Role of hydrothermal vent fluids for the formation
of ore deposits
When the hot metal-rich hydrothermal fluids mix with
ambient seawater, either in the sub-seafloor or when they
emanate at the seafloor, they precipitate large amounts of
its metal and sulfur load as minerals, forming massive sulfide deposits, mineral chimneys, and black smoke in the
rising plume (e.g., Hannington et al., 2005). The steep
temperature and geochemical gradients in the mixing
zones lead to a sequence of precipitation of sulfide minerals and others, such as anhydrite, carbonates, silicates,
and finally, in more oxidized zones, oxyhydroxides of Fe
and Mn. These deposits are an important sink of metals
mobilized from the oceanic crust by the hydrothermal
fluids and thus limit the amount of material that is
transported into the oceanic water column. They are very
rich in many valuable metals such as Au, Ag, Cu, and
Zn (e.g., de Ronde et al., 2011), depending on the composition of the ore-forming fluid. Hence, they may represent
important metal resources for our future needs in modern
technologies, although estimated overall quantities of
deposits are small in scale relative to terrestrial metal
resources (Hannington et al., 2011; Hein et al., 2013).
Role of hydrothermal vent fluids for supporting
deep-sea vent communities and the origin of life
Hydrothermal vent fluids do not only feed hydrothermal
ore deposits with precious metals but also nurture rich
hydrothermal ecosystems with their chemical energy and
material released in the fluids mixing with seawater
(e.g., van Dover, 2000). The members of these ecosystems
are both adapted to extreme conditions in the fluids, such
as low pH, high temperatures, and high metal contents,
and are able to sustain bioproductivity without any light
but based on the material and energy available in the vent
fluids (e.g., Tunnicliffe, 1992). During chemosynthesis,
microorganisms gain energy from redox reactions in the
mixing zone to build up biomass and feed a rich community of grazers, symbiotic mussels, shrimps, tube worms,
and other organisms. While the type and composition of
the fluid has an impact on the evolution of the ecosystem,
at the same time the organisms change the fluid composition in the mixing zone. They mediate redox reactions,
take up chemical components, and excrete others that
may influence the (bio)chemical reactivity and fluxes of
these metals. Hydrothermal fluids are also discussed to
be potential sites where life on Earth may have evolved.
While in early Earth history the surface of the Earth was
a rather hostile place, the seafloor was a more protected
place with hydrothermal fluids providing energy and
chemical components suitable to sustain microbial life
(Martin et al., 2008). However, also in the search for extraterrestrial life, the potential existence of hydrothermal
fluids plays a major role. Astrobiological research focuses
on geothermally active bodies in our solar system in which
water exists or may have existed, such as Mars and Venus,
or the icy satellites of the giant planets in the outer solar
system, such as Titan and Europa (e.g., Vance et al.,
2007). Hence, knowledge on the functions of hydrothermal fluids for life on Earth is an important prerequisite
for the search for life elsewhere in our solar system.
Role of hydrothermal vent fluids for heat and
energy transfer from the oceanic crust
into the ocean
Hydrothermal fluids are a crucial medium for the
exchange of heat and matter between the oceanic crust
and the ocean (Kadko et al., 1995; Baker, 2007). The globally widespread occurrence of hydrothermal fluid emanations on the seafloor, which has been operating for most of
Earth’s history, makes hydrothermal input one of the
main, but poorly quantified, sources of components such
as trace metals into the global oceans. The magnitude of
hydrothermal input seems to be on the order of riverine
input for some elements, such as Mn (Edmond et al.,
1979; Elderfield and Schulz, 1996). Among the many
problems that exist for a more precise assessment of
hydrothermal element fluxes to the ocean are the unknown
total number and size of hydrothermal vent fields and their
chemical variability, the role of poorly quantified or
undiscovered off-axis flow, and an incomplete understanding of the transformation processes of fluid components during mixing of fluid and ambient seawater and
within the rising and dispersing hydrothermal plume. Formation of mineral deposits, oxidation of Fe and Mn in the
plume, and scavenging of trace metals on Mn and Fe
oxides in the hydrothermal plumes (German et al., 1997)
significantly reduce the hydrothermal metal fluxes of
many elements, for example. However, modeling and
chemical speciation studies have shown that for Fe, the
hydrothermal metal flux must be significantly higher than
previously assumed, probably contributing to the biogeochemical cycling of Fe in the oceans (Tagliabue et al.,
2010). While the exact mechanisms of metal transport
and transformation from the vent sites into the ocean are
still unclear, several studies hint at a strong role of colloidal sulfides or hydroxides and organic metal complexes
(Sander and Koschinsky, 2011).
Summary and conclusions
Hydrothermal fluid emanations at the seafloor are a widespread phenomenon in volcanic, magmatic, or tectonically
active areas in the world oceans, such as mid-ocean ridges
and submarine volcanoes. Their wide range in emanation
temperatures (up to >400
C), chemical composition
(up to millimolar concentrations in reduced gases and
metals), and temporal variability reflect the specific nature
(such as water depth and host rock) of the site location.
Hydrothermal fluids are important on a global scale for
the formation of metal ore deposits on the seafloor, for
nurturing chemosynthetic ecosystems specifically adapted
to extreme conditions, and for metal fluxes into the water
column and the chemical budget of the ocean. However,
the exact nature and quantity of the related processes is
342
HYDROTHERMAL VENT FLUIDS (SEAFLOOR)
of ore deposits
When the hot metal-rich hydrothermal fluids mix with
ambient seawater, either in the sub-seafloor or when they
emanate at the seafloor, they precipitate large amounts of
its metal and sulfur load as minerals, forming massive sulfide deposits, mineral chimneys, and black smoke in the
rising plume (e.g., Hannington et al., 2005). The steep
temperature and geochemical gradients in the mixing
zones lead to a sequence of precipitation of sulfide minerals and others, such as anhydrite, carbonates, silicates,
and finally, in more oxidized zones, oxyhydroxides of Fe
and Mn. These deposits are an important sink of metals
mobilized from the oceanic crust by the hydrothermal
fluids and thus limit the amount of material that is
transported into the oceanic water column. They are very
rich in many valuable metals such as Au, Ag, Cu, and
Zn (e.g., de Ronde et al., 2011), depending on the composition of the ore-forming fluid. Hence, they may represent
important metal resources for our future needs in modern
technologies, although estimated overall quantities of
deposits are small in scale relative to terrestrial metal
resources (Hannington et al., 2011; Hein et al., 2013).
Role of hydrothermal vent fluids for supporting
deep-sea vent communities and the origin of life
Hydrothermal vent fluids do not only feed hydrothermal
ore deposits with precious metals but also nurture rich
hydrothermal ecosystems with their chemical energy and
material released in the fluids mixing with seawater
(e.g., van Dover, 2000). The members of these ecosystems
are both adapted to extreme conditions in the fluids, such
as low pH, high temperatures, and high metal contents,
and are able to sustain bioproductivity without any light
but based on the material and energy available in the vent
fluids (e.g., Tunnicliffe, 1992). During chemosynthesis,
microorganisms gain energy from redox reactions in the
mixing zone to build up biomass and feed a rich community of grazers, symbiotic mussels, shrimps, tube worms,
and other organisms. While the type and composition of
the fluid has an impact on the evolution of the ecosystem,
at the same time the organisms change the fluid composition in the mixing zone. They mediate redox reactions,
take up chemical components, and excrete others that
may influence the (bio)chemical reactivity and fluxes of
these metals. Hydrothermal fluids are also discussed to
be potential sites where life on Earth may have evolved.
While in early Earth history the surface of the Earth was
a rather hostile place, the seafloor was a more protected
place with hydrothermal fluids providing energy and
chemical components suitable to sustain microbial life
(Martin et al., 2008). However, also in the search for extraterrestrial life, the potential existence of hydrothermal
fluids plays a major role. Astrobiological research focuses
on geothermally active bodies in our solar system in which
water exists or may have existed, such as Mars and Venus,
or the icy satellites of the giant planets in the outer solar
system, such as Titan and Europa (e.g., Vance et al.,
2007). Hence, knowledge on the functions of hydrothermal fluids for life on Earth is an important prerequisite
for the search for life elsewhere in our solar system.
Role of hydrothermal vent fluids for heat and
energy transfer from the oceanic crust
into the ocean
Hydrothermal fluids are a crucial medium for the
exchange of heat and matter between the oceanic crust
and the ocean (Kadko et al., 1995; Baker, 2007). The globally widespread occurrence of hydrothermal fluid emanations on the seafloor, which has been operating for most of
Earth’s history, makes hydrothermal input one of the
main, but poorly quantified, sources of components such
as trace metals into the global oceans. The magnitude of
hydrothermal input seems to be on the order of riverine
input for some elements, such as Mn (Edmond et al.,
1979; Elderfield and Schulz, 1996). Among the many
problems that exist for a more precise assessment of
hydrothermal element fluxes to the ocean are the unknown
total number and size of hydrothermal vent fields and their
chemical variability, the role of poorly quantified or
undiscovered off-axis flow, and an incomplete understanding of the transformation processes of fluid components during mixing of fluid and ambient seawater and
within the rising and dispersing hydrothermal plume. Formation of mineral deposits, oxidation of Fe and Mn in the
plume, and scavenging of trace metals on Mn and Fe
oxides in the hydrothermal plumes (German et al., 1997)
significantly reduce the hydrothermal metal fluxes of
many elements, for example. However, modeling and
chemical speciation studies have shown that for Fe, the
hydrothermal metal flux must be significantly higher than
previously assumed, probably contributing to the biogeochemical cycling of Fe in the oceans (Tagliabue et al.,
2010). While the exact mechanisms of metal transport
and transformation from the vent sites into the ocean are
still unclear, several studies hint at a strong role of colloidal sulfides or hydroxides and organic metal complexes
(Sander and Koschinsky, 2011).
Summary and conclusions
Hydrothermal fluid emanations at the seafloor are a widespread phenomenon in volcanic, magmatic, or tectonically
active areas in the world oceans, such as mid-ocean ridges
and submarine volcanoes. Their wide range in emanation
temperatures (up to >400
C), chemical composition
(up to millimolar concentrations in reduced gases and
metals), and temporal variability reflect the specific nature
(such as water depth and host rock) of the site location.
Hydrothermal fluids are important on a global scale for
the formation of metal ore deposits on the seafloor, for
nurturing chemosynthetic ecosystems specifically adapted
to extreme conditions, and for metal fluxes into the water
column and the chemical budget of the ocean. However,
the exact nature and quantity of the related processes is
342
HYDROTHERMAL VENT FLUIDS (SEAFLOOR)