not only the absolute concentration but also its ratio
to chloride must be compared to the local ambient
sea water value. Hydrothermal fluids are also very
reducing, often containing large amounts of H 2 S, H 2 ,
and CH 4 . In most, but not all hydrothermal fluids
(fluids that are formed immediately after a volcanic
eruption being a major exception), Li, K, Rb, Cs, Ca,
Sr, Si, the transition metals in reduced forms, including Fe, Mn, Cu, and Zn, are enriched in hydrothermal fluids, the cause being water–rock
interaction. Sodium may be either enriched or depleted with respect to the chloride content of the
fluids, the loss being due to albitization, with a
concomitant gain in the calcium-content of the
fluids. It is the loss of magnesium to form magnesiumhydroxy silicates that, along with other aluminosilicate reactions, generates and then maintains
the acidity of the fluids. In the case of sulfate, some is
lost as anhydrite (CaSO 4 ) in the downflow zone,
while some is reduced to sulfide (as H 2 S).
The compositions of fluids exiting from a single
hydrothermal vent may vary widely over time. The
cases in which this has been observed are increasing,
and are usually associated with vents where a known
magmatic event has occurred. The variation in the
composition of a single vent can vary from vapor to
brine, and may encompass almost the entire range of
known compositions. In contrast, some sites of
venting are known where the fluid compositions have
been stable during the time interval over which they
have been sampled. None of these vents with constant compositions have known ‘magmatic’ or ‘tectonic’ events associated with them, although several
of these sites have now been sampled over times of
B15 years. Presumably, these vents are in a period of
steady-state venting, although we do not have adequate constraints to determine how long after an
eruptive event, or at what spreading rates, this may
occur. While our data has increased on the temperature and chemical characteristics of vents during
their early histories, few data exist for their waning
stage(s). Presumably all this variability – or lack
thereof – and the timescale(s) on which it occurs can
ultimately be tied to the nature of the heat source at a
given site. Aside from the most general characteristics related to the presence or absence of a seismic
low-velocity zone, and the depth at which it occurs,
little is known about the specifics of the heat sources
at sites that are hydrothermally active, especially in
contrast to those that are not.
The composition of a hydrothermal fluid cannot
be correlated to, or predicted by, such known physical parameters as the depth of the seafloor on which
it occurs, the spreading rate of the ridge on which it
occurs, and so on. As the fluid compositions in most
cases are probably due to the achievement of steadystate, if not true thermodynamic equilibrium, of the
fluids with the rock substrate, some of the measured
compositions can be tied to either the measured exit
temperature or the presumed in situ conditions
within the hydrothermal system itself. While there
has been some success, especially recently, with
understanding the chemical controls on these systems
using thermodynamic modeling, a major limitation
in many cases remains the proximity to the critical
point of both pure water and sea water.
The Flux Question
One of the driving questions for the study of seafloor
hydrothermal systems is to understand their net flux
to the ocean in terms of energy (thermal and chemical) and mass. The thermal, or heat, energy they
carry is believed to be relatively well constrained, as
various independent ways of estimating this flux
provide similar values. The mass of chemicals they
add and/or remove remains problematic. In some
cases whether hydrothermal activity is a net source
or sink for particular elements remains unresolved as
well. In addition to absolute concentrations, or
concentrations normalized to the chloride content,
the isotopic signature of various species can also be
used to constrain the source and sink terms, as well
as helping to identify the important processes occurring within the hydrothermal circulation cell.
Summary and Conclusions
Hydrothermal venting along the global midocean
ridge system is a process that is widespread
throughout the ocean basins and impacts all of the
oceanographic disciplines. Our studies of these systems remain in their infancy, however, and we do not
yet completely understand the controls on the
chemistry of these systems, the controls on the locations of individual vent sites, their overall importance to ocean chemistry, productivity, and
circulation, and their net effects on the structure and
composition of the oceanic crust.
See also
Hydrothermal Vent Deposits. Mid-Ocean Ridge
Geochemistry and Petrology.
Further Reading
Cowen JP and Baker ET (1998) Topical studies in
oceanography: detection of and response to mid-ocean
HYDROTHERMAL VENT FLUIDS, CHEMISTRY OF 87
to chloride must be compared to the local ambient
sea water value. Hydrothermal fluids are also very
reducing, often containing large amounts of H 2 S, H 2 ,
and CH 4 . In most, but not all hydrothermal fluids
(fluids that are formed immediately after a volcanic
eruption being a major exception), Li, K, Rb, Cs, Ca,
Sr, Si, the transition metals in reduced forms, including Fe, Mn, Cu, and Zn, are enriched in hydrothermal fluids, the cause being water–rock
interaction. Sodium may be either enriched or depleted with respect to the chloride content of the
fluids, the loss being due to albitization, with a
concomitant gain in the calcium-content of the
fluids. It is the loss of magnesium to form magnesiumhydroxy silicates that, along with other aluminosilicate reactions, generates and then maintains
the acidity of the fluids. In the case of sulfate, some is
lost as anhydrite (CaSO 4 ) in the downflow zone,
while some is reduced to sulfide (as H 2 S).
The compositions of fluids exiting from a single
hydrothermal vent may vary widely over time. The
cases in which this has been observed are increasing,
and are usually associated with vents where a known
magmatic event has occurred. The variation in the
composition of a single vent can vary from vapor to
brine, and may encompass almost the entire range of
known compositions. In contrast, some sites of
venting are known where the fluid compositions have
been stable during the time interval over which they
have been sampled. None of these vents with constant compositions have known ‘magmatic’ or ‘tectonic’ events associated with them, although several
of these sites have now been sampled over times of
B15 years. Presumably, these vents are in a period of
steady-state venting, although we do not have adequate constraints to determine how long after an
eruptive event, or at what spreading rates, this may
occur. While our data has increased on the temperature and chemical characteristics of vents during
their early histories, few data exist for their waning
stage(s). Presumably all this variability – or lack
thereof – and the timescale(s) on which it occurs can
ultimately be tied to the nature of the heat source at a
given site. Aside from the most general characteristics related to the presence or absence of a seismic
low-velocity zone, and the depth at which it occurs,
little is known about the specifics of the heat sources
at sites that are hydrothermally active, especially in
contrast to those that are not.
The composition of a hydrothermal fluid cannot
be correlated to, or predicted by, such known physical parameters as the depth of the seafloor on which
it occurs, the spreading rate of the ridge on which it
occurs, and so on. As the fluid compositions in most
cases are probably due to the achievement of steadystate, if not true thermodynamic equilibrium, of the
fluids with the rock substrate, some of the measured
compositions can be tied to either the measured exit
temperature or the presumed in situ conditions
within the hydrothermal system itself. While there
has been some success, especially recently, with
understanding the chemical controls on these systems
using thermodynamic modeling, a major limitation
in many cases remains the proximity to the critical
point of both pure water and sea water.
The Flux Question
One of the driving questions for the study of seafloor
hydrothermal systems is to understand their net flux
to the ocean in terms of energy (thermal and chemical) and mass. The thermal, or heat, energy they
carry is believed to be relatively well constrained, as
various independent ways of estimating this flux
provide similar values. The mass of chemicals they
add and/or remove remains problematic. In some
cases whether hydrothermal activity is a net source
or sink for particular elements remains unresolved as
well. In addition to absolute concentrations, or
concentrations normalized to the chloride content,
the isotopic signature of various species can also be
used to constrain the source and sink terms, as well
as helping to identify the important processes occurring within the hydrothermal circulation cell.
Summary and Conclusions
Hydrothermal venting along the global midocean
ridge system is a process that is widespread
throughout the ocean basins and impacts all of the
oceanographic disciplines. Our studies of these systems remain in their infancy, however, and we do not
yet completely understand the controls on the
chemistry of these systems, the controls on the locations of individual vent sites, their overall importance to ocean chemistry, productivity, and
circulation, and their net effects on the structure and
composition of the oceanic crust.
See also
Hydrothermal Vent Deposits. Mid-Ocean Ridge
Geochemistry and Petrology.
Further Reading
Cowen JP and Baker ET (1998) Topical studies in
oceanography: detection of and response to mid-ocean
HYDROTHERMAL VENT FLUIDS, CHEMISTRY OF 87