Osmium isotope compositions and highly siderophile element abundances in abyssal peridotites from the Southwest Indian Ridge: Implications for evolution of the oceanic upper mantle

Abstract

Abyssal peridotites are extensively exposed on the seafloor of amagmatic segments at the ultraslow spreading Southwest Indian Ridge (SWIR). This study presents abundances of highly siderophile elements (HSEs; Os, Ir, Ru, Pt, Pd and Re) and Re–Os isotope compositions as well as major and trace element concentrations of nine spinel-facies peridotites from the Dragon Bone amagmatic segment (~53°E) and the easternmost part (~63.5°E and 69.5°E) of the SWIR. These peridotites have experienced intense serpentinization and seafloor weathering in addition to variable degrees of melt depletion (bulk rock Al2O3 = 0.74–2.73 wt%). Low-temperature alterations, however, have a negligible effect on Os isotope compositions and concentrations of platinum-group elements. The 63.5°E peridotites displayed sub-chondritic Os/Ir and chondritic Pt/Ir ratios and slightly variable Re/Ir and Pd/Ir ratios. The observed positive covariance between 187Re/188Os ratios and γOs, Re and Pd abundances can be explained most likely by interaction of mantle residues with radiogenic basaltic melts, which is consistent with the petrographic observations indicative of melt infiltration. The 53°E peridotites, on the other hand, displayed larger variations in Os isotopes and relative HSE abundances, indicating a more complex evolutionary history. High Re abundances and 187Os/188Os ratios in some 53°E samples attest to Re and radiogenic Os addition, probably through reaction with S-undersaturated hydrous melts. Specifically, sample 21V-S9-D5‐2 exhibited a high 187Os/188Os ratio (0.148) and enrichment of Cu and Pt abundances, implying interaction with an oxidizing agent. Although the Re–Os isotopes of SWIR peridotites were superimposed by variable metasomatic processes, they preserve the ancient Os isotope signature (~1Ga) and record the melting episode at ~0.7 Ga during the closure of the Mozambique Ocean. We infer that ancient depleted and hence buoyant mantle exists beneath the SWIR, which probably supports the weakly magmatic rifted Marion Rise.