Evidence for a Widespread Tethyan Upper Mantle with Indian-Ocean-Type Isotopic Characteristics

Abstract

The mantle sources of Tethyan basalts and gabbros from Iran, Tibet, the eastern Himalayas, the seafloor off Australia, and possibly Albania were isotopically similar to those of present-day Indian Ocean ridges and hotspots. Alteration-resistant incompatible element compositions of many samples resemble those of ocean-ridge basalts, although ocean-island-like compositions are also present. Indian-Ocean-type mantle was widespread beneath the Neotethys in the Jurassic and Early Cretaceous, and present beneath at least parts of the Paleotethys as long ago as the Early Carboniferous. The mantle beneath the Indian Ocean today thus may be largely 'inherited' Tethyan mantle. Although some of the Tethyan rocks may have formed in intra-oceanic back-arcs or fore-arcs, contamination of the asthenosphere by material subducted shortly before magmatism cannot be a general explanation for their Indian-Ocean-ridge-like low-206 Pb/ 204 Pb signatures. Supply of low-206 Pb/ 204 Pb material to the asthenosphere via plumes is not supported by either presentday Indian Ocean hotspots or the ocean-island-like Tethyan rocks. Old continental lower crust or lithospheric mantle, including accreted, little-dehydrated marine sedimentary material, provides a potential low-206 Pb/ 204 Pb reservoir only if sufficient amounts of such material can be introduced into the asthenosphere over time. Anciently subducted marine sediment is a possible low-206 Pb/ 204 Pb source only if the large increase of U/Pb that occurs during subduction-related dewatering is somehow avoided. Fluxing of low-U/Pb fluids directly into the asthenosphere during ancient dewatering and introduction of ancient pyroxenitic lowercrustal restite or basaltic lower-arc crust into the asthenosphere provide two other means of creating Tethyan-Indian Ocean mantle, but these mechanisms, too, have potentially significant problems.