The current work concentrates on the iron isotope and elemental composition of the deep-sea hydrothermal vent Finn (Endeavour segment of the Juan de Fuca ridge, Pacific Ocean) in an attempt to clarify iron isotope systematics in a deep sea vent environment. We find that sulfide minerals from Finn are isotopically light relative to an “earth-moon average”, with d⁵⁶Fe values ranging from -1.9‰ in sphalerites to –0.3‰ in pyrites. The relatively large isotope fractionation in sphalerites is accompanied by varying amounts of iron in the same sphalerites, indicating that the mechanism of iron substitution in sphalerite may play a role in the observed isotopic fractionation.

The interactions between hot hydrothermal fluid and cold seawater were modeled using an equilibrium thermodynamic approach in order to understand the sequence of mineral precipitation and changes in iron speciation in the vent environment. The most favorable explanation for the observed mineral precipitation sequence involves cooling of 300°C hydrothermal fluid in the vent conduit and subsequent mixing with 2°C seawater. The modeling results indicate that isotope fractionation of iron might be caused by changes in iron speciation between various chlorides and hydroxides in solution during cooling and mixing. This equilibrium fractionation may then be locked into the various sulfide minerals, depending on which iron species form the reactants in sulfide precipitation reactions.