Tracing the Oxidizing State and Element-Mobilizing Fluids in Continental Subduction Zones: Insights From the Granitic Melt-Eclogite Interface

Abstract

Fluids in subduction zones significantly influence element mobility, isotope fractionation, and mass transfer. However, unraveling the source, composition, and redox state of fluids in continental subduction zones poses a significant challenge. This study focuses on a granitic melt-eclogite contact interface, along with adjacent granite and eclogite from the Sulu ultrahigh-pressure metamorphic belt in East China. The interface exhibits complex mineral assemblages, enriched rare earth elements (REEs), and high field strength elements (HFSEs). Zircon grains from the interface show an age of ∼217 ± 9 Ma, slightly later than peak metamorphism, along with the presence of coesite inclusions. These findings suggest that the interfacial fluid likely formed from the mixing of granitic anatectic melt and aqueous fluid from the eclogite during the initial exhumation of the Sulu terrane. The interaction resulted in the eclogite acquiring substantial REEs and HFSEs, suggesting the interfacial fluid's effective element-transporting capability and potential supercritical fluid properties. Zircon Ce anomaly and Fe³⁺/Fe²⁺ oxybarometer data indicate a highly oxidizing interfacial fluid, analogous to arc magmas in oxygen fugacity. This led to the preferential loss of isotopically heavier Cr from the eclogite during fluid-eclogite interaction, evidenced by heavier Cr isotopic compositions in the interface (δ⁵³Cr = −0.04 to −0.05‰) compared to adjacent eclogite (δ⁵³Cr as low as −0.11‰). In summary, our results highlight the presence of strong oxidizing and element-mobilizing fluids in continental subduction zones, offering insights into supercritical fluid recognition and the genesis of oxidizing arc magmas in subduction zones.