LEG 106

The Mid-Atlantic Ridge

Drilling in very young, largely sediment-free zones at spreading centers has proven traditionally difficult when the drill string required more than 100 m of sediment on basement for support. A major engineering effort by ODP has developed a method of drilling young, fresh volcanic rock that has little or no sediment cover. The main objectives of Leg 106 were to evaluate this new bare- rock drilling technique and to establish a hole in zero-age crust for future reentry. Among new engineering systems tested were a hard-rock guide base to confine the bit during initial spud-in on bare rock, an apparatus with a low-light TV camera for imaging the sea floor and monitoring drilling operations, and new downhole drilling and coring motors.

The Mid-Atlantic Ridge Rift Valley south of the Kane Fracture Zone appears typical of a plate boundary that accretes new material slowly, at less than 2 cmyr-1. Two sites (Sites 648 and 649) were drilled during Leg 106; the first about 70 km south of the fracture zone on a small volcano in the rift valley and the second in the Snake Pit hydrothermal deposit, an active hydrothermal vent field discovered during Leg 106 and located about 25 km south of the fracture zone.

Operations during Leg 106 demonstrated the feasibility of bare-rock spud-in drilling and coring in essentially new crust, even though penetration and recovery were less than desired. The rocks recovered from the small volcano are very fresh plagioclase-olivine, sparsely phyric basalt derived from parts of pillow lavas. The presence of plagioclase and olivine glomerocrysts and absence of chromian spinel suggests that the rocks are typical, moderately-evolved mid-ocean ridge basalts.

In the Snake Pit area, the first ever holes were drilled to investigate an active submarine hydrothermal system. Core recovery rates were lower than expected, but Leg 106 demonstrated that it is possible to drill at least shallow holes in a vent area and position them within a few meters of a high-temperature black smoker vent. The Snake Pit sulfide chimneys range up to several meters in diameter and more than 11 m in height and display the dendritic, tubular structures and ornamentation previously seen on the East Pacific Rise and Juan de Fuca Ridge. The hydrothermal deposits are thick (> 13 m at the foot of the black smoker; ~ 3-6 m at > 17 m distance) and are composed of mainly chalcopyrite, sphalerite, pyrite, marcasite, and pyrrhotite, and lenses of massive sulfide at the base of the black smoker. The biological community at the vent site is different from that observed in the Pacific, consisting of smaller and more mobile organisms.