Leg 153

Mid-Atlantic Ridge/Kane Fracture Zone

During Leg 153, five sites were drilled along the western flank of the Mid-Atlantic Ridge south of the Kane Transform (MARK) to investigate the chemical composition and evolution of the lower crust and mantle created at a slow-spreading ridge, the geometry of magmatic and deformation structures preserved in residual peridotite and in lower crustal rocks, and the intensity and diversity of metamorphism attending crustal evolution in this slow-spreading environment.

Drilling at Site 920, ~40 km south of the Kane Transform along the western wall of the MAR median valley, achieved the best penetration and recovery of any drilling efforts to date in massive serpentinized peridotite and sampled predominantly serpentinized harzburgite, interpreted to represent uplifted and altered suboceanic mantle. Despite intense alteration of the primary minerals to serpentine, primary mineralogy and textures are widely preserved in the ultramafic rocks. The dominant texture is coarse grained porphyroclastic with lobate grain boundaries and frequent triple junctions, suggesting efficient grain growth at close to solidus temperature. Most samples also display an anastomosing serpentine foliation that results from the preferred orientation of fine, discontinuous veins of serpentine and iron oxide minerals cutting the background mesh-textured serpentine. This anastomosing foliation probably dips moderately east, toward the MAR neovolcanic zone. Pyroxenite and gabbroic veins, and decimeter-thick gabbroic intervals are ubiquitous and cover a wide range of compositions from spinel-bearing interstitial melt segregations to altered, oxide mineral-, apatite-, and zircon-bearing gabbro.

Sites 921 through 924 are located in outcrops of gabbroic rocks, also along the western wall of the MAR median valley near its intersection with the Kane Transform, and on the flank of a broad, dome-like uplift (a ridge/transform intersection massif). Sites 921, 922, and 923 lie along a north-south-trending ridge axis parallel transect, such that variations in the igneous, metamorphic, and deformational features may reflect along-axis variability in the processes that create and modify lower crustal rocks. Site 924, offset toward the MAR, may, assuming a simple, symmetrical spreading history, have sampled younger gabbroic rocks. Lithologies recovered are predominantly gabbro and olivine gabbro which display a wide range of compositional and textural variability over lengths of centimeters to tens of meters. Igneous layering is commonly observed and crystal-plastic deformation, also common, is concentrated in centimeter- to decimeter-scale shear zones with dominantly normal displacements. Brittle deformation occurs in centimeter- to decimeter-scale cataclastic shear zones, which are likely to be associated with the east-dipping fault surfaces seen in the surrounding outcrops.