Geochemistry of metabasite in Mariana forearc serpentinite mudflows documents interactions between serpentinizing fluid and subducted seamount basalts.

Understanding the chemical composition of materials within the subducting slab is crucial for studying slab dehydration and melting mechanisms. Previous research has primarily focused on examining sediments and altered basalts collected near the seafloor, which has overlooked the chemical changes occurring within the forearc subduction channel. The Mariana forearc serpentinite mud volcanoes offer a unique opportunity to investigate metabasite xenoliths, providing valuable insights into the compositional changes of subducting slab materials at forearc depths. In this study, we present Pb-Nd-Hf isotopic data for blueschist-facies mafic clasts obtained from the Asùt Tesoru serpentinite seamount in the Mariana forearc. These data are combined with existing Sr isotope and trace element data to address the aforementioned issues. The mafic clasts display Nd Hf isotopic signatures reminiscent of the HIMU- and EM1-type basalts discovered in the Magellan seamounts. However, their Sr Pb isotopes display mixed signatures, indicating the influence of sediments and altered mid-ocean ridge basalts. These findings underscore the importance of serpentinizing fluids resulting from the interaction between sediment pore water and forearc mantle peridotite in altering the Sr Pb elemental and isotopic compositions of subducted slab materials. This process is achieved through interactions between slab materials and serpentinizing fluids within the subduction channel. It is important to emphasize that the blueschist-facies mafic clasts themselves do not directly serve as source materials for arc volcanism. However, their Sr Pb isotopes can undergo further modifications through exchanges with unaltered seamount basalts facilitated by serpentinizing fluids within the deep subduction channel, which extends from depths of 30 to 100 km. In this scenario, the mafic clasts with modified isotopic signatures could potentially contribute as source materials for arc volcanism. • The protolith of the metabasite is HIMU- and EM1-type seamount basalts. • The seamount basalts interacted with serpentinizing fluid in the subduction channel. • Serpentinizing fluid modified the Sr Pb isotopes of the metabasite. [ABSTRACT FROM AUTHOR]