plumes have
been found above one third of the submarine volcanoes,
which occur at intervals of ~70 km along the lengths of
the arcs (de Ronde et al., 2003). However, the incidence
and intensity of hydrothermal activity associated with
these volcanoes are highly variable and not yet well characterized. Some plumes detected within or above the summit calderas of arc volcanoes are related to hightemperature hydrothermal venting and black smoker
activity, but many of the plume signals appear to be due
to passive degassing or diffuse venting rather than hightemperature hydrothermal vents (Embley et al., 2006).
The major controls on vent fluid compositions in arc
and back-arc settings are the same as those at mid-ocean
ridges, although there is abundant evidence that magmas
directly supply a number of components, including CO 2
and metals, to the hydrothermal fluids and boiling affects
many of the hydrothermal systems at the volcanic fronts
352
HYDROTHERMALISM