Fore-arc metasomatism by hybrid slab fluids during subduction initiation: Sr–Mg–Ca isotopes of rodingite, western Yarlung Zangbo suture zone

    Subduction zone metasomatism is critical for Earth’s material exchanges, yet understanding slab dehydration, particularly deserpentinization beneath fore-arcs, remains challenging. Here, we present Sr–Mg–Ca isotopic data for the Purang rodingites in the western Yarlung-Zangbo suture zone (YZSZ). These rodingites, dominated by amphibolite- to greenschist-facies minerals like tremolite, magnesiohornblende, and chlorite, exhibit cumulate textures and rare earth element patterns resembling troctolites or gabbronorites, presumably formed beneath a seafloor spreading center. The rodingites are enriched in large ion lithophile elements and depleted in high field strength elements. They show higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7067–0.7075) and elevated  δ²⁶Mg values (–0.22 ± 0.07‰ to –0.13 ± 0.02‰) compared to pristine oceanic basalts, while their δ^44/40Ca values (0.72 ± 0.02‰ to 0.87 ± 0.03‰) correspond to mid-ocean ridge basalts (MORB). These features imply fore-arc mantle metasomatism of an original MOR-derived protolith at <40 km slab depth, driven by Sr- and Mg-rich fluids from clay-rich sediments and serpentinitized mantle, respectively. The measured Sr and Mg isotope compositions can be reproduced by mixing a MORB-like protolith with hybrid fluids derived from 70–30% clays and 30–70% serpentinites, with a fluid-to-rock ratio of approximately 1:3–1:5. Combined with studies on YZSZ metamorphic soles, we propose this metasomatism occurred during incipient subduction of the Neo-Tethys oceanic rocks. Our studies highlight the significance of deserpentinization at shallow fore-arc mantle settings during subduction initiation and suggest that subducting slabs exhibit more varied dehydration characteristics than previously recognized.