Whole-rock major and trace element compositions are presented for a suite of gabbroic samples formed at the Southwest Indian Ridge with the aim of constraining magmatic processes at ultra slow-spreading centres. The gabbros, together with subordinate basalts, dolerites and peridotites were collected from a ∼ 700 km2area around Atlantis Bank, adjacent to the Atlantis II Fracture Zone during cruise JR31 of the RRS James Clark Ross. The large area sampled, the abundance of gabbro, and the recovery of samples representative of all other levels in the oceanic lithosphere, allow an average lower crustal composition to be estimated. The estimated composition is not sufficiently primitive in terms of Mg/Fe or compatible trace element abundances (Ni, Cr) for the bulk crust to be in equilibrium with the mantle. This is probably due to compositional modifications within the mantle during melt extraction, although crystallisation within the crust in an unsampled area, either towards the segment centre or along a flow line, cannot be ruled out. Gabbro compositions show evidence for being mixtures of cumulate crystals and significant proportions of basalt, with the proportion of each end-member dependent on the distribution coefficient of the element in question. This suggests that the concept of 'trapped melt' cannot be used to understand the origin of these compositions and consideration of the migration of interstitial liquid within a crystal mush is necessary. The fractionation of incompatible elements between the upper and lower crust correlates with the observed degree of variation in trace element concentrations in basalts from the same spreading segment. This suggests that interstitial liquids are, in part, extracted from the crystal mush and mixed back into subsequently erupted basalts leading to their compositional modification. © 2001 Elsevier Science B.V. All rights reserved.
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