457
13 Input from the Deep:
Hot Vents and Cold Seeps
PETER M. HERZIG AND MARK D. HANNINGTON
The discovery of black smokers, massive sulfides
and vent biota at the crest of the East Pacific Rise
at 21°N in 1979 (Francheteau et al. 1979; Spiess et
al. 1980) confirmed that the formation of new
oceanic crust through seafloor spreading is intimately associated with hydrothermal venting and
the formation of metallic mineral deposits at the
seafloor. The 350°C hydrothermal fluids discharging from the black smoker chimneys at this site at
a water depth of about 2,600 m continuously
precipitate metal sulfides in response to mixing of
the high-temperature hydrothermal fluids with
ambient seawater. Seawater which penetrates
deeply into the oceanic crust at seafloor
spreading centers is being converted to a
hydrothermal fluid with low pH, low Eh, and high
temperature during water-rock interaction above a
high-level magma chamber. This fluid is capable of
leaching large amounts of metals and other
elements from the rocks. Metal sulfides, including
pyrite, sphalerite, and chalcopyrite which are
precipitated from the hydrothermal fluids,
gradually accumulate at and just below the
seafloor where they can form large sulfide
deposits. The TAG hydrothermal mound at the
Mid-Atlantic Ridge at 26°N, for example, has a
diameter of about 200 m and a height above
seafloor of about 50 m. Drilling during Leg 158 of
the Ocean Drilling Program indicated that the
deposit contains 2.7 Mt of massive sulfide,
containing 2 wt.% Cu, and 1.2 Mt of stockwork
mineralization extending 125 m below the seafloor
and containing 1 wt.% Cu (Hannington et al.
1998). The deposit is capped by a large black
smoker complex with as many as 100 black smoker
vents at a temperature of more than 360°C.
Seafloor hydrothermal activity at mid-ocean
ridges and back-arc spreading centers has a major
impact on the chemistry of the oceans (Edmond et
al. 1979a, 1982) and has been responsible for
extensive alteration of modern and ancient
oceanic crust (Alt 1995). It has been estimated
that 25-30% of the earth´s total heat flux is
transfered from the lithosphere to the hydrosphere by the circulation of seawater through
oceanic spreading centers (Lowell 1991; Stein and
Stein 1994). Early estimates of the total discharge
of hydrothermal vents at oceanic ridges based on
heat flow data were on the order of 5x10
6
L/s. More
recent estimates based on geochemical mass
balances and geophysical measurements are of
the same order of magnitude (Elderfield and
Schultz 1996). The calculated fluxes require that
the entire volume of the world’s oceans is
circulated through thermally active seafloor rift
zones every 5-11 Ma (Wolery and Sleep 1976;
Morton and Sleep 1985). If convective heat flux
from the flanks of the ridges is included in this
calculation, the cycling time for the world’s
oceans through the ridges is less than 1 million
years. Neglecting any component of diffuse flow,
the estimated flux of high-temperature fluids
would require at least one black smoker with a
mass flux of approxi-mately 1 kg/s and an
estimated power of 1.5 megawatts (Converse et al.
1984) for every 50 meters of ridge crest (55,000 km
in total). Off-axis diffuse flow accounts for as
much as 70-80% of the total heat loss at oceanic
ridges (Wheat and Mottl 1994; Stein and Stein
1994) and thus represents an important
component of seafloor hydrothermal activity,
although the actual chemical fluxes through the
ridge flanks have not yet been documented.
According to Wheat and Mottl (1994) and Ginster
et al. (1994), up to 90% of the heat in the axial rift
zones may be removed as a result of diffuse
discharge resulting from subseafloor mixing of
high-temperature hydro-thermal fluids with cold
seawater. Although the discharge of hydrothermal
fluids through ridge-crests is small by comparison
13 Input from the Deep:
Hot Vents and Cold Seeps
PETER M. HERZIG AND MARK D. HANNINGTON
The discovery of black smokers, massive sulfides
and vent biota at the crest of the East Pacific Rise
at 21°N in 1979 (Francheteau et al. 1979; Spiess et
al. 1980) confirmed that the formation of new
oceanic crust through seafloor spreading is intimately associated with hydrothermal venting and
the formation of metallic mineral deposits at the
seafloor. The 350°C hydrothermal fluids discharging from the black smoker chimneys at this site at
a water depth of about 2,600 m continuously
precipitate metal sulfides in response to mixing of
the high-temperature hydrothermal fluids with
ambient seawater. Seawater which penetrates
deeply into the oceanic crust at seafloor
spreading centers is being converted to a
hydrothermal fluid with low pH, low Eh, and high
temperature during water-rock interaction above a
high-level magma chamber. This fluid is capable of
leaching large amounts of metals and other
elements from the rocks. Metal sulfides, including
pyrite, sphalerite, and chalcopyrite which are
precipitated from the hydrothermal fluids,
gradually accumulate at and just below the
seafloor where they can form large sulfide
deposits. The TAG hydrothermal mound at the
Mid-Atlantic Ridge at 26°N, for example, has a
diameter of about 200 m and a height above
seafloor of about 50 m. Drilling during Leg 158 of
the Ocean Drilling Program indicated that the
deposit contains 2.7 Mt of massive sulfide,
containing 2 wt.% Cu, and 1.2 Mt of stockwork
mineralization extending 125 m below the seafloor
and containing 1 wt.% Cu (Hannington et al.
1998). The deposit is capped by a large black
smoker complex with as many as 100 black smoker
vents at a temperature of more than 360°C.
Seafloor hydrothermal activity at mid-ocean
ridges and back-arc spreading centers has a major
impact on the chemistry of the oceans (Edmond et
al. 1979a, 1982) and has been responsible for
extensive alteration of modern and ancient
oceanic crust (Alt 1995). It has been estimated
that 25-30% of the earth´s total heat flux is
transfered from the lithosphere to the hydrosphere by the circulation of seawater through
oceanic spreading centers (Lowell 1991; Stein and
Stein 1994). Early estimates of the total discharge
of hydrothermal vents at oceanic ridges based on
heat flow data were on the order of 5x10
6
L/s. More
recent estimates based on geochemical mass
balances and geophysical measurements are of
the same order of magnitude (Elderfield and
Schultz 1996). The calculated fluxes require that
the entire volume of the world’s oceans is
circulated through thermally active seafloor rift
zones every 5-11 Ma (Wolery and Sleep 1976;
Morton and Sleep 1985). If convective heat flux
from the flanks of the ridges is included in this
calculation, the cycling time for the world’s
oceans through the ridges is less than 1 million
years. Neglecting any component of diffuse flow,
the estimated flux of high-temperature fluids
would require at least one black smoker with a
mass flux of approxi-mately 1 kg/s and an
estimated power of 1.5 megawatts (Converse et al.
1984) for every 50 meters of ridge crest (55,000 km
in total). Off-axis diffuse flow accounts for as
much as 70-80% of the total heat loss at oceanic
ridges (Wheat and Mottl 1994; Stein and Stein
1994) and thus represents an important
component of seafloor hydrothermal activity,
although the actual chemical fluxes through the
ridge flanks have not yet been documented.
According to Wheat and Mottl (1994) and Ginster
et al. (1994), up to 90% of the heat in the axial rift
zones may be removed as a result of diffuse
discharge resulting from subseafloor mixing of
high-temperature hydro-thermal fluids with cold
seawater. Although the discharge of hydrothermal
fluids through ridge-crests is small by comparison