The water sampled above the sea floor was taken to shore in order to analyze
the helium content as well as other the compounds. Helium is an element found in
solar nebula and is considered as being a primordial element during the formation
of our planet. It is believed that the mantle is enriched in helium, which was
captured during the early formation of Earth. Thus, any helium that could be
measured was probably derived directly from the degassing of the Earth’s mantle
during volcanic and hydrothermal activity. Helium has a low mass density and
does not accumulate on the surface of the Earth, in addition to being only weakly
soluble in seawater. This is why helium is a good tracer for detecting hydrothermal
and volcanic activity.
The CLIPPERTON cruise served as a preparation ground for other more
detailed diving expeditions such as the GEOCYATHERM (1982) and the
GEOCYARISE (1984) cruises. Furthermore, this area has become a natural laboratory site for hydrothermal and biological in situ experimental work during the
past twenty years. The segment of the East Pacific Rise near 12°50
0 N is one of the
world’s ridge segments where the largest numbers of international submersible
dives (more than 60) and surface ship operations have taken place, between 1980
and 1990.
Low Velocity Zone Underneath the EPR
During a discussion on board the RV. JEAN CHARCOT, Felix Avedik had the
brilliant idea of using the combined SeaBeam bathymetry swath-mapping system
with his seismic water (1.1 l) gun to detect any sub-crustal discontinuities existing
underneath the spreading ridge axis. We could start our profile along the ridge axis
as soon as the various in situ stations were finished. Since we had seen from our
bathymetric coverage the capability of covering the entire 2 km wide axis of the
ridge with one single-pass bathymetry swath, by combining both systems we might
be able to make a radiographic subcrustal image of the material existing underneath the ridge axis. Thus, after completing our geological and hydrological work
at 13°N on the EPR, in the early morning at 4:30 AM on May 21, 1981 we started
the seismic profile along the ridge axis.
At the time, Felix did not know if the seismic signal would be strong enough to
see any distinct subcrustal features. However soon after we started, the signals that
came on board to the receivers were encouraging. For the first time, a combination
of multichannel bathymetry combined with towed seismic gear was able to show
deep seated (\2 km depth) subcrustal discontinuities in the form of a reflection of
the seismic waves. Such a discontinuity is related to a decrease in the seismic
velocity crossing a material of different density than the surrounding formation.
This experiment allowed us to calculate the size of an low velocity zone (LVZ)
along a stretch of the spreading center. The extent of the signal was proportional to
the length of the segment in the area of our experiment, which was less than 30 km
(Figs. 7.15, 7.17).
East Pacific Rise at 12°50
0 N
209
the helium content as well as other the compounds. Helium is an element found in
solar nebula and is considered as being a primordial element during the formation
of our planet. It is believed that the mantle is enriched in helium, which was
captured during the early formation of Earth. Thus, any helium that could be
measured was probably derived directly from the degassing of the Earth’s mantle
during volcanic and hydrothermal activity. Helium has a low mass density and
does not accumulate on the surface of the Earth, in addition to being only weakly
soluble in seawater. This is why helium is a good tracer for detecting hydrothermal
and volcanic activity.
The CLIPPERTON cruise served as a preparation ground for other more
detailed diving expeditions such as the GEOCYATHERM (1982) and the
GEOCYARISE (1984) cruises. Furthermore, this area has become a natural laboratory site for hydrothermal and biological in situ experimental work during the
past twenty years. The segment of the East Pacific Rise near 12°50
0 N is one of the
world’s ridge segments where the largest numbers of international submersible
dives (more than 60) and surface ship operations have taken place, between 1980
and 1990.
Low Velocity Zone Underneath the EPR
During a discussion on board the RV. JEAN CHARCOT, Felix Avedik had the
brilliant idea of using the combined SeaBeam bathymetry swath-mapping system
with his seismic water (1.1 l) gun to detect any sub-crustal discontinuities existing
underneath the spreading ridge axis. We could start our profile along the ridge axis
as soon as the various in situ stations were finished. Since we had seen from our
bathymetric coverage the capability of covering the entire 2 km wide axis of the
ridge with one single-pass bathymetry swath, by combining both systems we might
be able to make a radiographic subcrustal image of the material existing underneath the ridge axis. Thus, after completing our geological and hydrological work
at 13°N on the EPR, in the early morning at 4:30 AM on May 21, 1981 we started
the seismic profile along the ridge axis.
At the time, Felix did not know if the seismic signal would be strong enough to
see any distinct subcrustal features. However soon after we started, the signals that
came on board to the receivers were encouraging. For the first time, a combination
of multichannel bathymetry combined with towed seismic gear was able to show
deep seated (\2 km depth) subcrustal discontinuities in the form of a reflection of
the seismic waves. Such a discontinuity is related to a decrease in the seismic
velocity crossing a material of different density than the surrounding formation.
This experiment allowed us to calculate the size of an low velocity zone (LVZ)
along a stretch of the spreading center. The extent of the signal was proportional to
the length of the segment in the area of our experiment, which was less than 30 km
(Figs. 7.15, 7.17).
East Pacific Rise at 12°50
0 N
209