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

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 degrees 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 delta(56)Fe values and delta(34)S values, between -0.11 to -0.33 parts per thousand and 2.2 to 2.6 parts per thousand respectively. The delta(56)Fe values are, on average, slightly higher by 0.14 parts per thousand relative to coeval vent fluid composition while delta(34)S values suggest significant S-isotope fractionation (-0.6 +/- 0.2 parts per thousand) during chalcopyrite precipitation. In contrast, systematically lower delta(56)Fe and delta(34)S values relative to hydrothermal fluids, by up to 0.91 parts per thousand and 2.0 parts per thousand 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 H(2)S and seawater SO(4)(2-) 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, delta(56) Fe and delta(34)S 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 delta(56)Fe values in both high-temperature, Fe-rich black smokers and lower-temperature, Fe-depleted vents. (c) 2008 Elsevier B.V. All rights reserved.