Magnesium isotopes reveal multistage slab-derived fluid-rock interactions in subduction channels: Insight from vein-bearing high-pressure carbonated metabasites

Understanding the role of slab-derived fluids in slab-mantle interactions and their impact on arc magmatism is crucial for deciphering Earth’s deep material cycling. In this study, we combine petrology, bulk-rock geochemistry, Mg isotopes, and thermodynamic modeling to investigate fluid-rock interactions recorded in omphacite-rich vein-bearing blueschists and amphibolites from the southwestern Tianshan oceanic subduction zone, China. All high-pressure metabasites display uniformly heavy Mg isotopic compositions, with δ26Mg values ranging from −0.03‰ to +0.34‰ (compared to the mantle value of −0.25‰ ± 0.04‰) that correlate positively with the abundances of MgO, Ba, Rb, K, and (Cr + Ni), fingerprinting metasomatism by hybrid fluids enriched in heavy Mg isotopes. Petrological and isotopic constraints indicate these hybrid fluids were generated chiefly during prograde dehydration of serpentinite, with a subordinate contribution from metasediments. Despite their contrasting mineral assemblages, the omphacite-rich veins and their host rocks show no resolvable Mg isotopic variations (Δδ26Mgvein−host < ±0.03‰), indicating a dissolution-precipitation process during channelized fluid transport without measurable fractionation. Late-stage carbonation progressively shifts δ26Mg toward lower values, as evidenced by increasing loss-on-ignition contents and CaO/Al2O3 ratios and decreasing Ce/Pb ratios together with petrographic evidence of retrograde carbonation. We propose that slab-derived fluids, particularly those from serpentinite, can be both Mg rich and enriched in heavy Mg isotopes. In cold subduction zones, the Mg isotopic composition of fluids is primarily controlled by slab lithology and the corresponding devolatilization processes, while metasomatic fluids derived from carbonates appear to exert a limited influence. These results establish Mg isotopes as robust tracers of multisource, fluid-mediated reactive transport in slab-arc systems. Crucially, our findings provide key evidence that serpentinite-derived fluids are the principal carriers of heavy Mg isotopes to arc magma sources in cold subduction zones.