Leg 209

Mid-Atlantic Ridge Peridotite

A commonly held hypothesis of mid ocean ridge evolution is that mantle flow, or melt extraction from the mantle, or both, are focused in three dimensions toward the center of ridge segments, particularly at slow spreading ridges like the Mid-Atlantic Ridge. A direct test of this hypothesis would encompass drilling a transect of holes along a ridge segment where mantle peridotite is exposed on the seafloor. Leg 209 will sample the upper mantle in a magma-starved portion of a slow spreading ridge. The area to be drilled was identified at the 1996 Workshop on Oceanic Lithosphere and Scientific Drilling into the 21^st Century as the ideal region for this study. In this area, igneous crust is locally absent and the structure and composition of the mantle can be determined at sites over ~100 km along strike. The primary aim of drilling is to characterize the spatial variation of mantle deformation patterns, residual peridotite composition, melt migration features, and hydrothermal alteration along axis. A secondary aim is to provide a natural laboratory to test geophysical imaging techniques in a region underlain mainly by partially serpentinized peridotites. Potential operational issues include the difficulty of penetrating loose rubble on the fracture surface and hole cleaning problems in unstable basement. To reduce the risk of encountering these problems, all proposed sites have been inspected by submersible and are known to be underlain by lightly sedimented bedrock and to have a slope angle of less than 5°.

Seven holes will be cored to a minimum depth of 100 mbsf, all initiated in bare rock exposures identified by submersible expeditions. Deeper penetrations will be attempted where hole conditions and operations time allow. Three holes will be drilled in serpentinized peridotite exposed on the western wall of the Mid-Atlantic Ridge north of the 15°20'N fracture zone. Holes will be cored near the segment center at ~16°N, at an intermediate position between the segment center and the fracture zone, and at an exposure of peridotite near the segment end. Four holes will be drilled on the western wall of the ridge south of the fracture zone, from exposures of peridotite on the inside corner high, to intermediate locations with some gabbroic outcrops identified, to the segment center near 14°N.