example, the clay mineral sepiolite (Bonatti et al. 1983) was formed under
experimental conditions from the alteration of peridotite at a temperature of about
174 °C. Peridotite containing olivine fosterite (2Mg 2 SiO 4 ) is transformed into
serpentinite in the presence of water through the following two reactions:
1) Peridotite +2H 2 O = Mg2 ? (release) +SO 2 (release) +H 2 O
2) 2Mg 2 SiO
4+ (forsterite) +3H 2 O = Mg 3 Si 2 O 5 (OH) 4 (antigorite) +MgO (OH)
(brucite).
During the serpentinization process, the fluid becomes enriched in silica (SiO 2 )
as well hydrogen (H 2 ). The acidity (low pH) of the fluid will further increase
during the oxidation of the ferrous iron-bearing components in the minerals. This
is illustrated for the Fe-rich component (ferrosilite) of the pyroxene, which gives
rise to magnetite through the following reaction: 3FeSiO 3 (ferrosilite) +H 2 = Fe 3 O 4 (magnetite) +H 2 (aqueous) +3SiO 2 (aqueous).
Hydrogen and silica release during the alteration of peridotite also increases the
acidity of the fluid so it becomes more corrosive and facilitates the leaching of
metals from the rock (see Chap. 6). The density (3.30 g/cm
3 ) of fresh peridotite is
higher than that of the basalt, however when it is altered (serpentinized), its density
(2.550 g/cm
3 ) decreases below that of basalt (Christensen 1978; Miller and
Christensen 1997). Thus, serpentinization is accompanied by a decrease in the
weight of peridotite.
Emplacement and Distribution of Peridotite
The emplacement of residual and/or the un-fractionated mantle peridotite has been
found in different environments such as on the rift walls and rift-mountains of
magma-starved spreading ridge segments. It is also emplaced in fracture zones and
their transform zones where deep slices of the crust-upper mantle have been
exposed (Fig. 4.5). The classic example of oceanic peridotite exposure is when
tectonically governed processes lead to the formation of a sea floor basement
composed of mantle-derived peridotites and associated mafic intrusions. In
diverging plate boundaries at ridge spreading, which involves tectonic stretching,
the upper mantle peridotite is drawn upwards where it will undergo partial melting.
The emplacement of peridotite is also facilitated by the degree of serpentinization
due to magmatic fluid and seawater circulation in the lithosphere.
Hess (1955) postulated the idea that seismic layer 3 (located underneath the
basalt-dyke-gabbroic complex of the oceanic crust, which is in the lower crust)
consists of serpentinized peridotite. As mentioned before, serpentinization is
accompanied by a decrease in density of the serpentinized peridotite so it becomes
lighter. The upper mantle will rise during convection, but when cooled below
500 °C, it will become hydrated and give rise to the serpentinization of peridotite.
92
4 Sea Floor Rocks
experimental conditions from the alteration of peridotite at a temperature of about
174 °C. Peridotite containing olivine fosterite (2Mg 2 SiO 4 ) is transformed into
serpentinite in the presence of water through the following two reactions:
1) Peridotite +2H 2 O = Mg2 ? (release) +SO 2 (release) +H 2 O
2) 2Mg 2 SiO
4+ (forsterite) +3H 2 O = Mg 3 Si 2 O 5 (OH) 4 (antigorite) +MgO (OH)
(brucite).
During the serpentinization process, the fluid becomes enriched in silica (SiO 2 )
as well hydrogen (H 2 ). The acidity (low pH) of the fluid will further increase
during the oxidation of the ferrous iron-bearing components in the minerals. This
is illustrated for the Fe-rich component (ferrosilite) of the pyroxene, which gives
rise to magnetite through the following reaction: 3FeSiO 3 (ferrosilite) +H 2 = Fe 3 O 4 (magnetite) +H 2 (aqueous) +3SiO 2 (aqueous).
Hydrogen and silica release during the alteration of peridotite also increases the
acidity of the fluid so it becomes more corrosive and facilitates the leaching of
metals from the rock (see Chap. 6). The density (3.30 g/cm
3 ) of fresh peridotite is
higher than that of the basalt, however when it is altered (serpentinized), its density
(2.550 g/cm
3 ) decreases below that of basalt (Christensen 1978; Miller and
Christensen 1997). Thus, serpentinization is accompanied by a decrease in the
weight of peridotite.
Emplacement and Distribution of Peridotite
The emplacement of residual and/or the un-fractionated mantle peridotite has been
found in different environments such as on the rift walls and rift-mountains of
magma-starved spreading ridge segments. It is also emplaced in fracture zones and
their transform zones where deep slices of the crust-upper mantle have been
exposed (Fig. 4.5). The classic example of oceanic peridotite exposure is when
tectonically governed processes lead to the formation of a sea floor basement
composed of mantle-derived peridotites and associated mafic intrusions. In
diverging plate boundaries at ridge spreading, which involves tectonic stretching,
the upper mantle peridotite is drawn upwards where it will undergo partial melting.
The emplacement of peridotite is also facilitated by the degree of serpentinization
due to magmatic fluid and seawater circulation in the lithosphere.
Hess (1955) postulated the idea that seismic layer 3 (located underneath the
basalt-dyke-gabbroic complex of the oceanic crust, which is in the lower crust)
consists of serpentinized peridotite. As mentioned before, serpentinization is
accompanied by a decrease in density of the serpentinized peridotite so it becomes
lighter. The upper mantle will rise during convection, but when cooled below
500 °C, it will become hydrated and give rise to the serpentinization of peridotite.
92
4 Sea Floor Rocks