Sediment melts impart extremely light Mo isotopes to arc magmas of central Tibet

Subduction zones are pivotal in understanding the interaction between Earth's surface and deep materials. The markedly diverse Mo isotopic compositions observed in arc magmas provide a valuable opportunity to explore the material cycling processes within subduction zones. Arc magma sources altered by slab fluids exhibit heavy Mo isotopes, while the origin of arc lavas with light Mo isotopes remains contentious, which is hypothesized to originate from two potential sources: (1) the dehydrated oceanic crust and (2) subducting sediments. Although the former has been extensively recognized, the latter still poses an enigma. Here, we present the Mo-Sr-Nd-Hf isotopic and elemental data of Jiang Tso andesites in the central Tibetan Plateau to elucidate the chemical compositions of sediment melts. These andesites show elevated Mg# values, along with trace element characteristics reminiscent of typical sediment melts. Their Sr-Nd-Hf isotopic compositions (87Sr/86Sri = 0.710260–0.710671, εNd(t) = −10.63 to −8.97, and εHf(t) = −9.30 to −7.95) closely resemble those of contemporaneous sediments in the central Tibetan Plateau. They exhibit higher Ce/Mo ratios (396–587) and lower δ98/95Mo values (−1.62 ‰ to −0.69 ‰) compared to the depleted mantle and most arc lavas, suggesting a more plausible explanation lies in the involvement of dehydrated subducting sediments rather than dehydrated oceanic crust in the source. Our findings, integrated with existing research, suggest that the Mo isotopes of arc magmas, in conjunction with trace elemental ratios, can preliminarily constrain the different subduction components (fluid or melt) in their sources. In addition, arc rocks with extremely light Mo isotopes may not be exclusively derived from subducted oceanic crust. Instead, they could originate from dehydrated sediment residues that enter the sub-arc mantle. Sediments with light Mo isotopes that are subducted into and preserved within the continental lithospheric mantle are likely to form localized reservoir characterized by light Mo isotopic signatures. This reservoir could play a crucial role in reconciling the discrepancies in Mo isotopic compositions between the continental crust and the depleted mantle.