Molybdenum isotope evidence for forearc mantle recycling at the Tongan subduction zone

Molybdenum isotope ratios (δ98/95Mo) of marine sediments constitute an important tracer for paleoredox reconstructions of the ancient ocean. Due to its redox-sensitivity, significant mass-dependent Mo isotope fractionation is induced in present-day low temperature environments leading to distinct Mo concentrations and isotope compositions in different marine lithologies. Subduction and recycling of such fractionated material can thus be potentially traced in arc magmas. Indeed, Mo isotope variations are observed in mafic arc lavas that are attributed to reflect recycled crustal components, but open questions remain to what extent different subducted lithologies contribute to the Mo isotope signature of arc magmas. We present a comprehensive Mo isotope dataset covering input to output at the Tongan subduction zone, together with exhumed eclogite-facies oceanic crust and sediments from the Western Alps and Alpine Corsica. Pelagic Mn-rich metapelites and MORB-type eclogites reveal that Mo is largely lost during early subduction metamorphism. Moreover, rutile hosts most of the remaining isotopically light Mo in the slab at higher metamorphic degrees where it remains fixed during slab-dehydration processes at subarc depths. Thus, direct recycling of this fractionated material cannot account for the observed positive covariations of Mo/Ce and δ98/95Mo with fluid indices (e.g., Ba/Th) in Tongan arc lavas. We propose that Mo systematics in Tongan arc lavas are the result of shallow fluid-induced Mo mobilization and forearc mantle serpentinization during early stages of subduction. Subsequent mechanical transport and devolatiziation of this metasomatized forearc mantle material towards subarc regions is a plausible alternative process to recycle Mo and other metals.