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Seismic velocities have been measured at confining pressures as high as 600 MPa for lower crustal and upper mantle samples recovered during Ocean Drilling Program Leg 147. A comparison of porosity and density systematics with Southwest Indian Ridge Hole 735B and ophiolite data illustrates the more intense serpentinization of peridotites and more pervasive greenschist alteration characterizing the gabbros from Hess Deep. The degree of serpentinization and the olivine:orthopyroxene ratio of the Leg 147 samples control the density trends of the upper mantle rocks. The compressionaland shear-wave velocities show several types of seismic anisotropy caused by preferred mineral and microcrack orientations. Gabbroic samples are characterized by oriented plagioclase and olivine whereas the serpentinized peridotites show preferred horizontal microcrack orientation. Shear-wave splitting has been recorded and is attributed to preferred mineral orientation. The degree of serpentinization observed in the Hess Deep samples can be a significant factor in the reflectivity of the lower crust-upper mantle boundary. Gabbroic samples show lower Poisson's ratios than the highly serpentinized upper mantle rocks.

Velocity Behavior of Lower Crustal and Upper Mantle Rocks from a Fast-Spreading Ridge at Hess Deep