18
Y. Fouquet and D. Lacroix
Hydrogen has a high potential to combine with most elements of the periodic
table at high pressure and high temperature, forming metal hydrides, unstable in
the presence of water. Thanks to hydrogen, a large number of elements, transition
metals, lanthanoids and actinoids, of recognised metallogenic interest (titanium, vanadium, chromium, cobalt, molybdenum, tungsten, uranium, thorium, gold…) can
be transported in the mantle.
In hydrothermal circulation, hydrogen combined with sulphur (H 2 S) interacts
with the metals extracted from the rock to precipitate metal sulphides, forming
chimneys and hydrothermal mineral deposits along mid-ocean ridges and in backarc basins.
Hydrogen is also generated in large quantities during serpentinization of mantle
peridotites along slow- and ultraslow-spreading ridges and in subduction zones. We
now know that hydrogen and methane production are closely linked to ultramafic
rock outcrops on the ocean floor and on the walls of slow-spreading ridges. Hydrogen is produced abiotically at low temperatures (< 20 °C) by diffusion and degassing
of “inactive” serpentinized seamounts or at high temperatures (350 °C) at “active”
hydrothermal chimneys.
Since 1995, seven active high (> 350 °C) or moderate (~ 90 °C) temperature sites
have been discovered along the Mid-Atlantic Ridge in the mantle domain, at depths
ranging from 1,700 to 4,100 m, all producing large quantities of hydrogen. Recent
work has shown that the serpentinization phenomenon with hydrogen and methane
production was also present in many segments of the slow-spreading Arctic Ridge
as well as the Indian Ridge.
Global hydrogen flows obtained from oceanic serpentinization are as yet poorly
known. Current estimations of these flows vary between 90 and 190 billion moles
per year. These very preliminary calculations should be fine-tuned by continuous, in-depth exploration of slow-spreading ridges providing “field” data, but also
through experimentation and laboratory-based work, which will provide a better
understanding of reaction mechanisms, enable modelling of the natural productionmigration process, as well as the geochemical and thermodynamic processes implemented on a large scale.
Environmental Challenges
Deep-sea mineral and energy resources are located in highly contrasting areas of
the ocean. On ocean ridges or active systems of back-arc basins, active or past hydrothermal systems are at the origin of the production of sulphides rich in metals
and natural hydrogen. Crusts rich in cobalt and other metals are generally present
on seamounts formed from former volcanoes, but they can also be associated with
ocean ridges and plateaus. Finally, it is in abyssal plains that polymetallic nodules
are found. In these areas, the highly variable environmental conditions determine
the development of biological communities, which too are highly variable.
Y. Fouquet and D. Lacroix
Hydrogen has a high potential to combine with most elements of the periodic
table at high pressure and high temperature, forming metal hydrides, unstable in
the presence of water. Thanks to hydrogen, a large number of elements, transition
metals, lanthanoids and actinoids, of recognised metallogenic interest (titanium, vanadium, chromium, cobalt, molybdenum, tungsten, uranium, thorium, gold…) can
be transported in the mantle.
In hydrothermal circulation, hydrogen combined with sulphur (H 2 S) interacts
with the metals extracted from the rock to precipitate metal sulphides, forming
chimneys and hydrothermal mineral deposits along mid-ocean ridges and in backarc basins.
Hydrogen is also generated in large quantities during serpentinization of mantle
peridotites along slow- and ultraslow-spreading ridges and in subduction zones. We
now know that hydrogen and methane production are closely linked to ultramafic
rock outcrops on the ocean floor and on the walls of slow-spreading ridges. Hydrogen is produced abiotically at low temperatures (< 20 °C) by diffusion and degassing
of “inactive” serpentinized seamounts or at high temperatures (350 °C) at “active”
hydrothermal chimneys.
Since 1995, seven active high (> 350 °C) or moderate (~ 90 °C) temperature sites
have been discovered along the Mid-Atlantic Ridge in the mantle domain, at depths
ranging from 1,700 to 4,100 m, all producing large quantities of hydrogen. Recent
work has shown that the serpentinization phenomenon with hydrogen and methane
production was also present in many segments of the slow-spreading Arctic Ridge
as well as the Indian Ridge.
Global hydrogen flows obtained from oceanic serpentinization are as yet poorly
known. Current estimations of these flows vary between 90 and 190 billion moles
per year. These very preliminary calculations should be fine-tuned by continuous, in-depth exploration of slow-spreading ridges providing “field” data, but also
through experimentation and laboratory-based work, which will provide a better
understanding of reaction mechanisms, enable modelling of the natural productionmigration process, as well as the geochemical and thermodynamic processes implemented on a large scale.
Environmental Challenges
Deep-sea mineral and energy resources are located in highly contrasting areas of
the ocean. On ocean ridges or active systems of back-arc basins, active or past hydrothermal systems are at the origin of the production of sulphides rich in metals
and natural hydrogen. Crusts rich in cobalt and other metals are generally present
on seamounts formed from former volcanoes, but they can also be associated with
ocean ridges and plateaus. Finally, it is in abyssal plains that polymetallic nodules
are found. In these areas, the highly variable environmental conditions determine
the development of biological communities, which too are highly variable.