fluids over a wide range of temperatures (Allen and
Seyfried, 2004; Seyfried et al., 2011). Very high concentrations of H 2 (up to 16 mmol/kg) are mainly produced
by oxidation of ferrous iron in olivine during serpentinization. Large methane plumes are also common in the
water column above the exposed peridotite. The abundant
CH 4 is produced during serpentinization through a series
of redox reactions that convert carbon dioxide to CH 4
(i.e., CO 2 + 4H 2 ¼ CH 4 + 2H 2 O).
18 Mg 1:67 Fe 0:33
ð
Þ SiO 4 þ 23H 2 O ! 9Mg 3 Si 2 O 5 OH
ð Þ 4
Olivine solid solution
Serpentin
þ 3Mg OH
ð Þ 2 þ 2Fe 3 O 4 þ 2H 2
Brucite
Magnetite
The compositions of the hydrothermal fluids also appear
to be sensitive to small differences in the mineralogy of
the ultramafic rocks (e.g., different proportions of olivine
and pyroxene). For example, the low pH of the vent fluids
at Rainbow is interpreted to result mainly from Ca-Mg
exchange with pyroxene and the precipitation of tremolite
and talc in the gabbroic rocks (Allen and Seyfried, 2004).
Unusually high Fe concentrations in the end-member
fluids are believed to result from leaching of magnetite
by the low-pH fluids.
Hydrothermal discharge
Hydrothermal venting is the most recognizable feature of
hydrothermal systems. High-temperature, focused venting
produces black and white smokers and volcanogenic massive sulfide deposits that are often associated with diverse
chemosynthetic ecosystems.
The hot hydrothermal fluid that formed at depth has a
low density relative to cold seawater and thus rises buoyantly to the seafloor in a matter of minutes (at meters per
second) along focused zones of high permeability (e.g.,
deep normal faults). Whereas convecting seawater has a
residence time in the crust of several years from the point
of recharge to the reaction zone, the transit from the base
of the hydrothermal system to the seafloor is rapid
(Kadko and Moore, 1988; Kadko and Butterfield, 1998).
Observations of the rock record indicate that the rate of
ascent is so fast that the fluid does not reequilibrate with
the surrounding rock during ascent, although there may
be some reaction (e.g., silicification of the wall rock and
precipitation of metals). Minor amounts of sulfide are precipitated, but the major subseafloor deposition of sulfide
occurs only in well-developed “stockwork zones” or
feeder systems immediately beneath the seafloor vents
where entrainment of seawater (which reduces the fluid
temperature) and possibly boiling contribute to the precipitation of sulfide minerals (see below). Significant mixing
and precipitation of sulfides below the seafloor result in
the venting of clear fluids and a distinct lack of black or
white “smoke.”
Measured concentrations of most elements in fluids that
are discharged at the seafloor are somewhat lower than in
the reaction zone because of mineral precipitation and
dilution caused by mixing with seawater. However, the
concentrations in the unmixed fluids can be calculated
by assuming that Mg and SO 4 were quantitatively
removed from hydrothermal seawater at depth and any
Mg or SO 4 in the sample fluids must therefore be a result
of mixing with seawater at or below the seafloor. This is
the basis for the “end-member” fluid model (Edmond
et al., 1979; Von Damm et al., 1985). In this model, the
concentrations of major elements in samples collected at
different temperatures or degrees of mixing can be extrapolated back to their original values along mixing lines
between seawater and an undiluted, zero-Mg or zeroSO 4 fluid. Under some conditions, the uptake of Mg by
altered rocks can be reversed, and SO 4 may be added to
the fluids from other sources, so the zero-Mg, zero-SO 4
end-member model cannot be universally applied.
Venting of hydrothermal fluids results in the precipitation and accumulation of sulfide-rich chimneys and
mounds at the vent site. Chimneys that form at ~350
C
“black smoker” vents on mid-ocean ridges are composed
of an assemblage of Cu, Fe, and Zn sulfide minerals
including pyrite (Æmarcasite), pyrrhotite, chalcopyrite,
Fe-rich sphalerite, and minor isocubanite and bornite,
together with anhydrite. Lower-temperature vents
(so-called white smokers) are dominated by pyrite, marcasite, sphalerite, and minor galena and sulphosalts, together
with abundant amorphous silica, barite, and minor anhydrite; pyrrhotite and chalcopyrite are rare or absent. Mineral assemblages from extinct chimneys can be readily
ascribed to a high- or low-temperature origin by analogy
with the mineralogy of active vents. Mineralogical zoning
in the chimneys reflects strong gradients in fluid temperatures (e.g., Tivey, 2007). High-temperature fluid conduits
in black smokers commonly are lined by chalcopyrite
together with pyrrhotite, isocubanite, and locally bornite;
the outer portions of chimneys are commonly made up
of the minerals deposited at lower temperature, such as
sphalerite, pyrite, and marcasite. However, detailed examination reveals complex intergrowths, replacements, and
recrystallization of the minerals resulting from a dynamic
and locally chaotic environment of sulfide precipitation
within the chimneys.
Subseafloor stockwork mineralization
The feeder zones of most hydrothermal vent sites contain
vertically extensive networks of sulfide veins and disseminated sulfides, referred to as the “stockwork zone,” and
intensely altered rocks (i.e., the “alteration pipe”).
Stockwork mineralization can contain as much as
30–40 % of the total metal transported by hydrothermal
fluids. The main alteration minerals are quartz, chlorite,
illite, and kaolinite. These are produced by reaction of
the hydrothermal fluids with the host rocks and by entrainment of local seawater into the upflow zone. Pyrite, which
occurs as a replacement of Fe-bearing igneous phases, is
commonly associated with the intense clay alteration in
the central portion of the stockwork. Intense silicification
HYDROTHERMALISM
349
Seyfried, 2004; Seyfried et al., 2011). Very high concentrations of H 2 (up to 16 mmol/kg) are mainly produced
by oxidation of ferrous iron in olivine during serpentinization. Large methane plumes are also common in the
water column above the exposed peridotite. The abundant
CH 4 is produced during serpentinization through a series
of redox reactions that convert carbon dioxide to CH 4
(i.e., CO 2 + 4H 2 ¼ CH 4 + 2H 2 O).
18 Mg 1:67 Fe 0:33
ð
Þ SiO 4 þ 23H 2 O ! 9Mg 3 Si 2 O 5 OH
ð Þ 4
Olivine solid solution
Serpentin
þ 3Mg OH
ð Þ 2 þ 2Fe 3 O 4 þ 2H 2
Brucite
Magnetite
The compositions of the hydrothermal fluids also appear
to be sensitive to small differences in the mineralogy of
the ultramafic rocks (e.g., different proportions of olivine
and pyroxene). For example, the low pH of the vent fluids
at Rainbow is interpreted to result mainly from Ca-Mg
exchange with pyroxene and the precipitation of tremolite
and talc in the gabbroic rocks (Allen and Seyfried, 2004).
Unusually high Fe concentrations in the end-member
fluids are believed to result from leaching of magnetite
by the low-pH fluids.
Hydrothermal discharge
Hydrothermal venting is the most recognizable feature of
hydrothermal systems. High-temperature, focused venting
produces black and white smokers and volcanogenic massive sulfide deposits that are often associated with diverse
chemosynthetic ecosystems.
The hot hydrothermal fluid that formed at depth has a
low density relative to cold seawater and thus rises buoyantly to the seafloor in a matter of minutes (at meters per
second) along focused zones of high permeability (e.g.,
deep normal faults). Whereas convecting seawater has a
residence time in the crust of several years from the point
of recharge to the reaction zone, the transit from the base
of the hydrothermal system to the seafloor is rapid
(Kadko and Moore, 1988; Kadko and Butterfield, 1998).
Observations of the rock record indicate that the rate of
ascent is so fast that the fluid does not reequilibrate with
the surrounding rock during ascent, although there may
be some reaction (e.g., silicification of the wall rock and
precipitation of metals). Minor amounts of sulfide are precipitated, but the major subseafloor deposition of sulfide
occurs only in well-developed “stockwork zones” or
feeder systems immediately beneath the seafloor vents
where entrainment of seawater (which reduces the fluid
temperature) and possibly boiling contribute to the precipitation of sulfide minerals (see below). Significant mixing
and precipitation of sulfides below the seafloor result in
the venting of clear fluids and a distinct lack of black or
white “smoke.”
Measured concentrations of most elements in fluids that
are discharged at the seafloor are somewhat lower than in
the reaction zone because of mineral precipitation and
dilution caused by mixing with seawater. However, the
concentrations in the unmixed fluids can be calculated
by assuming that Mg and SO 4 were quantitatively
removed from hydrothermal seawater at depth and any
Mg or SO 4 in the sample fluids must therefore be a result
of mixing with seawater at or below the seafloor. This is
the basis for the “end-member” fluid model (Edmond
et al., 1979; Von Damm et al., 1985). In this model, the
concentrations of major elements in samples collected at
different temperatures or degrees of mixing can be extrapolated back to their original values along mixing lines
between seawater and an undiluted, zero-Mg or zeroSO 4 fluid. Under some conditions, the uptake of Mg by
altered rocks can be reversed, and SO 4 may be added to
the fluids from other sources, so the zero-Mg, zero-SO 4
end-member model cannot be universally applied.
Venting of hydrothermal fluids results in the precipitation and accumulation of sulfide-rich chimneys and
mounds at the vent site. Chimneys that form at ~350
C
“black smoker” vents on mid-ocean ridges are composed
of an assemblage of Cu, Fe, and Zn sulfide minerals
including pyrite (Æmarcasite), pyrrhotite, chalcopyrite,
Fe-rich sphalerite, and minor isocubanite and bornite,
together with anhydrite. Lower-temperature vents
(so-called white smokers) are dominated by pyrite, marcasite, sphalerite, and minor galena and sulphosalts, together
with abundant amorphous silica, barite, and minor anhydrite; pyrrhotite and chalcopyrite are rare or absent. Mineral assemblages from extinct chimneys can be readily
ascribed to a high- or low-temperature origin by analogy
with the mineralogy of active vents. Mineralogical zoning
in the chimneys reflects strong gradients in fluid temperatures (e.g., Tivey, 2007). High-temperature fluid conduits
in black smokers commonly are lined by chalcopyrite
together with pyrrhotite, isocubanite, and locally bornite;
the outer portions of chimneys are commonly made up
of the minerals deposited at lower temperature, such as
sphalerite, pyrite, and marcasite. However, detailed examination reveals complex intergrowths, replacements, and
recrystallization of the minerals resulting from a dynamic
and locally chaotic environment of sulfide precipitation
within the chimneys.
Subseafloor stockwork mineralization
The feeder zones of most hydrothermal vent sites contain
vertically extensive networks of sulfide veins and disseminated sulfides, referred to as the “stockwork zone,” and
intensely altered rocks (i.e., the “alteration pipe”).
Stockwork mineralization can contain as much as
30–40 % of the total metal transported by hydrothermal
fluids. The main alteration minerals are quartz, chlorite,
illite, and kaolinite. These are produced by reaction of
the hydrothermal fluids with the host rocks and by entrainment of local seawater into the upflow zone. Pyrite, which
occurs as a replacement of Fe-bearing igneous phases, is
commonly associated with the intense clay alteration in
the central portion of the stockwork. Intense silicification
HYDROTHERMALISM
349