Multi-stage growth and fluid evolution of a hydrothermal sulphide chimney in the East Pacific Ridge 1-2° S hydrothermal field: Constraints from in situ sulphur isotopes

Sulphur isotopes can be used as a powerful tool to trace fluid evolution and explore the formation of chimneys. To clarify the in situ S isotopic variations of sulphides at the micro-scale, we analyzed a sulphide chimney collected from the hydrothermal field in the East Pacific Rise 1-2° S using a sensitive high-mass-resolution ion micro-probe for stable isotopes (SHRIMP SI). Three mineral zones can be identified in the chimney: an external outer wall of porous anhydrite and colloform pyrite, an internal middle zone of sub-euhedral pyrite and massive chalcopyrite, and an inner zone of massive pyrite. The δ³⁴SV-CDT values of the sulphides fall within the range 1.83-7.51 ‰ (avg. 4.05 ‰, n = 16), and S isotopic values increase from the core (3.09 ‰, n = 3) to the middle (3.78 ‰, n = 11) to the edge (6.99 ‰, n = 2). These results illustrate mineral crystallization processes and the mixing between seawater-derived S and magmatic-hydrothermal fluids during the growth of the chimney. The zones from the edge to the core are characterized by crystal morphologies of colloform/anhedral pyrite to massive pyrite with decreasing δ³⁴S values, revealing multi-stage mineral deposition and sulphur isotopic fractionation. In contrast to the increase in δ³⁴S values from the core to the edge in one profile (profile A), anomalously low δ³⁴S values in fine-grained pyrite relative to chalcopyrite in another profile (profile B) in the middle zone result from S isotopic exchange between seawater SO4^2- and fluid H2S due to different fluid-seawater mixing, possibly caused by variations in permeability and porosity across the chimney.