Highly siderophile elements and Osmium isotopes in abyssal peridotites from the Southwest Indian Ridge

Abyssal peridotites are extensively exposed on the seafloor of amagmatic segments at the ultraslow spreading Southwest Indian Ridge (SWIR). We presents abundances of highly siderophile elements (HSEs; Os, Ir, Ru, Pt, Pd and Re) and Re-Os isotope compositions, major and trace element concentrations of nine spinel-facies peridotites from the Dragon Bone amagmatic segment (~53°E) and from the easternmost part (~63.5°E and 69.5°E) of SWIR. Although these peridotites experienced intense serpentinization and seafloor weathering, their Os isotopes and platinum-group elements are not significantly affected by the low-temperature alterations. The 63.5°E peridotites display 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, display larger variations in Os isotopes and relative HSE abundances, indicating a more complex evolutionary history. The enhancement of Re abundance and radiogenic Os in some 53°E samples can be attributed to the influence of melt-rock reaction, most probably with S-undersaturated hydrous melts. Specifically, sample 21V-S9-D5-2 shows superchondritic ¹⁸⁷Os/¹⁸⁸Os isotope ratio, as well as enrichment of Cu and Pt abundances, indicative of interaction with an oxidizing agency. Although the Re-Os isotopes of SWIR peridotites are superimposed by variable metasomatic processes, they preserve the ancient Os isotope signature (~1 Ga) and record the melting episode at ~0.7 Ga during the closure of the Mozambique Ocean. We infer that the ancient depleted and hence buoyant mantle exists beneath the SWIR, which may support the weakly magmatic rifted Marion Rise.