Iron isotope fractionation in a buoyant hydrothermal plume, 5°S Mid-Atlantic Ridge

Bennett, Sarah A., Rouxel, Olivier, Schmidt, Katja, Garbe-Schönberg, Dieter, Statham, Peter J. and German, Christopher R. (2009) Iron isotope fractionation in a buoyant hydrothermal plume, 5°S Mid-Atlantic Ridge. Geochimica et Cosmochimica Acta, 73 (19), 5619-5634. (doi:10.1016/j.gca.2009.06.027).

Abstract

Fe isotopes are a potential tool for tracing the biogeochemical redox cycle of Fe in the ocean. Specifically, it is hypothesized that Fe isotopes could enable estimation of the contributions from multiple Fe sources to the dissolved Fe budget, an issue that has received much attention in recent years. The first priority however, is to understand any Fe isotope fractionation processes that may occur as Fe enters the ocean, resulting in modification of original source compositions. In this study, we have investigated the Fe inputs from a basalt-hosted, deep-sea hydrothermal system and the fractionation processes that occur as the hot, chemically reduced and acidic vent fluids mix with cold, oxygen-rich seawater.
The samples collected were both end-member vent fluids taken from hydrothermal chimneys, and rising buoyant plume samples collected directly above the same vents at 5°S, Mid-Atlantic Ridge. Our analyzes of these samples reveal that, for the particulate Fe species within the buoyant plume, 25% of the Fe is precipitated as Fe-sulfides. The isotope fractionation caused by the formation of these Fe-sulfides is ?Fe(II)–FeS = +0.60 ± 0.12‰.
The source isotope composition for the buoyant plume samples collected above the Red Lion vents is calculated to be ?0.29 ± 0.05‰. This is identical to the value measured in end-member vent fluids collected from the underlying “Tannenbaum” chimney. The resulting isotope compositions of the Fe-sulfide and Fe-oxyhydroxide species in this buoyant plume are ?0.89 ± 0.11‰ and ?0.19 ± 0.09‰, respectively. From mass balance calculations, we have been able to calculate the isotope composition of the dissolved Fe fraction, and hypothesize that the isotope composition of any stabilised dissolved Fe species exported to the surrounding ocean may be heavier than the original vent fluid. Such species would be expected to travel some distance from areas of hydrothermal venting and, hence, contribute to not only the dissolved Fe budget of the deep-ocean but also it’s dissolved Fe isotope signature.

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