Hole 735B (32°43.39´S, 57°15.96´E) is located on the eastern flank of the Atlantis II Fracture Zone of the Southwest Indian Ridge. This part of the oceanic crust was formed at ~11 Ma beneath the Southwest Indian Ridge at a very slow half-spreading rate of 8 mm/yr (Fisher and Sclater, 1983; Dick et al., 2000). Coarse-grained lower crustal gabbros and mantle peridotites are predominantly exposed on a wave-cut terrace (Atlantis Bank) at a water depth of 700 m along a transverse ridge of the Atlantis II Fracture Zone (Dick et al., 1991b). Hole 735B is situated on this bank and was originally drilled ~500 m during Leg 118 in 1987 and was deepened to 1508 meters below seafloor (mbsf) in 1997 during Leg 176. The entire core drilled during these two legs is composed of crustal lithologies, and no mantle peridotite was sampled. Oceanic Layer 3 around this site is believed to be ~2 km thick (Muller et al., 1997). Therefore, the 1500-m-long section of Layer 3 gabbro with an exceptionally high recovery rate (~86%) from Hole 735B may provide us with one of the best opportunities for understanding the architecture and evolution of the oceanic crust, especially that formed at very slow-spreading ridges.
The major lithologic units in Hole 735B core include troctolite, olivine gabbro, gabbro, and gabbronorite. These show coarse-grained equigranular texture, in general, and constitute >90 vol% of the recovered core. Subordinate lithologic units cored during Leg 176 include microgabbros, oxide-rich gabbros, and felsic rocks. The microgabbros show equigranular, fine-grained textures and are observed as veins and dikelets intruding the olivine gabbros. The microgabbros appear to represent channels through which silicate melt migrated to the upper portion of the crust. Their compositions range from troctolite through olivine gabbro to oxide-rich gabbro and, in general, do not represent magmatic compositions in spite of their fine-grained textures (e.g., Dick, Natland, Miller, et al., 1999). The oxide-rich gabbros range in composition from oxide-rich troctolite through oxide-rich olivine gabbro to oxide-rich gabbros. The oxide-rich gabbros intrude into the olivine gabbros as dikes and veins usually associated with strong ductile deformation (e.g., Cannat, 1991). The felsic rocks are present as veins and dikelets and are dioritic, trondhjemitic, tonalitic, and granitic in composition. The felsic rocks include a diverse suite that may have originated from magmatic and hydrothermal processes. More detailed petrographical descriptions of the core recovered from Hole 735B are given in Dick, Natland, Miller, et al. (1999) and Robinson, Von Herzen, et al. (1989).
For the full depth of Hole 735B drilled during Legs 118 and 176, 12 major lithologic units were identified based on modal mineralogy and the relative abundance of rock types (Dick et al., 1991a, 2000; Dick, Natland, Miller, et al., 1999). The downhole sequence can be also subdivided into several units based on the whole-rock major element chemistry of the dominant lithologic unit, the olivine gabbros. Dick, Natland, Miller, et al. (1999) and Dick et al. (2000) recognized five units of ~200-300 m thickness. Each unit has a coherent chemical variation assumed for magmatic fractionation (e.g., see Fig. F1). This variation corresponds to a lithologic change from troctolite through olivine gabbro to gabbro and gabbronorite. These chemically defined units may represent an individual magmatic batch supplied by small and ephemeral magma chambers beneath the Southwest Indian Ridge (Dick, Natland, Miller, et al., 1999; Dick et al., 2000). Overall, there is no evidence for a large steady-state magma chamber in this downhole sequence.
Several kinds of veins have been described in cores from Hole 735B. In general, these veins are subdivided into two major categories, one crystallized from silicate melts and another the so-called hydrothermal or alteration veins. Clearly classified into the former magmatic veins are the microgabbros, oxide-rich gabbros, and also some trondhjemitic rocks (Stakes et al., 1991). Other veins are related to fluid migration and hydrothermal activity during metamorphic processes (e.g., Robinson, Von Herzen, et al., 1989; Robinson et al., 1991; Stakes et al., 1991; Magde et al., 1995). They include amphibole veins, plagioclase + amphibole veins, felsic veins of ambiguous origin, plagioclase + Ca-rich clinopyroxene (clinopyroxene, hereafter) veins, and other veins consisting of very low temperature minerals, such as clay minerals and carbonates (e.g., Magde et al., 1995). Information on the hydrothermal processes was also provided by fluid inclusions and oxygen, Sr, and Nd isotopic studies (e.g., Vanko and Stakes, 1991; Stakes et al., 1991; Kempton et al., 1991; Hart et al., 1999; Kelley and Früh-Green, 1999, in press).