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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 δ98/95Mo range (−0.31‰ to −1.03‰, average −0.47‰ ± 0.06‰, 2SD = 0.40‰, n = 13), while type II basalts have heavy and constant δ98/95Mo values (−0.11‰ to −0.17‰, average −0.14‰ ± 0.01‰, 2SD = 0.05‰, n = 6). The unique low δ98/95Mo 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. Plain Language Summary: Recycling of subducted material can exert a profound influence on the deep mantle. Tracing superficial substances in the mantle is crucial for understanding Earth's evolution. However, radiogenic isotopic compositions for different enriched mantle end‐members overlap, hindering our understanding of which specific recycled materials contribute to enriched mantle end‐members. In this paper, we solve this problem by studying the molybdenum (Mo) isotope system in Cenozoic mantle‐derived basalts from the southeastern margin of the Tibetan Plateau. Our findings show that although the studied basalts have similar ratios of radiogenic isotopes, they can be divided into two types based on petrographic and geochemical characteristics, especially Mo isotopes. We attribute recycled dehydrated oceanic crust with sediments to isotopically lighter Mo and serpentinized peridotites to isotopically heavier Mo in their mantle sources. This study shows the mantle heterogeneity for Mo isotopes, and in turn demonstrates that the Mo isotope system for mantle‐derived magmas is an effective tool to track substances in the Earth's interior. Key Points: Miocene Maguan basalts have similar Sr‐Nd‐Pb isotopic compositions but a large range of Mo isotopic compositionsRecycled serpentinized peridotites yield high δ98/95Mo values, while recycled oceanic crust and sediments yield low δ98/95Mo valuesMo isotopes are effective tools to trace complex slab fluxes generating mantle geochemical heterogeneity [ABSTRACT FROM AUTHOR]