Fe-Cu-Zn Isotopic Compositions in Polymetallic Sulfides from Hydrothermal Fields in the Ultraslow-Spreading Southwest Indian Ridge and Geological Inferences

Submarine hydrothermal sulfides from the ultraslow-spreading Southwest Indian Ridge (SWIR) were sampled from three hydrothermal fields, and the Fe-Cu-Zn isotopic compositions were analyzed in this study. The Fe isotopes ranged from -0.011 parts per thousand to -1.333 parts per thousand. We believe the processes controlling the Fe isotope variability in the hydrothermal systems include the sulfide precipitation process, the initial isotopic composition of the hydrothermal fluid, and the temperature during precipitation. Among these factors, the sulfide precipitation process is the dominant one. The Cu isotope compositions of the sulfides varied from -0.364 parts per thousand to 0.892 parts per thousand, indicating that the hydrothermal fluid preferentially leached Cu-65 in the early stages and that hydrothermal reworking led to decreases in the Cu isotopes in the later stages. In addition, because mass fractionation occurred during sulfide precipitation, the Zn isotope variations ranged from -0.060 parts per thousand to 0.422 parts per thousand. Combined with the S isotopic compositions, these results also implied that different Fe-Cu-Zn isotopic fractionation mechanisms prevailed for the different sample types. Based on these results, we are sure that the metallic elements, including Fe, Cu, and Zn, were derived from the mantle in the SWIR hydrothermal field, and the Fe-Cu isotope results indicated that these metallic elements were provided by fluid leaching processes. Using the isotopic fractionation and sulfide results, we calculated that the Fe-Cu-Zn isotopic compositions of the hydrothermal fluid in this field were delta Fe-56((fluid)): -0.8 similar to 0.0 parts per thousand; delta Cu-65((fluid)): 0.3 similar to 1.3 parts per thousand; and delta Zn-66((fluid)): 0 similar to 0.48 parts per thousand.