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

Abstract

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 ε_Nd of 94 and ε_Hf 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 ε_Hf, 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 ε_Nd −8 to 9.1 and ε_Hf −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.