Molybdenum Isotopic Signatures of Forearc Serpentinites: Origin and Contribution to the Subduction Zone Mo Cycle

Serpentinites, as important reservoirs for volatile and fluid‐mobile elements, have been increasingly recognized as playing a critical role in global geochemical cycles. However, direct evidence for their dominance in the molybdenum (Mo) cycle in subduction zones has been elusive. Here, we address this issue by comprehensively investigating the Mo isotope systematics of serpentinites and accompanying metabasites recovered from the Mariana forearc. Forearc serpentinites under low‐pressure conditions (blue‐ and lizardite‐serpentinites) show substantial enrichments in Mo/Ce ratios (0.42 to 27.34) with high δ98/95Mo values (−0.03‰ to 2.48‰), which arise from not only the incorporation of shallow forearc slab‐derived fluids but also late low‐temperature metasomatism by seawater during clast exhumation. Subducted oceanic crust represented by metabasites has low Mo/Ce ratios (0.004 to 0.049) and δ98/95Mo values (−1.13‰ to −0.61‰), indicating that the loss of Mo during oceanic crust alteration in the early or before subduction stage tends to decrease the Mo isotopic compositions. Combined Mo‐Sr‐Pb isotope systematics indicate that the high δ98/95Mo values in some arc lavas cannot be ascribed simply to the incorporation of slab‐derived fluids at subarc depths; instead, the injection of forearc serpentinite‐derived fluid into their mantle source is noteworthy. We propose that the mobilization of Mo by shallow slab‐derived fluids during the early subduction stage generated forearc lizardite‐bearing serpentinites enriched in heavy Mo isotopes. Subsequently, dehydration and melting of these serpentinites, dragged by the subducting slab down to the subarc region, might have contributed to the Mo systematics in arc lavas. This study demonstrated that forearc serpentinites may be important reservoirs for arc lavas with high Mo isotopic compositions and may contribute to the Mo cycling in subduction zones. Plain Language Summary Serpentinites, as important reservoirs of volatile elements and mobile elements, play key roles in geochemical cycles. However, direct evidence that these rocks dominate the Mo cycle in subduction zones has been elusive. In this paper, we solve this problem by studying the molybdenum (Mo) isotope system of serpentinites and associated metabasites recovered from the Mariana forearc. The forearc serpentinites have high δ98/95Mo values due to modification by slab‐derived fluids and seawater. The δ98/95Mo values of the metabasites are relatively low, ind