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 isotopesX. MENG AND OTHERSIn situ sulphur isotopes of hydrothermal sulphides

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 degrees 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 delta S-34(V-CDT) values of the sulphides fall within the range 1.83-7.51 parts per thousand (avg. 4.05 parts per thousand, n = 16), and S isotopic values increase from the core (3.09 parts per thousand, n = 3) to the middle (3.78 parts per thousand, n = 11) to the edge (6.99 parts per thousand, 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 delta S-34 values, revealing multi-stage mineral deposition and sulphur isotopic fractionation. In contrast to the increase in delta S-34 values from the core to the edge in one profile (profile A), anomalously low delta S-34 values in fine-grained pyrite relative to chalcopyrite in another profile (profile B) in the middle zone result from S isotopic exchange between seawater SO42- and fluid H2S due to different fluid-seawater mixing, possibly caused by variations in permeability and porosity across the chimney.