of the arcs. Nearly all of the volcanoes at the fronts of the
arcs have summit calderas at water depths of less than
1,600 m. This has a major impact on the temperatures of
the hydrothermal fluids due to boiling, and there is abundant visual evidence of phase separation at many of the
vents (Stoffers et al., 1999) (see section on phase separation for further details). Higher-temperature hydrothermal
vents are found mainly in the back-arc rifts or on volcanoes with large and deep calderas, which are below the
boiling depth of the hydrothermal fluids. The Lau Basin
was among the first of the Western Pacific arc- and backarc systems to be recognized as a young ocean basin
formed by the splitting of an island arc (Karig, 1970).
And, one of the first known occurrences of seafloor hydrothermal mineralization anywhere in the oceans was recovered by accident from the northern part of the Lau Basin in
1975, long before the discovery of black smoker deposits
on the East Pacific Rise (Bertine and Keene, 1975). Black
smoker activity has since been found on all of the spreading segments of the Lau Basin and in all of the other backarc spreading centers of the Western Pacific region.
Massive sulfide deposits in volcanic arc/back-arc environments also may contain abundant native gold, Ag-rich
galena (PbS), Pb-As-Sb sulphosalts (including tennantite
and tetrahedrite), stibnite (Sb 2 S 3 ), orpiment (As 2 S 3 ), realgar (AsS), and locally cinnabar (HgS). These differences
reflect the composition of the underlying volcanic rocks,
which commonly included more fractionated suites of
lava (andesite, dacite, rhyolite) and possibly direct contributions of metals from magmatic volatiles.
Sedimentary environments
About 5 % of the world’s active spreading centers are covered by sediment from nearby continental margins
(Hannington et al., 2005). These include sediment-covered
ridges such as Middle Valley and Escanaba Trough on the
Juan de Fuca and Gorda Ridges and rift grabens that have
become major depocenters for sediment (e.g., Guaymas
Basin in the Gulf of California). Subseafloor intrusions
(e.g., sill-sediment complexes) are common and are indicated by high heat flow in the sedimented valleys, where
high-temperature hydrothermal venting is locally focused
around the margins of buried sills (Einsele, 1985;
Zierenberg et al., 1993). The highest-temperature fluids
originate in the volcanic basement where the upper few
hundred meters of fractured basalt is several orders of magnitude more permeable than the overlying sediments and
can accommodate large-scale fluid flow.
Although the hydrothermal fluids originate in the basement, extensive interaction and buffering by sediments
occur en route to the seafloor. The fluids are generally
depleted in metals and enriched in the alkalies, boron,
ammonia, and organic-derived hydrocarbons compared
to seawater-basalt systems. They have a higher pH and
are more reduced than fluids that reacted only with basalt.
Sill intrusions into the sediments produce organic matter
derived hydrocarbons, mainly methane, as well as CO 2
and H 2 . Lower venting temperatures compared to
sediment-free mid-ocean ridges result from subseafloor
mixing and interaction with the sediments; together with
the higher pH, this accounts for the low metal concentrations at the seafloor (German and Von Damm, 2004).
The high pH and alkalinity of the vent fluids, combined
with high SiO 2 concentrations, results in the precipitation
of abundant Mg-silicates (e.g., talc and stevensite) and
carbonate during mixing with seawater, and chimneys
composed only of anhydrite, barite, amorphous silica,
smectite, or carbonate are common. Calcite, dolomite,
and manganiferous carbonates are also important constituents of the hydrothermal precipitates. Products of the thermal degradation of organic matter, including liquid
petroleum and solid bitumens, are commonly preserved
in the hydrothermal precipitates (Peter et al., 1991), and
higher hydrocarbons (e.g., C1 through C6) are also present
in the vent fluids (Simoneit, 1988). The high concentrations of ammonia in the vent fluids, which are several
orders of magnitude higher than in typical mid-ocean
ridge black smokers (e.g., 10–15 mmol/kg at Guaymas
Basin compared to <0.01 mmol/kg NH 4 at 21
N EPR),
originate from the breakdown of N-bearing organic compounds in the sediments, and this produces a strong pH
buffer via the reaction NH 3 + H
+
¼ NH 4
+
.
The massive sulfide deposits that form at sedimentcovered mid-ocean ridges (e.g., Escanaba Trough, Southern Gorda Ridge, and Guaymas Basin, Gulf of California)
are locally more complex than the black smoker chimneys
at sediment-starved mid-ocean ridges. Hydrothermal
fluids that interact with continent-derived turbiditic sediments leach Pb, Ba, and other elements from feldspar
and other detrital components in the sediments, and the
sulfide deposits include such minerals as arsenopyrite
(FeAsS), tetrahedrite (Cu 12 Sb 4 S 13 ), loellingite (FeAs 2 ),
stannite (Cu 2 FeSnS 4 ), jordanite (Pb 14 As 7 S 24 ), native bismuth, and other Pb-As-Sb sulphosalts (Koski et al.,
1988; Zierenberg et al., 1993). Due to the strongly reduced
nature of hydrothermal fluids that have reacted with
organic material in the sediment, pyrrhotite is also a common constituent of these deposits.
Many of the characteristics of hydrothermal systems
within sedimented ridge environments are also observed
in sedimented back-arc basins, such as in the Okinawa
Trough (Ishibashi et al., 2015). The basin sediments are
dominated by terrigenous material, commonly with a high
organic matter content. This can lead to concentrations of
hydrocarbons, including gas hydrates, in the sedimentfilled basins (Glasby and Notsu, 2003). Abundant carbonate minerals, similar to those found on sediment-covered
mid-ocean ridges, also occur in sedimented back-arc environments (Nakashima et al., 1995).
Plumes
A characteristic feature of high-temperature hydrothermal
vents is hydrothermal plumes that form above the active
vent-sources. The plumes, consisting of a mixture of
HYDROTHERMALISM
353
arcs have summit calderas at water depths of less than
1,600 m. This has a major impact on the temperatures of
the hydrothermal fluids due to boiling, and there is abundant visual evidence of phase separation at many of the
vents (Stoffers et al., 1999) (see section on phase separation for further details). Higher-temperature hydrothermal
vents are found mainly in the back-arc rifts or on volcanoes with large and deep calderas, which are below the
boiling depth of the hydrothermal fluids. The Lau Basin
was among the first of the Western Pacific arc- and backarc systems to be recognized as a young ocean basin
formed by the splitting of an island arc (Karig, 1970).
And, one of the first known occurrences of seafloor hydrothermal mineralization anywhere in the oceans was recovered by accident from the northern part of the Lau Basin in
1975, long before the discovery of black smoker deposits
on the East Pacific Rise (Bertine and Keene, 1975). Black
smoker activity has since been found on all of the spreading segments of the Lau Basin and in all of the other backarc spreading centers of the Western Pacific region.
Massive sulfide deposits in volcanic arc/back-arc environments also may contain abundant native gold, Ag-rich
galena (PbS), Pb-As-Sb sulphosalts (including tennantite
and tetrahedrite), stibnite (Sb 2 S 3 ), orpiment (As 2 S 3 ), realgar (AsS), and locally cinnabar (HgS). These differences
reflect the composition of the underlying volcanic rocks,
which commonly included more fractionated suites of
lava (andesite, dacite, rhyolite) and possibly direct contributions of metals from magmatic volatiles.
Sedimentary environments
About 5 % of the world’s active spreading centers are covered by sediment from nearby continental margins
(Hannington et al., 2005). These include sediment-covered
ridges such as Middle Valley and Escanaba Trough on the
Juan de Fuca and Gorda Ridges and rift grabens that have
become major depocenters for sediment (e.g., Guaymas
Basin in the Gulf of California). Subseafloor intrusions
(e.g., sill-sediment complexes) are common and are indicated by high heat flow in the sedimented valleys, where
high-temperature hydrothermal venting is locally focused
around the margins of buried sills (Einsele, 1985;
Zierenberg et al., 1993). The highest-temperature fluids
originate in the volcanic basement where the upper few
hundred meters of fractured basalt is several orders of magnitude more permeable than the overlying sediments and
can accommodate large-scale fluid flow.
Although the hydrothermal fluids originate in the basement, extensive interaction and buffering by sediments
occur en route to the seafloor. The fluids are generally
depleted in metals and enriched in the alkalies, boron,
ammonia, and organic-derived hydrocarbons compared
to seawater-basalt systems. They have a higher pH and
are more reduced than fluids that reacted only with basalt.
Sill intrusions into the sediments produce organic matter
derived hydrocarbons, mainly methane, as well as CO 2
and H 2 . Lower venting temperatures compared to
sediment-free mid-ocean ridges result from subseafloor
mixing and interaction with the sediments; together with
the higher pH, this accounts for the low metal concentrations at the seafloor (German and Von Damm, 2004).
The high pH and alkalinity of the vent fluids, combined
with high SiO 2 concentrations, results in the precipitation
of abundant Mg-silicates (e.g., talc and stevensite) and
carbonate during mixing with seawater, and chimneys
composed only of anhydrite, barite, amorphous silica,
smectite, or carbonate are common. Calcite, dolomite,
and manganiferous carbonates are also important constituents of the hydrothermal precipitates. Products of the thermal degradation of organic matter, including liquid
petroleum and solid bitumens, are commonly preserved
in the hydrothermal precipitates (Peter et al., 1991), and
higher hydrocarbons (e.g., C1 through C6) are also present
in the vent fluids (Simoneit, 1988). The high concentrations of ammonia in the vent fluids, which are several
orders of magnitude higher than in typical mid-ocean
ridge black smokers (e.g., 10–15 mmol/kg at Guaymas
Basin compared to <0.01 mmol/kg NH 4 at 21
N EPR),
originate from the breakdown of N-bearing organic compounds in the sediments, and this produces a strong pH
buffer via the reaction NH 3 + H
+
¼ NH 4
+
.
The massive sulfide deposits that form at sedimentcovered mid-ocean ridges (e.g., Escanaba Trough, Southern Gorda Ridge, and Guaymas Basin, Gulf of California)
are locally more complex than the black smoker chimneys
at sediment-starved mid-ocean ridges. Hydrothermal
fluids that interact with continent-derived turbiditic sediments leach Pb, Ba, and other elements from feldspar
and other detrital components in the sediments, and the
sulfide deposits include such minerals as arsenopyrite
(FeAsS), tetrahedrite (Cu 12 Sb 4 S 13 ), loellingite (FeAs 2 ),
stannite (Cu 2 FeSnS 4 ), jordanite (Pb 14 As 7 S 24 ), native bismuth, and other Pb-As-Sb sulphosalts (Koski et al.,
1988; Zierenberg et al., 1993). Due to the strongly reduced
nature of hydrothermal fluids that have reacted with
organic material in the sediment, pyrrhotite is also a common constituent of these deposits.
Many of the characteristics of hydrothermal systems
within sedimented ridge environments are also observed
in sedimented back-arc basins, such as in the Okinawa
Trough (Ishibashi et al., 2015). The basin sediments are
dominated by terrigenous material, commonly with a high
organic matter content. This can lead to concentrations of
hydrocarbons, including gas hydrates, in the sedimentfilled basins (Glasby and Notsu, 2003). Abundant carbonate minerals, similar to those found on sediment-covered
mid-ocean ridges, also occur in sedimented back-arc environments (Nakashima et al., 1995).
Plumes
A characteristic feature of high-temperature hydrothermal
vents is hydrothermal plumes that form above the active
vent-sources. The plumes, consisting of a mixture of
HYDROTHERMALISM
353