Geochemistry and isotopic composition of gabbros from layer 3 of the Indian Ocean crust, Hole 735B

Abstract

Isotopic compositions of unaltered Leg 118 gabbros are distinct from Indian Ocean mid-ocean ridge basalts (MORB) in having higher 143Nd/144Nd and lower 206Pb/204Pb values; 87Sr/86Sr values overlap those of SWIR basalts, but are generally lower than MORBs from the southeast Indian Ridge or the Rodrigues Triple Junction. More than one magma composition may have been introduced into the magma chamber during its crystallization history. Whole-rock gabbro and plagioclase mineral separate 87Sr/86Sr values are uniformly low. By contrast, 87Sr/86Sr values for clinopyroxene in the upper half of Hole 735B are higher than coexisting plagioclase and reflect extensive replacement of clinopyroxene by amphibole. Hydrothermal veins and breccias have elevated 87Sr/86Sr values and indicate enhanced local introduction of seawater strontium. O-isotope disequilibrium between coexisting pyroxene and plagioclase pairs from throughout the stratigraphic column indicates that seawater interacted with much of the gabbro section, but at relatively low water/rock ratios. -from Authors