Geochemistry of gabbroic rocks, Hole 1105A, Leg 179 Atlantis II fracture zone, Southwest Indian Ridge (SWIR): Formation of oceanic crust at an ultraslow spreading ridge

ODP Hole 1105A was drilled on a gabbroic massif exposed along the Atlantis II fracture zone on Southwest Indian Ridge. Hole 1105A penetrated to a depth of 158 meters on the Atlantis Bank. The core was divided into four broad units based on the presence and absence of oxides. Gabbro, oxide gabbro, oxide olivine gabbro and felsic veins were the four broad categories of rocks described by the shipboard scientists. The recovered core had close juxtaposition of primitive gabbros with evolved oxide rich gabbros. These gabbroic rocks posed some important questions regarding the formation of the lower oceanic crust at ultra slow spreading ridges. Some of the most important questions were: (a) Are the oxide rich and primitive gabbros in the stratigraphic order of formation, or are the Fe rich gabbros formed out of late stage infiltrating melts? (b) Are the Fe-Ti opaque oxides associated with the gabbroic rocks in equilibrium with other silicate phases within the rocks? (c) Are the gabbroic rocks congealed melt or cumulate with variable trapped melt? (d) What role did boundary layer (heterogeneous) crystallization play in the formation of cumulates?; Plagioclase, clinopyroxene, olivine, Fe-Ti oxides were the main cumulus minerals found in the rocks. Plagioclase and clinopyroxenes are found in all 148 samples, olivine has been reported from 89 samples and opaque minerals are found in 59 samples in this study. Very few samples have orthopyroxene and inverted pigeonite and they are associated with oxide rich rocks. Apatite and zircon occur as accessory phases in some highly evolved rocks. Felsic veins cut across the whole core at various stratigraphic levels. Gabbroic samples in this study show a wide range of composition from very primitive to extremely fractionated. Clinopyroxene Mg# ranges from (53--87), olivine ranges in Fo% from (36--77) and plagioclase ranges in An% from (14--67). Crossplots of various types emphasize the role of fractionation of melt as an important process in the formation of the gabbroic rocks. Downhole plots demonstrate cryptic chemical variation in the core. Study of the clinopyroxenes revealed a lithologic break at Mg# 75 that divided the oxide free samples from the oxide rich samples. This break corresponds to the concentration of TiO2 in melt that signifies maximum Fe-Ti enrichment and subsequent decrease in Fe-Ti components due to precipitation of Fe-Ti oxide. (Abstract shortened by UMI.)...