significant increase in the temperature of the circulating
fluids in proximity to the discharge site. Recent numerical
modeling indicates that about two thirds of the recharge
occurs on-axis by warm (up to 300
C) recharge close to
the discharge sites, while one third occurs as colder and
broader recharge up to several kilometers away from the
axis (Hasenclever et al., 2014).
The following is a brief description of the evolution of
hydrothermal fluid as it circulates through the oceanic lithosphere and the fluid-rock interactions that occur. The
reader is directed to the entry on hydrothermal vent fluids
for a more detailed description of the physical and chemical properties and evolution of hydrothermal fluids.
Hydrothermal recharge
Seawater undergoes important chemical changes when
heated. These changes were first established experimentally by Bischoff and Seyfried (1978) and Bischoff and
Rosenbauer (1988) and are in agreement with the first
measurements of hydrothermal fluids (Edmond et al.,
1979). At ~130
C, anhydrite, which has retrograde solubility, will begin to precipitate from dissolved Ca
2+ and
SO 4
2À in the seawater, removing almost all of the Ca
2+
and about one third of the sulfate. Beginning at about
25
C, magnesium begins to precipitate from seawater in
a brucite-like molecule [Mg(OH) 2 ] as part of the smectite
structure. When seawater is heated slowly in the recharge
zone, Ca
2+ is released at the same rate that Mg
2+ is taken
up (Elderfield et al., 1999) and the pH remains near neutral. At temperatures in excess of 150–250
C, calcium
is then lost from the fluid, when Ca silicates (prehnite,
clinozoisite, tremolite) become stable. If seawater is
heated rapidly (i.e., in chimney walls), protons may be
released in exchange for Mg when magnesium hydroxide
sulfate hydrate (MHSH) may precipitate and the pH drops.
Considerable acidity is also generated when protons are
released into solution in exchange for Ca
2+ . The released
Ca
2+ also combines with the remaining sulfate to form
more anhydrite. Leftover sulfate is thermochemically
reduced to hydrogen sulfide by reaction with ferrous iron
in the host rock. As a result, SO 4
2À concentrations
decrease to nearly zero by the time fluids reach their maximum temperature.
Sequential water-rock reactions along the hydrothermal
fluid flow path are responsible for the exchange of different elements between the fluid and substrate at progressively higher temperatures. At temperatures of about
40–60
C, basaltic glass in the substrate is altered to clay
minerals, zeolites, and Fe oxyhydroxides. Alkali elements
such as K, Rb, and Cs are precipitated out of seawater and
fixed in the rock at temperatures up to about 100
C.
Initially, this exchange is balanced by the leaching of Fe
and Si from volcanic glass. At 150
C, the alkali elements
that were originally fixed in the rocks at low temperatures
are leached, and Ca, Mg, and Na are added to the rock to
maintain charge balance. Magnesium is fixed first as
Mg-rich smectite (<200
C) and then as chlorite
(>200
C). The H
+ released during Ca fixation is partly
consumed by the hydrolysis of the igneous minerals, but
contributes to the overall acidity of the fluids and promotes metal leaching from the rock. At about 250
C,
Na
+ is fixed as albite, partly replacing the remaining plagioclase. In the highest-temperature parts of the hydrothermal system (>350
C), all Mg is lost from the
hydrothermal fluid, and Ca fixation causes the formation
of secondary plagioclase and epidote and the release of
even more H
+ , which is then buffered in a system that
has become rock-dominated (e.g., Seyfried, 1987). At
the same time, reaction of water with Fe-bearing minerals
in the rock (e.g., olivine, pyroxene, Fe sulfides) liberates
metals and H 2 and results in major reduction of the fluids.
Thus, for a system hosted in basaltic/gabbroic oceanic
crust, the highest-temperature fluids are in equilibrium
with an assemblage of plagioclase-epidote-quartzmagnetite-anhydrite-pyrite, referred to as the PEQMAP
buffer (Pester et al., 2012). The major sources of Fe, Mn,
Zn, and Cu in hydrothermal fluids are immiscible sulfides
and ferromagnesian minerals in basalt. Feldspar is a major
source of Pb and Ba especially in more felsic volcanic
rocks associated with hydrothermal systems related to volcanic arcs and in back-arc regions.
High-temperature reaction zones in different
geological environments
The sum of the reactions occurring along the path of
hydrothermal circulation at basaltic mid-ocean ridge segments results in a fluid that is slightly acidic, reduced,
silica- and alkali-rich, and Mg-poor relative to seawater.
Metals and reduced sulfur reach maximum concentrations
in the high-temperature “reaction zone,” at temperatures in
excess of ~400
C and pressures of 400–500 bars (~2 km
depth). Here, fluids in equilibrium with the PEQMAP
buffer assemblage have a narrow range of aH 2 S and
aH 2 , and elements such as Si and Fe become so enriched
that they are major constituents of the hydrothermal fluids.
Under these conditions, SiO 2 concentrations quickly rise
to near quartz saturation. Because quartz solubility is pressure dependent, silica concentrations measured in the
highest-temperature fluids at the seafloor may provide an
indication of the depth of the reaction zone, which commonly reaches the top of the crystallizing subvolcanic
intrusions.
In subduction-related environments, the more felsic
composition of the melts results in higher concentrations
of alkali elements and other trace elements such as Ba
and Pb (Hannington et al., 2005). Higher volatile contents
of the fluids (e.g., CO 2 and SO 2 ) are also common, relative
to mid-ocean ridge fluids, and likely reflect direct contributions from volcanic degassing.
Where ultramafic rocks are exposed to hydrothermal
circulation of seawater, a number of different reactions
lead to fluid compositions that are dramatically different
from those of ordinary mid-ocean ridges, including both
low-pH, metal-rich fluids and high-pH, metal-depleted
348
HYDROTHERMALISM
fluids in proximity to the discharge site. Recent numerical
modeling indicates that about two thirds of the recharge
occurs on-axis by warm (up to 300
C) recharge close to
the discharge sites, while one third occurs as colder and
broader recharge up to several kilometers away from the
axis (Hasenclever et al., 2014).
The following is a brief description of the evolution of
hydrothermal fluid as it circulates through the oceanic lithosphere and the fluid-rock interactions that occur. The
reader is directed to the entry on hydrothermal vent fluids
for a more detailed description of the physical and chemical properties and evolution of hydrothermal fluids.
Hydrothermal recharge
Seawater undergoes important chemical changes when
heated. These changes were first established experimentally by Bischoff and Seyfried (1978) and Bischoff and
Rosenbauer (1988) and are in agreement with the first
measurements of hydrothermal fluids (Edmond et al.,
1979). At ~130
C, anhydrite, which has retrograde solubility, will begin to precipitate from dissolved Ca
2+ and
SO 4
2À in the seawater, removing almost all of the Ca
2+
and about one third of the sulfate. Beginning at about
25
C, magnesium begins to precipitate from seawater in
a brucite-like molecule [Mg(OH) 2 ] as part of the smectite
structure. When seawater is heated slowly in the recharge
zone, Ca
2+ is released at the same rate that Mg
2+ is taken
up (Elderfield et al., 1999) and the pH remains near neutral. At temperatures in excess of 150–250
C, calcium
is then lost from the fluid, when Ca silicates (prehnite,
clinozoisite, tremolite) become stable. If seawater is
heated rapidly (i.e., in chimney walls), protons may be
released in exchange for Mg when magnesium hydroxide
sulfate hydrate (MHSH) may precipitate and the pH drops.
Considerable acidity is also generated when protons are
released into solution in exchange for Ca
2+ . The released
Ca
2+ also combines with the remaining sulfate to form
more anhydrite. Leftover sulfate is thermochemically
reduced to hydrogen sulfide by reaction with ferrous iron
in the host rock. As a result, SO 4
2À concentrations
decrease to nearly zero by the time fluids reach their maximum temperature.
Sequential water-rock reactions along the hydrothermal
fluid flow path are responsible for the exchange of different elements between the fluid and substrate at progressively higher temperatures. At temperatures of about
40–60
C, basaltic glass in the substrate is altered to clay
minerals, zeolites, and Fe oxyhydroxides. Alkali elements
such as K, Rb, and Cs are precipitated out of seawater and
fixed in the rock at temperatures up to about 100
C.
Initially, this exchange is balanced by the leaching of Fe
and Si from volcanic glass. At 150
C, the alkali elements
that were originally fixed in the rocks at low temperatures
are leached, and Ca, Mg, and Na are added to the rock to
maintain charge balance. Magnesium is fixed first as
Mg-rich smectite (<200
C) and then as chlorite
(>200
C). The H
+ released during Ca fixation is partly
consumed by the hydrolysis of the igneous minerals, but
contributes to the overall acidity of the fluids and promotes metal leaching from the rock. At about 250
C,
Na
+ is fixed as albite, partly replacing the remaining plagioclase. In the highest-temperature parts of the hydrothermal system (>350
C), all Mg is lost from the
hydrothermal fluid, and Ca fixation causes the formation
of secondary plagioclase and epidote and the release of
even more H
+ , which is then buffered in a system that
has become rock-dominated (e.g., Seyfried, 1987). At
the same time, reaction of water with Fe-bearing minerals
in the rock (e.g., olivine, pyroxene, Fe sulfides) liberates
metals and H 2 and results in major reduction of the fluids.
Thus, for a system hosted in basaltic/gabbroic oceanic
crust, the highest-temperature fluids are in equilibrium
with an assemblage of plagioclase-epidote-quartzmagnetite-anhydrite-pyrite, referred to as the PEQMAP
buffer (Pester et al., 2012). The major sources of Fe, Mn,
Zn, and Cu in hydrothermal fluids are immiscible sulfides
and ferromagnesian minerals in basalt. Feldspar is a major
source of Pb and Ba especially in more felsic volcanic
rocks associated with hydrothermal systems related to volcanic arcs and in back-arc regions.
High-temperature reaction zones in different
geological environments
The sum of the reactions occurring along the path of
hydrothermal circulation at basaltic mid-ocean ridge segments results in a fluid that is slightly acidic, reduced,
silica- and alkali-rich, and Mg-poor relative to seawater.
Metals and reduced sulfur reach maximum concentrations
in the high-temperature “reaction zone,” at temperatures in
excess of ~400
C and pressures of 400–500 bars (~2 km
depth). Here, fluids in equilibrium with the PEQMAP
buffer assemblage have a narrow range of aH 2 S and
aH 2 , and elements such as Si and Fe become so enriched
that they are major constituents of the hydrothermal fluids.
Under these conditions, SiO 2 concentrations quickly rise
to near quartz saturation. Because quartz solubility is pressure dependent, silica concentrations measured in the
highest-temperature fluids at the seafloor may provide an
indication of the depth of the reaction zone, which commonly reaches the top of the crystallizing subvolcanic
intrusions.
In subduction-related environments, the more felsic
composition of the melts results in higher concentrations
of alkali elements and other trace elements such as Ba
and Pb (Hannington et al., 2005). Higher volatile contents
of the fluids (e.g., CO 2 and SO 2 ) are also common, relative
to mid-ocean ridge fluids, and likely reflect direct contributions from volcanic degassing.
Where ultramafic rocks are exposed to hydrothermal
circulation of seawater, a number of different reactions
lead to fluid compositions that are dramatically different
from those of ordinary mid-ocean ridges, including both
low-pH, metal-rich fluids and high-pH, metal-depleted
348
HYDROTHERMALISM