78
Y. Fouquet
at 21 °N. Certain researchers predicted that activity would be particularly intense
further south, on the fastest-spreading portion of the ridge (spread: 17 cm a year),
between latitudes 15 and 20 °S. Numerous campaigns were carried out in the 1980s
and 1990s, in particular by American, French and German teams. Many discoveries were made along the ridge, from Mexico to Easter Island: 13 °N, 11 °N, 9 °50’
N, 17 °30’ S, 18 °15’ S, 18 °30’ S, 21 °30’ S … (Fig. 4.2). During a single campaign
with the Nautile submersible, 70 hydrothermal sites were discovered between 17
and 19 °S, on the world’s fastest-spreading ridge. This demonstrated that the degree
of activity depends on the spreading rate.
However, the number as well as the very small size (a few hundred square metres) of the hydrothermal fields indicates that these are extremely unstable systems,
due to the frequency of tectonic movements and the high number of volcanic eruptions. This configuration is incompatible with the formation of mineral accumulations of economic importance. Sulphide concentrations at the axis are insignificant
in terms of mineral resources. The only high volume deposits on fast-spreading
ridges are located on volcanoes a few kilometres away from the ridge’s axis; thanks
to the high degree of stability of these hydrothermal systems over a few thousand
years, several million tonnes of sulphide clusters have been able to form on these
volcanoes.
Slow-Spreading Ridges: Few Yet Vast Hydrothermal Sites For several years, it was
believed that black smokers did not exist on slow-spreading ridges, such as the MidAtlantic Ridge. Magma chambers, more local and deeper, were thought not to allow
the hottest fluids to reach the surface. In 1985, the discovery, just a few months
apart, of two fields of black smokers at 26 °N and 23 °N on the Mid-Atlantic Ridge
showed that hot hydrothermal activity was a general phenomenon on ridges, whatever their spread rate. Since then, many discoveries have been made, and around
fifteen sulphide sites are now known along the Mid-Atlantic Ridge (Fig. 4.7). Age
measurements taken on the TAG site at 26 °N show that several successive episodes
of activity occurred over a period of around 40,000 years. This figure is taken in
comparison with the duration of a few dozen years, which is the general rule on the
axis of fast-spreading ridges.
The combination of thermal and topographical effects focuses hydrothermal convections on high, hot spots of volcanic segments (for instance the Lucky Strike site).
However, as sites are known at the base and summit of the walls of the axial rift,
their position is therefore controlled not by volcanic activity, but by tectonics. Other
sites are located at segment extremities (Rainbow, Saldanha and MenezHom), in
contexts in which the low surface magmatic productivity lets mantle rocks show
through. These discoveries considerably extend the potential scope of investigation,
as now not only volcanic summits are to be explored, but also the base and summit
of rift walls; they also show that the volumes of sulphides at slow-spreading ridges
are higher than those of fast-spreading ridges.
Sediment-Covered Ridges A specific case concerns the portions of sediment-covered ridges, due to proximity with continents. In these environments, hydrothermal
convective flows successively affect the basaltic rock of the oceanic crust, then
Y. Fouquet
at 21 °N. Certain researchers predicted that activity would be particularly intense
further south, on the fastest-spreading portion of the ridge (spread: 17 cm a year),
between latitudes 15 and 20 °S. Numerous campaigns were carried out in the 1980s
and 1990s, in particular by American, French and German teams. Many discoveries were made along the ridge, from Mexico to Easter Island: 13 °N, 11 °N, 9 °50’
N, 17 °30’ S, 18 °15’ S, 18 °30’ S, 21 °30’ S … (Fig. 4.2). During a single campaign
with the Nautile submersible, 70 hydrothermal sites were discovered between 17
and 19 °S, on the world’s fastest-spreading ridge. This demonstrated that the degree
of activity depends on the spreading rate.
However, the number as well as the very small size (a few hundred square metres) of the hydrothermal fields indicates that these are extremely unstable systems,
due to the frequency of tectonic movements and the high number of volcanic eruptions. This configuration is incompatible with the formation of mineral accumulations of economic importance. Sulphide concentrations at the axis are insignificant
in terms of mineral resources. The only high volume deposits on fast-spreading
ridges are located on volcanoes a few kilometres away from the ridge’s axis; thanks
to the high degree of stability of these hydrothermal systems over a few thousand
years, several million tonnes of sulphide clusters have been able to form on these
volcanoes.
Slow-Spreading Ridges: Few Yet Vast Hydrothermal Sites For several years, it was
believed that black smokers did not exist on slow-spreading ridges, such as the MidAtlantic Ridge. Magma chambers, more local and deeper, were thought not to allow
the hottest fluids to reach the surface. In 1985, the discovery, just a few months
apart, of two fields of black smokers at 26 °N and 23 °N on the Mid-Atlantic Ridge
showed that hot hydrothermal activity was a general phenomenon on ridges, whatever their spread rate. Since then, many discoveries have been made, and around
fifteen sulphide sites are now known along the Mid-Atlantic Ridge (Fig. 4.7). Age
measurements taken on the TAG site at 26 °N show that several successive episodes
of activity occurred over a period of around 40,000 years. This figure is taken in
comparison with the duration of a few dozen years, which is the general rule on the
axis of fast-spreading ridges.
The combination of thermal and topographical effects focuses hydrothermal convections on high, hot spots of volcanic segments (for instance the Lucky Strike site).
However, as sites are known at the base and summit of the walls of the axial rift,
their position is therefore controlled not by volcanic activity, but by tectonics. Other
sites are located at segment extremities (Rainbow, Saldanha and MenezHom), in
contexts in which the low surface magmatic productivity lets mantle rocks show
through. These discoveries considerably extend the potential scope of investigation,
as now not only volcanic summits are to be explored, but also the base and summit
of rift walls; they also show that the volumes of sulphides at slow-spreading ridges
are higher than those of fast-spreading ridges.
Sediment-Covered Ridges A specific case concerns the portions of sediment-covered ridges, due to proximity with continents. In these environments, hydrothermal
convective flows successively affect the basaltic rock of the oceanic crust, then