Trace Element Partitioning Between Saline Aqueous Fluids and Subducted Metasediments, and the Origin of the “Sediment Fingerprint” in Arc Magmas

Abstract Isotopic and elemental signatures in arc magmas bear similarities to marine sediments from the forearc. Partial melts of subducted sediments were previously invoked as the predominant carrier of material between the slab and mantle wedge, whereas aqueous fluids were deemed too inefficient at mobilizing trace elements to account for arc magma compositions. However, trace element solubility in aqueous fluids may be significantly affected by the formation of chloride complexes. We introduced NaCl into the sediment‐H2O system, and re‐investigated trace element partitioning between aqueous fluids and metasediments in a series of piston cylinder experiments at 2.5–4.5 GPa and 600–700°C. The diamond trap method was employed to separate the fluids, which were analyzed in the frozen state. Trace element compositions of fluids and all solid phases were obtained by LA‐ICP‐MS. We combined our data with published literature to demonstrate that saline fluids and sediment melts have a comparable efficiency in extracting incompatible elements from subducted sediments, producing similar element fractionation during mobile phase expulsion. Accessory phases such as allanite may only be stable in particularly Rare Earth Elements‐enriched lithologies. The Sr, Pb and Nd isotopic signatures of arc magma suggest that the mantle wedge is infiltrated by mobile phases derived from both the sediment and basalt sections of the subducted oceanic crust. Up to several percent of either sediment‐derived saline fluid or melt can account for the isotopic and elemental compositions of most arc magma. Geochemical similarities between arc magma and forearc sediments are not necessarily evidence of slab melting