Abyssal peridotites: Rosetta Stone for recycled mantle materials in the asthenosphere

Decompressional melting of the asthenosphere gives rise of mid-ocean ridge basalts (MORB) that are extracted to generate ocean crust, and also leaves mantle residues that are represented by abyssal peridotites. Thus, both MORB and abyssal peridotites can be utilized to constrain the compositional characteristics of the asthenosphere. Numerous studies on MORB have widely demonstrated that they are from a relatively homogenous and geochemically depleted mantle source. The homogeneity of the asthenosphere has been commonly attributed to the efficiency of mantle convection. Nonetheless, geochemical compositions of global abyssal peridotites show highly variable compositions and a wide range of isotopic spectrum, clearly reflecting that the asthenosphere is compositionally heterogeneous. Mantle peridotites memorizing evolutionary histories at different tectonic settings, including sub-continental lithospheric mantle, mantle wedge and oceanic mantle, can be recycled into the asthenosphere, which might be eventually popped up at ocean ridges where they are sampled by abyssal peridotites. Different types of recycled mantle materials can be discriminated using geochemical tools. Our recent studies on abyssal peridotites dredged at different segments along the Southwest Indian Ridge (SWIR) have shown the occurrence of diverse types of recycled mantle, i.e., Archean cratonic mantle in its western segment, mantle wedge in its central segment, and oceanic mantle in this eastern segment. Such a spatial distribution is genetically related to the assembly and breakup of the Gondwana supercontinent since the Cambrian. Therefore, systematic studies on abyssal peridotites outcropped along the ocean ridges can decipher the compositionally characteristics and evolutionary histories of different mantle domains within the asthenosphere.