Publication Date:
2022-05-26
Description:
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Chemical Geology 252 (2008): 214-227, doi:10.1016/j.chemgeo.2008.03.009.
Description:
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.
Description:
Support for W. Bach and
K. Edwards was provided by NSF grant OCE-0241791 and support for O. Rouxel was provided
by funding from the WHOI Deep Ocean Exploration Institute and NSF grant OCE-0622982 and
OCE-0647948.
Keywords:
Iron isotopes
;
Sulfur isotopes
;
Hydrothermal systems
;
Sulfide deposits
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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