The eighth Nautile dive occurred on August 16th. It has been devoted to the exploration of a massif elongated along an E-W direction, located slightly to the South of the Romanche fracture zone, close to its intersection with the ridge axis.
We reached the bottom (-3860 m) at 10H43 UT on the southern flank of the massif and we could start almost immediately the cross section to the North. Rare blocks of different nature were scattered on the sediments. They included big flat stones characteristic of mylonitic peridotite, more rounded stones that happened to be basalts or andesites, and irregular soft blocks partly disaggregated in orange dust typical of iron hydroxydes. Some of these blocks were coated with bright blue material typical of copper sulphate or carbonate. A closer observation of these rocks onboard confirmed that they were made of aggregates of sulphides embedded in a greenish mud.
To the North, we reached a steeper slope (30° à 40°) that we could interpret as a fault plane whose present outcrop was made possible due to a giant collapse, the resulting debris forming a characteristic deposit at the foothill visible on the bathymetric map. Where the sedimentary cover was absent, whitish serpentine plate with down the dip striations revealed the fault mirror. Under the fault plane, the rock had a characteristic pale green-blue colour of low temperature alteration of serpentinites.
At a depth of 3440 m, we reached a crest marking the end of the outcrop of the fault plane. There, we were able to observe an outcrop where highly foliated serpentine were interlayered with altered sulphides. The schistosity plane was parallel to the slope. There the coating of copper sulphate was particularly widespread and we possibly observed faint emanations of hot water (although this observation needs to be confirmed).
After crossing this crest, no more altered sulphides were observed. At a depth of 3402 m we observed stacked shells stacked on the slope, which provides an additional evidence of a relatively recent hydrothermal activity along this fault plane.
The rest of the section was made of alternating debris and cliffs of peridotite. The cliff was locally foliated, the orientation of the foliation being mostly E-W with a northward dip of 70° to the North. We reached the top of the cliff at a depth of 2984 m. We left the bottom a 15H45.
By. G. Ceuleneer