Through dredging and shallow drilling, a great deal has been learned over the last 30 yr about the lithology and geochemistry of the basaltic rocks that make up seismic Layer 2 of the ocean basins. However, relatively little is known about the lower crust because these rocks are rarely exposed on the seafloor and thus are difficult to dredge or drill. Based on sparse dredge hauls and comparisons with ophiolites, seismic Layer 3 is believed to consist primarily of mafic plutonic rocks, although in Hole 504B, the boundary between Layers 2 and 3 appears to lie within the sheeted dike section (Detrick et al., 1994).
During Ocean Drilling Program Leg 118 in 1987, foliated gabbros were found exposed on the seafloor in 720 m of water atop Atlantis Bank on the Southwest Indian Ridge (Fig. F1). Atlantis Bank is a flat-topped feature ~9 km long in a north-south direction and 4 km wide (Dick et al., 1991b) that was unroofed by a detachment fault (Dick et al., 1991c, 1992). The JOIDES Resolution spudded Hole 735B on this outcrop and drilled to a depth of 504.7 meters below seafloor (mbsf) before the hole was terminated for lack of time. The hole penetrated a sequence of troctolite, gabbro, olivine gabbro, gabbronorite, and Fe-Ti oxide gabbro with physical properties and seismic velocities appropriate for oceanic Layer 3 (Robinson, Von Herzen, et al., 1989; Iturrino et al., 1991). Although similar in some respects to typical ophiolites, the drilled section was characterized by a high degree of deformation, an abundance of Fe-Ti oxide gabbro, and local, high-grade metamorphism (Von Herzen, Robinson, et al., 1991).
In 1997, the JOIDES Resolution returned to Hole 735B and deepened it to 1508 mbsf before the drill pipe broke and could not be retrieved. Core recovery on both legs was 87%, thus providing an almost continuous 1.5-km-thick section of the lower crust. The overall lithology of the core and its general character is described in Robinson, Von Herzen, et al. (1989) and Shipboard Scientific Party (1999). In this report, we present results on the petrography, mineralogy, and geochemistry of this section of lower ocean crust. The tables and figures contain new data for samples collected during both Legs 118 and 176, as well shipboard geochemical data produced during Leg 176 and phase chemistry previously published by Hébert et al. (1991), Ozawa et al. (1991), and Natland et al. (1991).