A variety of recent observations on slow-spreading ridges, including the Mid-Atlantic Ridge, suggests that the crust in these settings is a complicated mixture of gabbroic plutons and partially serpentinized peridotite (review in Cannat, 1996). Serpentinized mantle peridotite is known to crop out along both flanks of the Mid-Atlantic Ridge from at least 14°40´ to 15°40´N (Fig. F1). In some cases, lava flows lie directly over mantle peridotite without intervening gabbroic "lower crust." Thus this region has been interpreted as "magma starved," an end-member compared to the "robust" East Pacific Rise.
Seismic surveys of regions of slow-spreading ridges with abundant peridotite outcrops generally yield significant crustal thicknesses, if crust is defined as material with a seismic P-wave velocity of <8 km/s. This is true, for example, for the Mid-Atlantic Ridge just north of the 15°20´ Fracture Zone, within the Leg 209 drilling area (Fig. F5) (J.A. Collins and R.S. Detrick, pers. comm., 1998). In general, seismic data have been used to determine an average crustal thickness of 6–7 km for oceanic crust formed far from mantle hotspots, independent of spreading rate (e.g., White et al., 1992). This paradox represents a first-order problem in studies of the global ridge system.
Drilling at Sites 1268, 1270, 1271, and 1272 recovered 25% gabbroic rocks and 75% residual mantle peridotite. Core from Site 1274 is mainly residual peridotite, with a few meter-scale gabbroic intrusions. Core from Site 1275 is mainly gabbroic but contains 24% poikilitic lherzolite interpreted as residual peridotite "impregnated" by plagioclase and pyroxene crystallized from melt migrating along olivine grain boundaries. Impregnated peridotites are also common at Site 1271 and present at Sites 1268 and 1270. The overall proportion of gabbroic rocks vs. residual peridotites from these six sites is similar to previous dredging and submersible sampling in the area (Fig. F1). Thus, despite the variable proportions of peridotite vs. gabbroic rocks in individual sample areas, we conclude that the seafloor in the 14°40´–15°40´N region is underlain by 20%–40% gabbroic rocks intruding residual mantle peridotite.
It is difficult to be certain of the result that gabbroic rocks form tens of percent of the seismic crust and shallow mantle along this part of the Mid-Atlantic Ridge or to determine how general this result may be for other regions in which abundant peridotite is exposed on the seafloor. If possible, it will be very important to develop a geophysical technique for distinguishing between partially serpentinized peridotite and plutonic gabbroic rocks, even where these have the same seismic velocity and density (e.g., Christensen and Salisbury, 1975; Miller and Christensen, 1997). Obtaining extensive drill core of altered mantle peridotite from well below the surface weathering horizon in the 15°N area, together with prior geophysical characterization of this area and downhole logging, was a first step in resolving this problem. Physical properties of the samples measured in the laboratory (remanent magnetization, density, seismic velocities and attenuation, and electrical conductivity) can be compared with geophysical data in order to calibrate the large-scale surface techniques used worldwide. As postcruise data become available, a combination of lithologic observations on core and geophysical measurements made at true seismic wavelengths will be used to seek out features in the geophysical signals that are characteristic of partially serpentinized peridotite and truly measurable in the field.