The effect of iron sulfidation on the stability of the trisulfur radical ion S3•‾ in subduction zone fluids

Abstract

The sulfur cycle in subduction zones is crucial for modulating the redox evolution of the mantle wedge. The trisulfur radical ion (S₃•‾) has been proposed as a dominant carrier of ore–forming elements in subduction zone fluids. However, existing high P–T experiments have primarily focused on S₃•‾ stability in pure aqueous sulfide–sulfate systems. The role of iron sulfidation on S–rich fluids in subduction zone rocks, which may influence S₃•‾ stability, remains poorly understood. In this study, we systematically investigated the impact of iron/iron oxides on S₃•‾ stability in subduction-zone fluids using in situ high P–T experiments and thermodynamic simulations. Our results reveal that in pure aqueous systems containing both SO₄²⁻ and S²⁻, S₃•‾ is the dominant sulfur species at temperatures above 350 °C along typical subduction P–T paths. However, the introduction of Fe or FeO into these S₃•‾–rich fluids triggers sulfidation reactions that lead to the precipitation of sulfide minerals (e.g., pyrite, pyrrhotite), substantially reducing S₃•‾ stability in the fluid phase. In contrast, Fe₂O₃ has little effect on S₃•‾ stability due to the formation of a passivating pyrite layer, which inhibits further sulfidation. Additional thermodynamic modeling suggests that the mobility of S₃•‾ and associated metal ions in subduction zones is only feasible under high fluid/rock (F/R) ratios and may be tightly regulated by channelized fluid flow.