A textural and geochemical investigation of high level gabbros from the Oman ophiolite: implications for the role of the axial magma chamber at fast-spreading ridges

Abstract

Geophysical studies at fast-spreading ridges have identified a magma sill at, or slightly below, the base of the sheeted dyke complex. The role of this sill, the so-called axial magma chamber (AMC), in mid-ocean ridge basalt (MORB) differentiation and lower crustal accretion at fast-spreading ridges is unclear. Using the Oman ophiolite as an analogue for crust formed at a fast-spreading ridge, we investigate the composition of magmas delivered to the AMC using the compositions and textures of samples from the uppermost plutonics immediately below the sheeted dyke complex. The cores of clinopyroxene crystals are commonly as primitive as those found in deeper parts of the crust. This is true for major (Mg #), compatible trace (Cr) and incompatible trace (REEs) elements. Plagioclase crystal size distributions are ln-linear at all but the smallest crystal sizes suggesting a single crystal population. This indicates that the primitive crystal compositions are not carried to the AMC as xenocrysts formed deeper in the crust. Instead, these data demonstrate that primitive magmas that have previously undergone little or no differentiation can be delivered to the AMC. This supports models in which crystal subsidence from this body plays an important role in the accretion of the lower crust.