meantime, Peter joined me in the lab and right away, without any hesitation, he
went to the bridge and asked the officer in charge to turn the ship around and go
back to our last sampling location, so we could try again to dredge until we were
successful in recovering some samples. This decision turned out to be very
important because we were able to obtain a considerable amount of a variety of
rock types from a depth of 5100 m (Fig. 8.12c). This diversity revealed that the
wall of the transform was constructed with sequential formations, which probably
reflected layers of the upper-mantle and lower-crust. Later on, this finding would
be confirmed by submersible observations during the ‘‘Pito cruise’’ (see below).
Thus, the first dredge station carried out in the Terevaka transform fault was
made at the intersection of the north facing wall and the transform valley at
4850–4200 m depths during the SONNE cruise (leg 80) and brought up a variety of
rocks consisting of peridotite, gabbro and diabase (intrusive dyke fragments).
These rocks were thoroughly described by Marc Constantin, a PhD student at
UBO, with the collaboration of Peter Stoffers and Dietrich Ackerman. The results
were published in a special volume concerning mantle rocks edited by R.L.M.
Vissers and A. Nicolas (1995).
Based on more than 250 samples, it was found that the dredge contained 8 %
diabase, 15 % massive gabbros and 71 % peridotite. Among the 71 % peridotite
samples, there were a large number (about 35 %) of ultramafic rocks with veins of
gabbro, which indicated the percolation of magmatic melt within the peridotite
formation. This is very significant when compared to other regions of the Pacific
Ocean containing lower-crust/upper-mantle ultramafics, and where similar associations are observed, such as in the Garrett transform and in the Hess Deep.
The textural observations made by Marc Constantin on the minerals of these
ultramafic rocks indicated that different sequences of melt circulation had taken
place within the rock. By looking at the rocks’ thin sections, Marc found that the
first event took place during a regime of ductile deformation due to stress release
when basaltic liquid circulated between fractured joints and crystal grain boundaries in the harzburgite rocks. These liquids percolated between the planes of
foliation, and had crystallized clinopyroxenite. The next event is marked by melt
injection from plagioclase bearing dunite during an alteration by metasomatism
(transformation) of the peridotite host. Finally, the emplacement of gabbroic veins
forming a reticulated network of ferrogabbros indicates the last event of melt
injection (Constantin et al. 1995).
Diving in Terevaka Transform
The 1992 mission of the FS SONNE, headed by Peter Stoffers from the University
of Kiel (Germany), provided valuable information from the dredge haul rocks.
Thus, a diving expedition with the submersible Nautile was planned to take place
in 1993 during a French expedition (Pito cruise) organized by Jean Francheteau
(Francheteau et al.1994a, b).
First Samples from Terevaka Transform
275
went to the bridge and asked the officer in charge to turn the ship around and go
back to our last sampling location, so we could try again to dredge until we were
successful in recovering some samples. This decision turned out to be very
important because we were able to obtain a considerable amount of a variety of
rock types from a depth of 5100 m (Fig. 8.12c). This diversity revealed that the
wall of the transform was constructed with sequential formations, which probably
reflected layers of the upper-mantle and lower-crust. Later on, this finding would
be confirmed by submersible observations during the ‘‘Pito cruise’’ (see below).
Thus, the first dredge station carried out in the Terevaka transform fault was
made at the intersection of the north facing wall and the transform valley at
4850–4200 m depths during the SONNE cruise (leg 80) and brought up a variety of
rocks consisting of peridotite, gabbro and diabase (intrusive dyke fragments).
These rocks were thoroughly described by Marc Constantin, a PhD student at
UBO, with the collaboration of Peter Stoffers and Dietrich Ackerman. The results
were published in a special volume concerning mantle rocks edited by R.L.M.
Vissers and A. Nicolas (1995).
Based on more than 250 samples, it was found that the dredge contained 8 %
diabase, 15 % massive gabbros and 71 % peridotite. Among the 71 % peridotite
samples, there were a large number (about 35 %) of ultramafic rocks with veins of
gabbro, which indicated the percolation of magmatic melt within the peridotite
formation. This is very significant when compared to other regions of the Pacific
Ocean containing lower-crust/upper-mantle ultramafics, and where similar associations are observed, such as in the Garrett transform and in the Hess Deep.
The textural observations made by Marc Constantin on the minerals of these
ultramafic rocks indicated that different sequences of melt circulation had taken
place within the rock. By looking at the rocks’ thin sections, Marc found that the
first event took place during a regime of ductile deformation due to stress release
when basaltic liquid circulated between fractured joints and crystal grain boundaries in the harzburgite rocks. These liquids percolated between the planes of
foliation, and had crystallized clinopyroxenite. The next event is marked by melt
injection from plagioclase bearing dunite during an alteration by metasomatism
(transformation) of the peridotite host. Finally, the emplacement of gabbroic veins
forming a reticulated network of ferrogabbros indicates the last event of melt
injection (Constantin et al. 1995).
Diving in Terevaka Transform
The 1992 mission of the FS SONNE, headed by Peter Stoffers from the University
of Kiel (Germany), provided valuable information from the dredge haul rocks.
Thus, a diving expedition with the submersible Nautile was planned to take place
in 1993 during a French expedition (Pito cruise) organized by Jean Francheteau
(Francheteau et al.1994a, b).
First Samples from Terevaka Transform
275