Subduction-related transfer of sulfur and chalcophile elements recorded in continental mantle wedge peridotites

Abstract

Fluids or melts liberated from subducting slabs promote metasomatism of the overlying mantle wedge, enriching it in volatiles, such as sulfur and carbon, and possibly ore-forming chalcophile metals. Despite the genetic links, the redox evolution of arc mantle, mineralization at convergent margins, the S species in slab fluids and related metal transfer remain poorly understood, also because physicochemical properties vary at different stages of subduction. In this study, we conduct a comprehensive investigation in sulfide petrology, in-situ and bulk-rock S isotope compositions, PGE-Au-Cu-Ag-S abundances, and Re-Os isotope compositions of orogenic peridotites from the Sulu ultrahigh-pressure (UHP) metamorphic unit, documenting the three-stage transfer of S and of chalcophile and siderophile elements within the mantle wedge.

Refractory peridotites have Paleoproterozoic Re depletion ages (1.7–1.9 Ga) and show strong depletion of S, PPGE, Cu, Ag and Au, indicating that they are melt residues of the subcontinental lithosphere mantle beneath the North China Craton. Sulfur isotope signatures combined with detailed petrographic observations identify three distinct pulses of crustal S release and associated metal transfer at variable depths. First, mantle refertilization at UHP conditions (Stage 1) led to the precipitation of sulfides (δ³⁴S: +0.15 ± 0.48 ‰) in garnet lherzolite and replenished PPGE, Cu, Ag, Au, and S contents to a level similar to the fertile mantle. The peridotites were further metasomatized by H₂S-bearing crustal melts/fluids during early exhumation within or near the slab (Stage 2), as indicated by the formation of matrix sulfides (δ³⁴S: +1.63 ± 0.32 ‰), amphiboles, and carbonates as well as by elevated bulk-rock ¹⁸⁷Os/¹⁸⁸Os isotope ratios (up to 0.13927). During serpentinization (Stage 3), samples were metasomatized by CO₂-SO₄^2--bearing, H₂O-dominated fluids, causing elevated bulk S contents (161–713 μg/g, largely incorporated into serpentine) and high bulk-rock Fe³⁺/∑Fe ratios (0.39–0.59). This was accompanied by partial replacement of earlier-precipitated pentlandite grains (δ³⁴S: +2.98 to + 5.24 ‰) by magnetite. Combined with moderately elevated δ³⁴S, non-zero Δ³³S_sulfide values (up to + 0.05 ‰) suggest the contribution of surficial S to a mantle-S dominated reservoir. In contrast, the transfer of metals in COHS-bearing fluids in the exhumation channel (Stages 2–3) appears limited.

The results suggest that the efficacy of volatile and metal mobilization during metasomatic processes in continental subduction zones is depth-dependent along the slab-mantle interface. At high pressure, refertilization introduces reduced S and a modest amount of metals into the mantle wedge, while circulation of shallow CO₂-SO₄^2--bearing, H₂O-rich fluids, though metal-poor, provide a vector for metal transport and endowment at convergent margins. Evidence for fluids dominated by oxidized S is restricted to the shallow subduction channel associated with exhumation, suggesting a negligible role of sulfate-S in continental mantle wedge oxidation at subarc depths.