Integrated Fe- and S-isotope study of seafloor hydrothermal vents at East Pacific Rise 9–10°N

Abstract

In this study, we report on coupled Fe- and S-isotope systematics of hydrothermal fluids and sulfide deposits from the East Pacific Rise at 9-10°N to better constrain processes affecting Fe- isotope fractionation in hydrothermal environments. We aim to address three fundamental questions: (1) is there significant Fe isotope fractionation during sulfide precipitation? (2) Is there significant variability of Fe-isotope composition of the hydrothermal fluids reflecting sulfide precipitation in subsurface environments? (3) Are there any systematics between Fe- and S- isotopes in sulfide minerals? The results show that chalcopyrite, precipitating in the interior wall of a hydrothermal chimney displays a limited range of δ56Fe values and δ34S values, between –0.11 to –0.33‰ and 2.2 to 2.6‰ respectively. The δ56Fe values are, on average, slightly higher by 0.14‰ relative to coeval vent fluid composition while δ34S values suggest significant S-isotope fractionation (-0.6±0.2‰) during chalcopyrite precipitation. In contrast, systematically lower δ56Fe and δ34S values relative to hydrothermal fluids, by up to 0.91‰ and 2.0‰ respectively, are observed in pyrite and marcasite precipitating in the interior of active chimneys. These results suggest isotope disequilibrium in both Fe- and S-isotopes due to S-isotopic exchange between hydrothermal H2S and seawater SO42- followed by rapid formation of pyrite from FeS precursors, thus preserving the effects of a strong kinetic Fe-isotope fractionation during FeS precipitation. In contrast, δ56Fe and δ34S values of pyrite from inactive massive sulfides, which show evidence of extensive late-stage reworking, are essentially similar to the hydrothermal fluids. Multiple stages of remineralization of ancient chimney deposits at the seafloor appear to produce minimal Fe-isotope fractionation. Similar affects are indicated during subsurface sulfide precipitation as demonstrated by the lack of systematic differences between δ56Fe values in both high-temperature, Fe-rich black smokers and lower temperature, Fe-depleted vents.