The fate of molybdenum in subducted oceanic crust: Dehydration-controlled fractionation and cycling

Molybdenum (Mo) isotopes are powerful tracers of subduction-zone material cycling, yet their fractionation mechanisms and transport pathways during slab dehydration remain poorly constrained. This study integrates Mo concentrations and isotopic compositions from multiple global reservoirs with thermodynamics and numerical simulations, aiming to clarify Mo isotopic fractionation behavior and develop a Mo cycling model for subduction zones. Mass‑balance calculations yield mean δ 98 Mo values of 0.22 ± 0.04‰ (2σ) for sediments, −0.15 ± 0.04‰ (2σ) for the underlying altered mafic oceanic crust (AMOC), and 0.15 ± 0.04‰ (2σ) for the bulk altered oceanic crust (AOC). Isotope fractionation modeling indicates that upon complete dehydration, residual sediment δ 98 Mo can be reduced to −1.79‰ (α = 1.0015) and −1.88‰ (α = 1.002), while AMOC may decrease to −0.32‰ (α = 1.0015) and −0.38‰ (α = 1.002). The contribution from residual slabs after dehydration can explain pronounced negative δ 98 Mo observed in some arc lavas. Monte Carlo simulations demonstrate that slab residues retain substantial Mo after extreme subduction dehydration, with 33.9 % retention in sediments and 85.3 % in AMOC, highlighting their important contribution to Mo enrichment in deep subduction settings. Magmatic mixing models further indicate that subduction-modified, high-δ 98 Mo mid‑upper mantle can yield enriched mid-ocean ridge basalts (MORBs), while the deeper mantle metasomatized by low-δ 98 Mo dehydrated slabs can source ocean island basalts (OIBs). High-δ 98 Mo arc magmas in hot arcs form through fluid- or AOC-melt-dominated metasomatism, while cold subduction zones favor fluid-dominated metasomatism. Conversely, low-δ 98 Mo back-arc magmas mainly originate from dehydrated AOC melts. These results demonstrate that pre-melting slab dehydration controls Mo isotope fractionation and recycling in subduction systems.