lower temperatures (e.g., Au, Ag, As, Sb, Hg, Tl, Pb, and
Zn) will be transported to the seafloor above the boiling
zone and deposited in the seafloor precipitates
(Hannington et al., 1999). The extent of boiling is therefore a major control on the bulk composition of the hydrothermal deposits forming at the seafloor and could lead to
abundant subseafloor vein mineralization over a large vertical depth range coincident with the boiling zone.
Global distribution
Mid-ocean ridges
On mid-ocean ridges spreading at between 50 and
150 mm/year, the occurrence and distribution of
hydrothermalism is roughly proportional to the spreading
rate (Baker and German, 2004). The frequency of
hydrothermal vents along the magmatically robust fastspreading ridges is high, relative to the more magmastarved slow-spreading ridges (Beaulieu et al., 2013).
However, associated hydrothermal sulfide deposits on
the fast-spreading ridges tend to be small, and the fewer
deposits on slow-spreading ridges are commonly larger,
reflecting longer-lived hydrothermal deposition. The incidence of large hydrothermal plumes is unexpectedly high
at ultraslow-spreading ridges. Although most hightemperature hydrothermal systems occur in the axial
zones of the ridges, a number of important hydrothermal
systems also have developed at ridge-axis volcanoes and
at the intersections of ridges with major transform faults
or hot spots (mantle plumes). Widespread but typically
low-temperature hydrothermal activity also has been
found on off-axis volcanoes. Curiously, although intraplate hotspot volcanoes account for a large proportion of
seafloor volcanism (about 12 % of the Earth’s magmatic
budget (Schmidt and Schmincke, 2000), almost no hightemperature hydrothermal activity has been reported that
is associated with these volcanoes, except in proximity
to ridges. This apparent contradiction may be due to the
great depth of the heat source and the narrow magma conduits beneath hotspot volcanoes (Hannington et al., 2005).
The lack of documented hydrothermalism in other parts of
the oceans (e.g., the polar regions and the Southern Ocean,
Figure 3) mainly reflects the difficulties of marine research
at these latitudes. Discoveries of hydrothermal plumes and
massive sulfide deposits in the high Arctic (e.g., Gakkel
Ridge) and in Antarctica (e.g., Bransfield Strait, East Scotia Ridge) confirm that seafloor hydrothermal activity in
remote parts of the oceans is little different from that
observed elsewhere. Along slow- and ultraslow-spreading
ridges (<20 mm/year full spreading rate), at high latitudes
and elsewhere, hydrothermal activity appears to be
demonstrably more abundant than predicted based on the
relation between magma budget and hydrothermal activity
established for ridges spreading at faster rates.
A variety of geophysical measurements indicate that
about 3 to 6 Â 10
13 kg/year of seawater must be circulated
through the axial zones of the world’s mid-ocean ridges
and heated to a temperature of at least 350
C to remove
the heat from newly formed crust (Schultz and Elderfield,
1997; Alt, 2003; Mottl, 2003). Other estimates, based on
geochemical mass balances, have ranged as high as
1.5 Â 10
14 kg/year (Baker et al., 1996) and as low as
7 Â 10
12 kg/year (Nielsen et al., 2006). These calculations
assume that the component of lower-temperature diffuse
flow (responsible for up to 90 % of the total heat discharge) represents end-member fluid in which the elements have been conservatively diluted with seawater.
The combined flux of low- and high-temperature fluids
is >10
14 kg/year, enough to cycle the entire mass of the
world’s oceans (1.37 Â 10
21 kg) through the axis of the
mid-ocean ridges every 5–10 Ma (Elderfield and Schultz,
1996). If the flux of heat from the flanks of the ridges is
included in this calculation (crust older than 1 Ma but
<65 Ma), the cycling time for the worlds’ oceans through
the ridges is less than 1 Ma.
Based on an assumption that fast-ridge EPR-style
venting was globally representative, Mottl (2003) and
Sinha and Evans (2004) estimated that the heat transport
associated with high-temperature convection at ridge axes
is 1.8 + 0.3 Â 10
12 W. As noted above, only about 10 % of
this heat is discharged at black smoker temperatures
because of mixing and conductive cooling of fluids on
their way to the seafloor (Ginster et al., 1994). Conversely,
German et al. (2010) argue that closer to 50 % of the total
on-axis heat flow on slow-spreading ridges may be in
large black smoker systems. Assuming a heat flux of
2–5 MW for a single black smoker vent (Converse et al.,
1984; Bemis et al., 1993; Ginster et al., 1994), between
50,000 and 100,000 black smokers would be required to
account for the high-temperature flux (10 % of
1.8 + 0.3 Â 10
12 W) or a density of at least one black
smoker for every 1 km of ridge axis. Because their distribution is far from uniform, it is more likely that this heat
is removed by fewer larger-scale hydrothermal systems.
For example, some large vent fields contain as many as
100 black smoker vents with a heat output equivalent to
200–500 MW (Becker and von Herzen, 1996; Kelley
et al., 2002). Only 1,000 high-temperature vent fields of
this magnitude would be required to account for the
high-temperature flux, equivalent to one large hydrothermal system every 100 km along the ridges. Fewer than
100 vent fields of this size are presently known, but only
about 20 % of the mid-ocean ridge system has been
explored in detail so far. The annual flux of metals and sulfur from these vents is estimated to be on the order of 10
6
tonnes per year, assuming a discharge rate of
200–500 kg/s and total combined metal and sulfur concentrations of about 250 mg/kg. This may be only a small
fraction of the metal and sulfur originally mobilized by
high-temperature fluids at depth, if a large proportion is
retained in the crust as a result of cooling, mixing, and
water-rock interaction in the subseafloor (Alt, 2003).
Independent estimates based on the actual distribution
of known vent sites suggest that the spacing of sulfide
occurrences is quite regular. Hannington and Monecke
(2009) examined the spacing of large vent fields on
HYDROTHERMALISM
351
Zn) will be transported to the seafloor above the boiling
zone and deposited in the seafloor precipitates
(Hannington et al., 1999). The extent of boiling is therefore a major control on the bulk composition of the hydrothermal deposits forming at the seafloor and could lead to
abundant subseafloor vein mineralization over a large vertical depth range coincident with the boiling zone.
Global distribution
Mid-ocean ridges
On mid-ocean ridges spreading at between 50 and
150 mm/year, the occurrence and distribution of
hydrothermalism is roughly proportional to the spreading
rate (Baker and German, 2004). The frequency of
hydrothermal vents along the magmatically robust fastspreading ridges is high, relative to the more magmastarved slow-spreading ridges (Beaulieu et al., 2013).
However, associated hydrothermal sulfide deposits on
the fast-spreading ridges tend to be small, and the fewer
deposits on slow-spreading ridges are commonly larger,
reflecting longer-lived hydrothermal deposition. The incidence of large hydrothermal plumes is unexpectedly high
at ultraslow-spreading ridges. Although most hightemperature hydrothermal systems occur in the axial
zones of the ridges, a number of important hydrothermal
systems also have developed at ridge-axis volcanoes and
at the intersections of ridges with major transform faults
or hot spots (mantle plumes). Widespread but typically
low-temperature hydrothermal activity also has been
found on off-axis volcanoes. Curiously, although intraplate hotspot volcanoes account for a large proportion of
seafloor volcanism (about 12 % of the Earth’s magmatic
budget (Schmidt and Schmincke, 2000), almost no hightemperature hydrothermal activity has been reported that
is associated with these volcanoes, except in proximity
to ridges. This apparent contradiction may be due to the
great depth of the heat source and the narrow magma conduits beneath hotspot volcanoes (Hannington et al., 2005).
The lack of documented hydrothermalism in other parts of
the oceans (e.g., the polar regions and the Southern Ocean,
Figure 3) mainly reflects the difficulties of marine research
at these latitudes. Discoveries of hydrothermal plumes and
massive sulfide deposits in the high Arctic (e.g., Gakkel
Ridge) and in Antarctica (e.g., Bransfield Strait, East Scotia Ridge) confirm that seafloor hydrothermal activity in
remote parts of the oceans is little different from that
observed elsewhere. Along slow- and ultraslow-spreading
ridges (<20 mm/year full spreading rate), at high latitudes
and elsewhere, hydrothermal activity appears to be
demonstrably more abundant than predicted based on the
relation between magma budget and hydrothermal activity
established for ridges spreading at faster rates.
A variety of geophysical measurements indicate that
about 3 to 6 Â 10
13 kg/year of seawater must be circulated
through the axial zones of the world’s mid-ocean ridges
and heated to a temperature of at least 350
C to remove
the heat from newly formed crust (Schultz and Elderfield,
1997; Alt, 2003; Mottl, 2003). Other estimates, based on
geochemical mass balances, have ranged as high as
1.5 Â 10
14 kg/year (Baker et al., 1996) and as low as
7 Â 10
12 kg/year (Nielsen et al., 2006). These calculations
assume that the component of lower-temperature diffuse
flow (responsible for up to 90 % of the total heat discharge) represents end-member fluid in which the elements have been conservatively diluted with seawater.
The combined flux of low- and high-temperature fluids
is >10
14 kg/year, enough to cycle the entire mass of the
world’s oceans (1.37 Â 10
21 kg) through the axis of the
mid-ocean ridges every 5–10 Ma (Elderfield and Schultz,
1996). If the flux of heat from the flanks of the ridges is
included in this calculation (crust older than 1 Ma but
<65 Ma), the cycling time for the worlds’ oceans through
the ridges is less than 1 Ma.
Based on an assumption that fast-ridge EPR-style
venting was globally representative, Mottl (2003) and
Sinha and Evans (2004) estimated that the heat transport
associated with high-temperature convection at ridge axes
is 1.8 + 0.3 Â 10
12 W. As noted above, only about 10 % of
this heat is discharged at black smoker temperatures
because of mixing and conductive cooling of fluids on
their way to the seafloor (Ginster et al., 1994). Conversely,
German et al. (2010) argue that closer to 50 % of the total
on-axis heat flow on slow-spreading ridges may be in
large black smoker systems. Assuming a heat flux of
2–5 MW for a single black smoker vent (Converse et al.,
1984; Bemis et al., 1993; Ginster et al., 1994), between
50,000 and 100,000 black smokers would be required to
account for the high-temperature flux (10 % of
1.8 + 0.3 Â 10
12 W) or a density of at least one black
smoker for every 1 km of ridge axis. Because their distribution is far from uniform, it is more likely that this heat
is removed by fewer larger-scale hydrothermal systems.
For example, some large vent fields contain as many as
100 black smoker vents with a heat output equivalent to
200–500 MW (Becker and von Herzen, 1996; Kelley
et al., 2002). Only 1,000 high-temperature vent fields of
this magnitude would be required to account for the
high-temperature flux, equivalent to one large hydrothermal system every 100 km along the ridges. Fewer than
100 vent fields of this size are presently known, but only
about 20 % of the mid-ocean ridge system has been
explored in detail so far. The annual flux of metals and sulfur from these vents is estimated to be on the order of 10
6
tonnes per year, assuming a discharge rate of
200–500 kg/s and total combined metal and sulfur concentrations of about 250 mg/kg. This may be only a small
fraction of the metal and sulfur originally mobilized by
high-temperature fluids at depth, if a large proportion is
retained in the crust as a result of cooling, mixing, and
water-rock interaction in the subseafloor (Alt, 2003).
Independent estimates based on the actual distribution
of known vent sites suggest that the spacing of sulfide
occurrences is quite regular. Hannington and Monecke
(2009) examined the spacing of large vent fields on
HYDROTHERMALISM
351