461
reduction to H 2 S. Although reduction of seawater
SO 4
2contributes some H 2 S to the hydrothermal
fluid, but most of the reduced S is derived from
the rock. The fluids become strongly reduced by
reaction of igneous pyrrhotite to secondary
pyrite and oxidation of Fe
2+
to Fe
3+
in the basalt
(Alt 1995).
This highly corrosive fluid is now capable of
leaching elements such as Li, K, Rb, Ca, Ba, the
transition metals Fe, Mn, Cu, Zn, together with
Au, Ag and some Si from the oceanic crust (Mottl
1983; Table 13.2). Metals and S are leached mainly
from immiscible sulfide droplets trapped in the
crystallized magma (e.g., Keays 1987; Shanks
2001). The metals are transported as chloride
complexes at high temperatures and, in some
cases, as bisulfide complexes (in particular Au) at
lower temperatures.
Due to its buoyancy at high temperatures, the
hydrothermal fluid rises rapidly from the deepseated reaction zone to the seafloor along major
faults and fractures within the rift valley or close
to the flanks of the rift. In particular the intersections of faults running parallel and perpendicular to the ridge axis are the loci of highvelocity discharge, black smokers and massive
sulfide mounds. The sulfide precipitation within
the upflow zone (stockwork) and at the seafloor
(massive sulfides) is mainly a consequence of
mixing of high-temperature (ca. 350°C), metal-rich
hydrothermal fluids with cold (about 2°C),
oxygen-bearing seawater.
Temperature
Basalt
Seawater /
Hydrothermal Fluid
< 150°C
K, Rb, Li,
←
B,
18
O
> 70°C
Mg
←
> 350°C
Ca
←
>> 150°C
→
Fe, Mn, Zn, Cu, Ba,
Si, Ca, Rb, Li, K, B,
H
+
,
18
O, S
all T
H 2 O, CO 2 ,
←
Na
3.5 km
Seafloor
Magma Chamber
Fig. 13.3 Seismic profile showing the top of a high-level magma chamber at a depth of 3.5 km below the seafloor at
a spreading center in the southwest Pacific (after Collier and Sinha 1992; provided by M.C. Sinha).
Table 13.2 Temperature dependence of water/rock interaction in the oceanic crust.
13.1
Hydrothermal Convection and Generation of Hydrohermal Fluids at Mid-Ocean Ridges
reduction to H 2 S. Although reduction of seawater
SO 4
2contributes some H 2 S to the hydrothermal
fluid, but most of the reduced S is derived from
the rock. The fluids become strongly reduced by
reaction of igneous pyrrhotite to secondary
pyrite and oxidation of Fe
2+
to Fe
3+
in the basalt
(Alt 1995).
This highly corrosive fluid is now capable of
leaching elements such as Li, K, Rb, Ca, Ba, the
transition metals Fe, Mn, Cu, Zn, together with
Au, Ag and some Si from the oceanic crust (Mottl
1983; Table 13.2). Metals and S are leached mainly
from immiscible sulfide droplets trapped in the
crystallized magma (e.g., Keays 1987; Shanks
2001). The metals are transported as chloride
complexes at high temperatures and, in some
cases, as bisulfide complexes (in particular Au) at
lower temperatures.
Due to its buoyancy at high temperatures, the
hydrothermal fluid rises rapidly from the deepseated reaction zone to the seafloor along major
faults and fractures within the rift valley or close
to the flanks of the rift. In particular the intersections of faults running parallel and perpendicular to the ridge axis are the loci of highvelocity discharge, black smokers and massive
sulfide mounds. The sulfide precipitation within
the upflow zone (stockwork) and at the seafloor
(massive sulfides) is mainly a consequence of
mixing of high-temperature (ca. 350°C), metal-rich
hydrothermal fluids with cold (about 2°C),
oxygen-bearing seawater.
Temperature
Basalt
Seawater /
Hydrothermal Fluid
< 150°C
K, Rb, Li,
←
B,
18
O
> 70°C
Mg
←
> 350°C
Ca
←
>> 150°C
→
Fe, Mn, Zn, Cu, Ba,
Si, Ca, Rb, Li, K, B,
H
+
,
18
O, S
all T
H 2 O, CO 2 ,
←
Na
3.5 km
Seafloor
Magma Chamber
Fig. 13.3 Seismic profile showing the top of a high-level magma chamber at a depth of 3.5 km below the seafloor at
a spreading center in the southwest Pacific (after Collier and Sinha 1992; provided by M.C. Sinha).
Table 13.2 Temperature dependence of water/rock interaction in the oceanic crust.
13.1
Hydrothermal Convection and Generation of Hydrohermal Fluids at Mid-Ocean Ridges