(Table T1) Olivine-gabbro sequence description of ODP Hole 118-735B

Olivine gabbros, allied troctolitic gabbros, and troctolites, together making up the olivine gabbro suite, comprise some 76% of the section cored in Hole 735B on the eastern transverse ridge of the Atlantis II Fracture Zone, Southwest Indian Ridge. These rocks hold the keys to the processes of crustal accretion and magmatic differentiation beneath the floor of the rift valley. We develop the sequential stratigraphy of the olivine gabbro suite by itself, stripping out all later, crosscutting, usually more deformed, and more strongly differentiated oxide gabbros and felsic veins that comprise the remaining 24% of the section. The fundamental unit of crustal construction in the olivine gabbro suite appears as alternations in grain size that we term olivine gabbro sequences. Uniformly textured masses of coarse-grained olivine gabbro range from meters to tens of meters thick and alternate with finer-grained and usually more olivine-rich rocks that have higher Mg# and Ni, such as troctolite and microtroctolite. There are 97 such sequences in the core. However, only 35 are more than 10 m thick and contribute substantially to the section. Several sequences are >40 m thick, and one is 120 m thick. We consider that each sequence represents either a single pulse of magma, in which olivine tended to concentrate toward the base during injection, or several pulses of magma, mostly marked by sutured, interpenetrative igneous contacts and small changes in grain size, but all injected quickly and at one place in the section.