The objective of this study was to determine whether the effects of low Mg content and/or cracks that typically populate olivine grains can explain why P and S wave velocities in samples of oceanic gabbro do not increase with increasing olivine content. The Mg content of olivine in these rocks is known to be appreciably less than Fo90, and optical, SEM, and electron microprobe analyses indicate that the “olivine” is actually an aggregate of olivine and 5–15% crack fill consisting of talc and/or serpentine plus magnetite. Voigt average models show that there is a strong, positive correlation between P wave velocities and Fo90 content but no correlation with Fo73. Because of its Mg content, olivine that is typical of oceanic gabbros does not influence seismic velocities. Adding 11% alteration of olivine to serpentine plus magnetite in the form of crack fill to the Fo73 model yields a nearly exact match between the model and measured P and S wave velocities. The fact that the crack fill consists largely of talc and/or serpentine indicates that these minerals formed by hydrothermal alteration in young, hot (>150°C) lower crust. Hence our results apply to gabbros in situ, as well as to the laboratory samples; seismic velocities in lower oceanic crust produced at high and intermediate spreading rates are controlled principally by the degree of hydrothermal alteration of the gabbros that compose the lower crust.