Abundant Ancient Melt-Depleted Peridotite Beneath the Marion Rise, Southwest Indian Ocean, Effects on Basalt Composition and Dynamic Topography

The Marion Rise (MR) at the central Southwest Indian Ridge (SWIR) is an ultra-slow spreading ridge with thin crust, shallow ridge depth, sparse basaltic coverage, and exposed peridotite. Clinopyroxenes from the MR peridotites have highly variable Hf-Nd isotopic composition extending to extreme εNd of 94 and εHf of 417, which requires extensive melting and evolution with high Lu/Hf for more than 1 Ga. The Yb content of clinopyroxenes is negatively correlated with the Cr# (molar Cr/Cr + Al) of spinel, but not with εHf, indicating a multi-stage evolution of depletion and melt-rock reaction. The highly variable Hf-Nd isotopic compositions of the MR basalts are not systematically correlated and range from εNd −8 to 9.1 and εHf −10 to 32. Therefore, the basalts are probably a mixture of melts from several lithologies, for example, a recycled crustal component with exceptionally low Hf-Nd isotope ratios, in addition to melts from the volumetrically predominant, isotopically highly variable peridotites. The ancient melt-depletion of the MR peridotites with high Hf isotope ratios also reduced their density. A peridotitic mantle melted to <10% can support the Marion Rise without the need of increased mantle temperature. Ultra-depleted peridotites like those from the MR ones have been documented at multiple localities, indicating that they are ubiquitous in the sub-ridge mantle. Hence, melts from such ultra depleted peridotite influences mid-ocean ridge basalt (MORB) compositions and variably melt depleted sub-ridge peridotites should be considered when evaluating ridge depth variations.