Sulfur isotopes as a new indicator for the maturation of sulfide chimneys in submarine hydrothermal systems

Understanding the relationship between sulfide chimney maturity and fluid evolution in submarine hydrothermal systems is essential for deciphering mineralogical changes, sulfur dynamics, and fluid flux over time. We investigate this link through sulfur isotopic analysis of sulfides from three maturity stages of sulfide chimneys at the Niaochao hydrothermal field (East Pacific Rise, 1–2°S), integrated with δ³⁴S values from diverse global hydrothermal fields. Based on mineral associations and sulfur isotopes, the sulfide chimneys are classified into Zn-Fe-rich, Fe-Cu-rich, and Fe-Cu-Zn-rich chimneys, representing the increasing maturity. A systematic increase in δ³⁴S values (from 1.54‰ to 7.22‰) from chimney cores to rims indicates the progressive reduction of seawater sulfate during fluid-seawater mixing in open systems. Fluctuations in δ³⁴S values within chimney zones are attributed to the interaction between hydrothermal fluids and pre-existing minerals. As chimneys grow, rapid sulfur isotopic exchanges occur between seawater sulfate and fluid H₂S in localized semi-closed systems, leading to a significant drop in δ³⁴S values of pyrites, decreasing by up to 4.82‰. A new chimney evolutionary model has been developed utilizing mineralogical and sulfur isotopic data of sulfide chimneys from global hydrothermal vents. This model encompasses birth, growth, and waning stages, accompanied by an elevation of mean δ³⁴S values from 2.1‰ (n = 66) to 3.2‰ (n = 435) and 3.8‰ (n = 61) due to ongoing seawater sulfate reduction. Conversely, the infusion of magmatic sulfur during chimney growth contributes to a decrease in δ³⁴S values of sulfides. This study underscores the significance of sulfur isotopes as indicators of sulfide chimney maturity, which enhances our understanding of the evolution and formation processes of hydrothermal systems.