Seismic velocities of basalts, dikes, and gabbros from an intact section of in situ upper oceanic crust

We examine the seismic structure of the upper oceanic crust at ODP/IODP Site 1256 using high-pressure laboratory measurements of mini-cores. Drilled in 15 Ma old crust of the Cocos plate formed by super-fast spreading, Hole 1256D is the first hole drilled continuously through the sediments, basalts, dikes and into the gabbros of the upper oceanic crust. To determine seismic structure and answer whether Hole 1256D has reached seismic Layer 3, we measured compressional wave velocities for 46 water-saturated mini-core samples at pressures up to 600 MPa. Velocities generally increase rapidly up to 100 MPa and then linearly above 200 MPa (~0.02 km/s per 100 MPa). Samples from 277 to 1487 meters below the seafloor have compressional wave velocities of 5.3 to 6.9 km/s, at lithostatic pressures of 42 to 76 MPa. Basalt samples average 5.9 ± 0.3 km/s, while dikes average 6.3 ± 0.2 km/s and gabbros average 6.7 ± 0.2 km/s. Other samples from this site, measured at sea under atmospheric conditions, indicate seismic velocities <6.5 km/s (Wilson et al., 2006), typical of Layer 2. Once mini-cores of the gabbros were measured at in situ lithostatic pressure, they exhibited classic Layer 3 velocities (~6.7 km/s). Likewise, several intervals of the downhole compressional wave velocity log reach Layer 3 velocities in the dike section. Unfortunately, the logs are not complete through the drilled sections of gabbro, so a comparison with laboratory data could not be completed in the bottom of the hole. The hand samples we selected are some of the most coherent pieces of the recovered core and thus tend to yield higher velocities than logs, which integrate cracks and other water-filled spaces in the crust. Therefore, while the compressional wave velocities of these samples at lithostatic pressures are consistent with Layer 3 velocities, if cracks are present in the gabbros or pore pressures are significant, then in situ velocities in the uppermost gabbros may be lower than those typical of Layer 3.