Leg 118

Southwest Indian Ridge

Four sites (Site 732 to Site 735) were drilled during Leg 118. At Site 735, a large intact 500-m section of gabbros was recovered from the Atlantis II Fracture Zone on the Southwest Indian Ridge, providing insight into the structure, composition, and evolution of lower ocean crust and magma-chamber evolution at a slow-spreading ridge. The section was formed by continuous intrusion and reintrusion of numerous small, rapidly crystallized bodies of magma, not in a large steady-state magma chamber. New batches of magma are intruded into and initially supercooled by a lower ocean crust consisting of wholly crystalline rock and semi-solidified crystal mush. This leads to undercooling and rapid initial crystallization of new magmas to form a highly viscous or rigid crystal mush preventing the formation of magmatic sediments, followed by a longer, petrologically more important period of intercumulus melt evolution in a highly viscous crystal mush or rigid melt crystal aggregate. Wall rock assimilation played a major role in the chemical evolution of the section and thus in the chemistry of the erupted basalt.

When they became sufficiently rigid to support a shear stress, deformation and ductile faulting of the still partially molten gabbros produced numerous small and large shear zones into which the late intercumulus melt moved, transforming the gabbro there into oxide-rich ferrogabbros. The net effect of these magmatic and tectonic processes was the production of a complex igneous stratigraphy with undeformed oxide-free olivine gabbros and microgabbros criss-crossed by bands of sheared ferrogabbro. Ductile deformation and shearing continued into the sub-solidus regime, causing recrystallization of the primary igneous assemblage under granulite facies conditions and the formation of amphibole-rich shear zones. Formation of ductile shear zones localized late fluid flow, with the most intense alteration occurring in the ductile faults. Undeformed sections of gabbro also underwent enhanced alteration at this time, principally by replacement of pyroxene and olivine by amphibole. In the middle amphibolite facies, shearing and ductile deformation ceased. Mineral vein assemblages changed from amphibole-rich to diopside-rich, reflecting different fluid chemistry. Continued alteration and cooling to low temperature occurred under static conditions similar to those found for large layered intrusions. These changes likely occurred due to an inward jump of the master faults defining the rift valley walls, thus transferring the section out of the zone of extension and lithospheric necking beneath the rift valley into a zone of simple block uplift in the adjoining rift mountains. Ongoing hydrothermal circulation, no longer enhanced by stresses related to extension, was greatly reduced, driven only by thermal dilation cracking as the section cooled to ambient temperature.