Identifying Recycled Materials Using Mo Isotopes in Intraplate Alkali Basalts From the Southeastern Margin of Tibetan Plateau

Abstract

Mantle heterogeneity in lithology and geochemistry is often attributed to recycled subducted materials. While distinct mantle end-members are identified by radiogenic isotopes, the specific recycled materials contributing to this heterogeneity remain debated. This study presents Mo-Sr-Nd-Pb isotopic data for OIB-like alkali basalts from the Maguan area in the southeastern Tibetan Plateau, focusing on slab inputs' role in mantle heterogeneity. The Miocene (ca. 13 Ma) Maguan alkali basalts are divided into two types based on petrographic and geochemical characteristics, showing similar Sr-Nd-Pb isotopic signatures but different Mo isotopic compositions. Type I basalts exhibit a wide δ^98/95Mo range (−0.31‰ to −1.03‰, average −0.47‰ ± 0.06‰, 2SD = 0.40‰, n = 13), while type II basalts have heavy and constant δ^98/95Mo values (−0.11‰ to −0.17‰, average −0.14‰ ± 0.01‰, 2SD = 0.05‰, n = 6). The unique low δ^98/95Mo value (−1.03‰) in type I basalts is among the lowest reported in OIB-like continental basalts. Type I basalts likely originate from an enriched asthenospheric mantle metasomatized by melts from recycled dehydrated oceanic crust and sediments, whereas type II basalts are derived from partial melting of an enriched asthenospheric mantle metasomatized by melts from recycled serpentinized peridotites. The residual Tethys oceanic slabs in the deep mantle significantly contribute to the mantle source of the Maguan basalts. The formation of Maguan Miocene magmas may be linked to mantle upwelling induced by the subduction of the West Burma plate. This study highlights the Mo isotopic system's utility in tracing complex slab fluxes generating mantle geochemical heterogeneity.