J.-L. Charlou et al.
90
(Table 5.1) and for which accurate flow calculations remain to be made. Diffuse low
temperature fluids enriched in H 2 and CH 4 are emitted from many diapiric serpentized seamounts. These emissions occur at various depths (up to 4,100 m at Ashadze
at 13° N). In all cases, the fluids have a relatively uniform composition, controlled
by the phase separation process. They are derived from seawater interaction with
mantle peridotites, as many serpentized rocks are found at the outcrop.
This table indicates the mole percentages of each gas (from Charlou et al. 2010).
What Laboratory Experimentation Tells Us
The physical and chemical conditions which lead to the formation of natural hydrogen in the ocean environment can be reproduced in the laboratory (Fig. 5.2). An experimental approach enables us to independently survey and quantify the effect of
each of the important parameters (temperature, pressure, duration, chemistry of seawater, chemistry and mineralogy of solid reagents, mineral grain size, water-rock
interaction, etc.) involved in the natural process. However, laboratory experiments
have a time scale around 10,000 times shorter than that of the equivalent process
in the natural environment. For this reason, experiments are generally carried out
in optimal reaction conditions, for small-scale systems (finely ground solids, rock
samples measuring only centimetres) and water quantities that are often higher than
those available in the natural environment. Thanks to experimental work performed
on interactions between peridotites and seawater between 200 and 450 °C and between 0.5 and 3 kilobars (1 kilobar = around 1,000 times atmospheric pressure), the
complexity of natural chemical reactions resulting in hydrogen production has been
greatly elucidated. The rate of these reactions (chemical kinetics) is well known
for hydrogen formation, but remains relatively poorly defined for the production of
hydrocarbon gases.
We know today that the primary ferromagnesian minerals that make up peridotites are destabilised in the presence of seawater at temperatures below 400 °C,
to form magnesium hydrated minerals such as serpentine (Fig. 5.3a), simplified
formula Mg 3 Si 2 O 5 (OH) 4 , and possibly talc, Mg 3 Si 4 O 10 (OH) 2 or brucite, Mg(OH) 2 ,
together with magnetite (Fe 3 O 4 ) (Fig. 5.3b). While iron is present in primary minerals (before hydration) in ferrous form (Fe
2+
), it is 1/3 ferrous and 2/3 ferric (Fe
3+
)
Table 5.1 Hydrogen enrichment, observed on hydrothermal sites discovered in the Earth’s mantle
in the North Atlantic
Hydrothermal site
CO 2
CH 4
H 2
N 2
Lost city (30° N)
–
9.8
82.5
–
Rainbow (36° 14′N)
42.7
6.6
42.7
4.8
Logatchev I (14° 45′ N)
18.8
11.1
53.5
–
Logatchev II (14° 45′ N)
26.3
5.1
47.0
–
Ashadze I (12° 58′ N)
12.9
3.9
62.4
–
Ashadze II (12° 58′ N)
n.d.
2.3
76.5
–
90
(Table 5.1) and for which accurate flow calculations remain to be made. Diffuse low
temperature fluids enriched in H 2 and CH 4 are emitted from many diapiric serpentized seamounts. These emissions occur at various depths (up to 4,100 m at Ashadze
at 13° N). In all cases, the fluids have a relatively uniform composition, controlled
by the phase separation process. They are derived from seawater interaction with
mantle peridotites, as many serpentized rocks are found at the outcrop.
This table indicates the mole percentages of each gas (from Charlou et al. 2010).
What Laboratory Experimentation Tells Us
The physical and chemical conditions which lead to the formation of natural hydrogen in the ocean environment can be reproduced in the laboratory (Fig. 5.2). An experimental approach enables us to independently survey and quantify the effect of
each of the important parameters (temperature, pressure, duration, chemistry of seawater, chemistry and mineralogy of solid reagents, mineral grain size, water-rock
interaction, etc.) involved in the natural process. However, laboratory experiments
have a time scale around 10,000 times shorter than that of the equivalent process
in the natural environment. For this reason, experiments are generally carried out
in optimal reaction conditions, for small-scale systems (finely ground solids, rock
samples measuring only centimetres) and water quantities that are often higher than
those available in the natural environment. Thanks to experimental work performed
on interactions between peridotites and seawater between 200 and 450 °C and between 0.5 and 3 kilobars (1 kilobar = around 1,000 times atmospheric pressure), the
complexity of natural chemical reactions resulting in hydrogen production has been
greatly elucidated. The rate of these reactions (chemical kinetics) is well known
for hydrogen formation, but remains relatively poorly defined for the production of
hydrocarbon gases.
We know today that the primary ferromagnesian minerals that make up peridotites are destabilised in the presence of seawater at temperatures below 400 °C,
to form magnesium hydrated minerals such as serpentine (Fig. 5.3a), simplified
formula Mg 3 Si 2 O 5 (OH) 4 , and possibly talc, Mg 3 Si 4 O 10 (OH) 2 or brucite, Mg(OH) 2 ,
together with magnetite (Fe 3 O 4 ) (Fig. 5.3b). While iron is present in primary minerals (before hydration) in ferrous form (Fe
2+
), it is 1/3 ferrous and 2/3 ferric (Fe
3+
)
Table 5.1 Hydrogen enrichment, observed on hydrothermal sites discovered in the Earth’s mantle
in the North Atlantic
Hydrothermal site
CO 2
CH 4
H 2
N 2
Lost city (30° N)
–
9.8
82.5
–
Rainbow (36° 14′N)
42.7
6.6
42.7
4.8
Logatchev I (14° 45′ N)
18.8
11.1
53.5
–
Logatchev II (14° 45′ N)
26.3
5.1
47.0
–
Ashadze I (12° 58′ N)
12.9
3.9
62.4
–
Ashadze II (12° 58′ N)
n.d.
2.3
76.5
–