We present a multidisciplinary approach to investigating the physical properties of rocks interpreted to have been exhumed from the lower oceanic crust and upper mantle. Laboratory-measured compressional-wave velocities have been examined with respect to geochemical and petrological characteristics to help explain the seismic velocity structure of the oceanic lithosphere beneath the fast-spreading ridge near Hess Deep. Samples analyzed indicate that no bulk-rock chemical indices exhibit apparent correlation with velocity behavior. Differences between mean atomic weight predicted from velocity-density systematics and calculated from bulk-rock geochemistry can be ascribed to alteration effects and preferred mineral orientation. The primary controls on velocity behavior are similar in the gabbroic rocks sampled at Site 894 and the ultramafic rocks sampled at Site 895. In the gabbroic rocks, velocity behavior is controlled by alteration of clinopyroxene and possibly preferred mineral orientation. In the serpentinized peridotites, the primary control on velocity behavior is intensity of serpentinization. Comparison of velocity-depth profiles and mineral composition profiles highlights compositional trends that are not obvious from compositional data alone. Similar-scale, high-velocity gradients in cores from the fast-spreading ridge near Hess Deep, and the slow-spreading Southwest Indian Ridge, suggest that similar scale structural controls (on ~150 m wavelength) may be present in both environments.

Date of initial receipt: 20 January 1995
Date of acceptance: 17 May 1995

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