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 delta 98/95Mo values (-0.03 parts per thousand to 2.48 parts per thousand), 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 delta 98/95Mo values (-1.13 parts per thousand to -0.61 parts per thousand), 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 delta 98/95Mo 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. 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 delta 98/95Mo values due to modification by slab-derived fluids and seawater. The delta 98/95Mo values of the metabasites are relatively low, indicating that Mo isotope changes occurred during oceanic crust alteration. The forearc serpentinites with heavy Mo isotopes that experiencing dehydration and melting were dragged below the arc by the subducting slab, which have contributed to the Mo system in arc lava. This study showed that forearc serpentinite may act as an indispensable reservoir in the Mo cycle in subduction zones. Forearc serpentinites show high Mo/Ce ratios and delta 98/95Mo values Lizardite-serpentinites are influenced by shallow slab-derived fluid, while blue-serpentinites are influenced by late seawater metasomatism Forearc serpentinites may contribute significantly to the Mo systematics in arc lavas