Geoscience Reference
In-Depth Information
water
(7.6)
, and that it is totally devoid of magnesium. This can be explained by reactions
of seawater with common basalt minerals:
Mg
2
+
2H
+
Mg
2
SiO
4
+
Si(OH)
4
+
H
2
O
+
⇔
Mg
3
Si
2
O
5
(OH)
4
+
(olivine)
(solution)
(solution)
(serpentine)
(solution)
whose effect is to exchange the abundant Mg
2
+
in seawater for protons and therefore to
reduce its alkalinity. In such reactions, the silica in the solution comes from dissolution of
various enclosing igneous rocks (basalt, gabbro).
The redox reactions between seawater and hot basaltic rock first explains why hydro-
gen, methane, and ammonia continuously seep out from mid-ocean ridges and in aquifers
located in ultramafic rocks, notably those of ophiolitic environments. The most important
reactions are probably the oxidation of the ferrous component of Fe-olivine (fayalite) by
water and its complement, the serpentinization of Mg-olivine (forsterite), which we dis-
cussed in the previous chapter; and the reaction of hydrogen with CO
2
and N
2
to produce
methane and ammonia.
As the state of oxidation of the oceanic crust at the time it is subducted also dictates the
long-term evolution of the redox state of the mantle, similar reactions involving iron and
sulfur are also important. The two following half-reactions of oxidation-reduction:
Fe
2
+
⇔
Fe
3
+
+
e
−
(10.14)
S
2
−
+
SO
2
4
8H
+
+
8e
−
4H
2
O
⇔
+
(10.15)
can be represented schematically by oxidation in an acidic medium of the ferrous iron of
magmatic rocks offset by reduction of marine sulfate into sulfide ions (see above for the
origin of H
+
ions). By multiplying the first equation by eight and subtracting the second,
we obtain:
SO
2
4
8H
+
8Fe
2
+
⇔
S
2
−
8Fe
3
+
+
+
+
4H
2
O
+
(seawater)
(seawater)
(basalt)
(solution)
(solution)
Ferric ion precipitates as hydroxide Fe(OH)
3
and S
2
−
as sulfide minerals of iron and
copper (chalcopyrite), and zinc (sphalerite) forming the mineralized submarine chim-
neys of the black smokers. Sulfur isotopes indicate, however, that, within the mid-ocean
ridge hydrothermal systems, sulfur derived from seawater through the previous reaction is
dominated by sulfur leached from the basaltic rocks. The sulfate-reduction reaction also
transforms part of the ferrous iron of the oceanic crust into ferric iron stored as oxides
or silicates (epidote). Subduction of the oceanic crust therefore controls not only mantle
hydration but also its relative proportions of ferric and ferrous iron, and its electrical con-
ductivity. The Earth's mantle, into which enormous amounts of oxidized crust have been
recycled over the Earth's history, is thus far more oxidized than the Moon's mantle.
