Down Hole Variation in the Chemistry of Gabbroic Rocks From Atlantis Bank, ODP Hole 1105A, Southwest Indian Ridge: Magma Chamber Processes

Geochemical studies on cored sections have been conducted on gabbroic rocks recovered from ODP Leg 179 Hole 1105A. Hole 1105A was drilled on the Atlantis Bank through the same gabbroic massif cored at Hole 735B (Leg 118 and 176). Both holes are located along the Atlantis II Transform, Southwest Indian Ridge. Hole 1105A is offset 1.2 Km from Hole 735B, the deepest hole ever drilled in to the plutonic foundation of the oceanic crust. Preliminary shipboard magnetic, resistivity, lithologic and geochemical data suggest a possible correlation of lithostratigraphic units identified in both holes. Four major rock types have been identified in the cored section of Hole 1105A. They are the gabbros, olivine gabbros, oxide olivine gabbros and the oxide gabbros. The cored section is divided into 4 major units based on the presence and absence of oxide minerals in the gabbroic rocks. The major rock types are distinguished on the basis of the presence of cumulus phases olivine, clinopyroxene, plagioclase. Whole rock Mg numbers range from 83.5 to 19.6 indicating extensive fractionation of magma. The oxide free gabbroic rocks have a very low abundance of incompatible elements like P2O5, TiO2, Zr and LREE indicating that they crystallized as adcumulates or mesocumulates with little to no trapped melt retained. The oxide gabbro layers have a higher abundance of Fe2O3 and TiO2.Textural variation is observed with changes in Mg number down hole. Clinopyroxenes show a wide range of composition, Mg number ranging from 50.86-83.78. The mineral chemistry matches closely with the whole rock data and shows down hole cryptic chemical variation. Variations in Mg numbers of CPX occur on thin section scale as well as in meter scales. Down hole cryptic chemical variations suggest periodic replenishment of the magma chamber, fractionation and magma mixing. The bulk rock major, trace, REE and the mineral chemistry suggest that gabbroic rocks formed as cumulates with low to negligible trap melt and none can be shown to resemble congealed melt likely to form as a chamber chilled margin. Sharp contrasts in equilibrium melt compositions over short distances, may indicate infiltration of fractionated melts into a relatively primitive adcumulus assemblage. It appears that these fractionated melts result in localized adcumulus crystal growth.