hydrothermal fluid diluted by seawater, rise several hundred meters above the seafloor until they attain neutral
buoyancy, at which point the plume spreads laterally along
the direction of the local currents (see “Hydrothermal
Plumes”). Plumes can readily be detected for several kilometers away from their source, from measurements of
physical and chemical parameters in the water column
(e.g., positive temperature,
3
He, CH 4 and turbidity anomalies, and negative Eh anomalies). The detection of
plumes in the water column has been the primary method
for detecting active hydrothermal venting on the seafloor.
While it was long believed that most transition metals
from hydrothermal vents are deposited near the vent sites,
hydrothermal plumes have recently been recognized as a
potentially important source of dissolved iron throughout
the oceans (Tagliabue et al., 2010).
The minerals that precipitate out of hydrothermal fluids
at a vent site and are entrained into the buoyant ascending
plume (i.e., the “smoke”) are commonly dispersed over a
wide area by near-bottom and mid-depth currents and do
not contribute to the growth of sulfide deposits, except in
rare cases. At typical black smoker vents, at least 90 %
of the metals and sulfur discharged at the seafloor is lost
to the hydrothermal plume. Sulfide particles larger than
about 50 mm in size settle out of the plume within a kilometer of the vent, but finer particles, typically less than
20 mm in size, are dispersed (Baker et al., 1985; Mottl
and McConachy, 1990; Feely et al., 1994). The finer particles account for most of the particle load of the plumes,
but a large proportion of these may oxidize and dissolve
before settling, releasing metals back into seawater
(Feely et al., 1987; Metz and Trefry, 1993). However, up
to 10 % of total Fe within a plume may be in the form of
nanoparticles (<200 nM size fraction) that are
resilient to both oxidation and particle settling and can
therefore be dispersed significant distances off-axis
(Yucel et al., 2011).
The metalliferous sediments in the Atlantis II Deep of
the Red Sea are an exceptional example of metal deposition from hydrothermal fluids vented into the water
column. In the Atlantis II Deep, metal-rich fluids are
injected into the base of a highly stratified brine pool by
hydrothermal vents located within the deep (Zierenberg
and Shanks, 1983; Scholten et al., 2000). These metalbearing brines have salinities, which are many times
greater than vent fluids on the mid-ocean ridges, most
likely as a consequence of seawater circulation through
Miocene evaporites on the flanks of the rift. The brines
sink, rather than rise as buoyant hydrothermal plumes, settling into several deep anoxic basins and depositing the
metals as thick horizons of oxides and sulfides on the
basin floor.
Summary
The circulation of seawater through oceanic lithosphere
and the associated fluid-rock thermal and chemical
exchange that occurs in the subsurface has a major effect
on the composition of both seawater and oceanic lithosphere. The wide range of different mineralogical and geochemical characteristics of hydrothermal systems is a
result of a combination of source rock composition,
pressure- and temperature-dependent solubility controls,
and possible direct contributions of metals and gases from
subvolcanic magmas. These characteristics provide clues
to the geology and chemical processes that are occurring
in the underlying crust. The heating and subsequent
venting of circulated seawater is the major mechanism
for cooling hot, newly formed oceanic crust. The resulting
chemical exchange includes the leaching and transport of
base and precious metals from the underlying rock to the
seafloor. The mixing of hot, metal-rich hydrothermal
fluids with cold seawater at sites of focused black or white
smoker discharge on the seafloor result in the formation of
volcanogenic massive sulfide deposits that often host
chemosynthetic-based ecosystems. The massive sulfide
deposits that formed in ancient oceans and are now
exposed on land are an important source for Cu, Zn, Pb,
Au, and Ag and are mined for their metal contents. In
the future, hydrothermal sulfide deposits on the modern
seafloor may similarly be exploited for their rich metal
content.
Bibliography
Allen, D. E., and Seyfried, W. E., 2004. Serpentinization and heat
generation: constraints from Lost City and rainbow hydrothermal systems. Geochimica Et Cosmochimica Acta, 68,
1347–1354.
Alt, J. C., 2003. Hydrothermal fluxes at mid-ocean ridges and on
ridge flanks. Comptes Rendus Geoscience, 335, 853–864.
Alt, J. C., and Teagle, D. A. H., 2003. Hydrothermal alteration of
upper oceanic crust formed at a fast-spreading ridge: mineral,
chemical, and isotopic evidence from ODP Site 801. Chemical
Geology, 201, 191–211.
Bach, W., and Frueh-Green, G. L., 2010. Alteration of the oceanic
lithosphere and implications for seafloor processes. Elements,
6, 173–178.
Baker, E. T., 2007. Hydrothermal cooling of midocean ridge axes:
do measured and modeled heat fluxes agree? Earth and Planetary Science Letters, 263, 140–150.
Baker, E. T., and German, C. R., 2004. On the global distribution of
hydrothermal vent fields. Mid-Ocean Ridges, 148, 245–266.
Baker, E. T., Lavelle, J. W., and Massoth, G. J., 1985. Hydrothermal
particle plumes over the southern Juan de Fuca Ridge. Nature,
316, 342–344.
Baker, E. T., Chen, Y. J., and Morgan, J. P., 1996. The relationship
between near-axis hydrothermal cooling and the spreading rate
of mid-ocean ridges. Earth and Planetary Science Letters, 142,
137–145.
Beaulieu, S. E., Baker, E. T., German, C. R., and Maffei, A., 2013.
An authoritative global database for active submarine hydrothermal vent fields. Geochemistry Geophysics Geosystems, 14,
4892–4905.
Becker, K., and von Herzen, R. P., 1996. Pre-Drilling Observations
of Conductive Heat Flow at the TAG Active Mound Using Alvin.
Texas A & M University, Ocean Drilling Program, College Station, TX, USA.
Bemis, K. G., Vonherzen, R. P., and Mottl, M. J., 1993. Geothermal
heat-flux from hydrothermal plumes on the Juan de Fuca Ridge.
Journal of Geophysical Research-Solid Earth, 98, 6351–6365.
354
HYDROTHERMALISM
buoyancy, at which point the plume spreads laterally along
the direction of the local currents (see “Hydrothermal
Plumes”). Plumes can readily be detected for several kilometers away from their source, from measurements of
physical and chemical parameters in the water column
(e.g., positive temperature,
3
He, CH 4 and turbidity anomalies, and negative Eh anomalies). The detection of
plumes in the water column has been the primary method
for detecting active hydrothermal venting on the seafloor.
While it was long believed that most transition metals
from hydrothermal vents are deposited near the vent sites,
hydrothermal plumes have recently been recognized as a
potentially important source of dissolved iron throughout
the oceans (Tagliabue et al., 2010).
The minerals that precipitate out of hydrothermal fluids
at a vent site and are entrained into the buoyant ascending
plume (i.e., the “smoke”) are commonly dispersed over a
wide area by near-bottom and mid-depth currents and do
not contribute to the growth of sulfide deposits, except in
rare cases. At typical black smoker vents, at least 90 %
of the metals and sulfur discharged at the seafloor is lost
to the hydrothermal plume. Sulfide particles larger than
about 50 mm in size settle out of the plume within a kilometer of the vent, but finer particles, typically less than
20 mm in size, are dispersed (Baker et al., 1985; Mottl
and McConachy, 1990; Feely et al., 1994). The finer particles account for most of the particle load of the plumes,
but a large proportion of these may oxidize and dissolve
before settling, releasing metals back into seawater
(Feely et al., 1987; Metz and Trefry, 1993). However, up
to 10 % of total Fe within a plume may be in the form of
nanoparticles (<200 nM size fraction) that are
resilient to both oxidation and particle settling and can
therefore be dispersed significant distances off-axis
(Yucel et al., 2011).
The metalliferous sediments in the Atlantis II Deep of
the Red Sea are an exceptional example of metal deposition from hydrothermal fluids vented into the water
column. In the Atlantis II Deep, metal-rich fluids are
injected into the base of a highly stratified brine pool by
hydrothermal vents located within the deep (Zierenberg
and Shanks, 1983; Scholten et al., 2000). These metalbearing brines have salinities, which are many times
greater than vent fluids on the mid-ocean ridges, most
likely as a consequence of seawater circulation through
Miocene evaporites on the flanks of the rift. The brines
sink, rather than rise as buoyant hydrothermal plumes, settling into several deep anoxic basins and depositing the
metals as thick horizons of oxides and sulfides on the
basin floor.
Summary
The circulation of seawater through oceanic lithosphere
and the associated fluid-rock thermal and chemical
exchange that occurs in the subsurface has a major effect
on the composition of both seawater and oceanic lithosphere. The wide range of different mineralogical and geochemical characteristics of hydrothermal systems is a
result of a combination of source rock composition,
pressure- and temperature-dependent solubility controls,
and possible direct contributions of metals and gases from
subvolcanic magmas. These characteristics provide clues
to the geology and chemical processes that are occurring
in the underlying crust. The heating and subsequent
venting of circulated seawater is the major mechanism
for cooling hot, newly formed oceanic crust. The resulting
chemical exchange includes the leaching and transport of
base and precious metals from the underlying rock to the
seafloor. The mixing of hot, metal-rich hydrothermal
fluids with cold seawater at sites of focused black or white
smoker discharge on the seafloor result in the formation of
volcanogenic massive sulfide deposits that often host
chemosynthetic-based ecosystems. The massive sulfide
deposits that formed in ancient oceans and are now
exposed on land are an important source for Cu, Zn, Pb,
Au, and Ag and are mined for their metal contents. In
the future, hydrothermal sulfide deposits on the modern
seafloor may similarly be exploited for their rich metal
content.
Bibliography
Allen, D. E., and Seyfried, W. E., 2004. Serpentinization and heat
generation: constraints from Lost City and rainbow hydrothermal systems. Geochimica Et Cosmochimica Acta, 68,
1347–1354.
Alt, J. C., 2003. Hydrothermal fluxes at mid-ocean ridges and on
ridge flanks. Comptes Rendus Geoscience, 335, 853–864.
Alt, J. C., and Teagle, D. A. H., 2003. Hydrothermal alteration of
upper oceanic crust formed at a fast-spreading ridge: mineral,
chemical, and isotopic evidence from ODP Site 801. Chemical
Geology, 201, 191–211.
Bach, W., and Frueh-Green, G. L., 2010. Alteration of the oceanic
lithosphere and implications for seafloor processes. Elements,
6, 173–178.
Baker, E. T., 2007. Hydrothermal cooling of midocean ridge axes:
do measured and modeled heat fluxes agree? Earth and Planetary Science Letters, 263, 140–150.
Baker, E. T., and German, C. R., 2004. On the global distribution of
hydrothermal vent fields. Mid-Ocean Ridges, 148, 245–266.
Baker, E. T., Lavelle, J. W., and Massoth, G. J., 1985. Hydrothermal
particle plumes over the southern Juan de Fuca Ridge. Nature,
316, 342–344.
Baker, E. T., Chen, Y. J., and Morgan, J. P., 1996. The relationship
between near-axis hydrothermal cooling and the spreading rate
of mid-ocean ridges. Earth and Planetary Science Letters, 142,
137–145.
Beaulieu, S. E., Baker, E. T., German, C. R., and Maffei, A., 2013.
An authoritative global database for active submarine hydrothermal vent fields. Geochemistry Geophysics Geosystems, 14,
4892–4905.
Becker, K., and von Herzen, R. P., 1996. Pre-Drilling Observations
of Conductive Heat Flow at the TAG Active Mound Using Alvin.
Texas A & M University, Ocean Drilling Program, College Station, TX, USA.
Bemis, K. G., Vonherzen, R. P., and Mottl, M. J., 1993. Geothermal
heat-flux from hydrothermal plumes on the Juan de Fuca Ridge.
Journal of Geophysical Research-Solid Earth, 98, 6351–6365.
354
HYDROTHERMALISM