Deformation and Melt Transport at Slow Spreading Ridges:
Initial Results from Ocean Drilling Program Leg 209.

Ulrich Faul and Leg 209 Shipboard Scientific Party

Our understanding of how mid-ocean ridges work are in large part based on observations from obducted bodies of mafic and ultramafic rocks called ophiolites. Models of mid-ocean ridges derived from these observations predict that the mantle upwells and undergoes decompression melting beneath the ridge, before the flow turns horizontal away from the ridge axis. This type of flow should result in distinct deformation patterns in the mantle rocks. Leg 209 was one of the first legs of the Ocean Drilling Program to directly test this hypothesis by drilling a series of holes along strike of the Mid-Atlantic Ridge between 14 and 16ºN. Previous observations from this area from dredges and submersible sampling had shown numerous peridotite outcrops at the sea floor.

During Leg 209 a total of 20 holes at 8 sites were drilled in the target area. Of those, two sites were drilled in basalt; one site on top of a "Megamullion" (thought to be the surface of a long-lived detachment fault) yielded gabbroic rocks and impregnated dunites, while peridotites with some interlayed gabbroic rocks were recovered from five sites. Initial analysis of the gabbroic rocks indicates that they crystallised at depths between 12 and 20 km. Gabbroic veins in the peridotites indicate continuously evolving compositions that were intruded as the host rock cooled in the thermal boundary layer.

Most of the deformation during uplift and emplacement was taken up by relatively large shear zones, up to several metres thick in the recovered cores. Surprisingly, no signs of the expected high temperature deformation in the peridotites are observable, for example foliation, subgrain boundaries or trails of spinel grains are absent. Scanning electron microscope (SEM) based electron-backscatter diffraction will be used to investigate whether signs of deformation were simply annealed, but a lattice preferred orientation of olivine and pyroxene is still present. The results will shed light on the dynamics of slow spreading ridges.

Peridotite textures from Leg 209, Hole 1274A are characterized by interstitial orthopyroxene grains with elongate "fingers" protruding between adjacent olivine grains.

Spinel grains (brown) show complex shapes and are often associated with clinopyroxene (blue-yellow-red). This assemblage could be due to exsolution and/or open system crystallisation of migrating melts.