Hydrothermal vent fluid-seawater mixing and the origins of Archean iron formation

Precambrian iron formation provides valuable windows onto ancient marine environments, but exactly how these Fe-and Si-rich chemical sedimentary rocks formed have remained a matter of debate. Traditional models envisage Fe(III)-oxide formation through chemical and/or biological oxidation of soluble Fe2+, but sedimentological and textural data show that, throughout much of Archean stratigraphy, the finest-grained Fe(III)-oxides, previously interpreted to reflect primary components, were largely derived through the oxidation of the Fe(II)-silicate mineral greenalite. These observations have formed the basis for an alternative model where Fe2+ and SiO2(aq) combined to precipitate greenalite upon the mixing of hydrothermal fluids with ambient seawater. All models for iron formation genesis invoke hydrothermal alteration of seafloor basalt as the principal source of soluble Fe2+, and its subsequent transport in an anoxic deep ocean. However, the processes and products associated with the venting of near-axis hydrothermal fluids into Precambrian seawater, and implications for Archean iron formation, have not been investigated. In order to test models for Archean iron formation genesis, we used reaction path models to investigate the interactions between ancient hydrothermal vent fluids and anoxic seawater. We assembled available thermodynamic data and executed reaction path models to examine: (1) the equilibration between anoxic SO4-free seawater and basalt/gabbro at subseafloor hydrothermal conditions (400 oC and 400-500 bar), (2) the cooling and decompression of these fluids during their ascent through the oceanic crust, and (3) their subsequent mixing with anoxic SO4-free seawater. Our results confirm previous suggestions that the effective lack of SO4 in Archean seawater would have allowed fayalite-magnetite-pyrrhotite-quartz equilibria to buffer fluid chemistry, in turn leading to reaction zone fluids characterised by high Fe/H2S ratios. Cooling and decompression of these reducing fluids upon ascent, and further reaction with mafic wall rock, leads to significant Fe-chlorite and quartz precipitation with little to no sulfide precipitation. As these vent fluids mix with cold seawater directly, or with conductively-heated seawater, mineral assemblages are dominated by pyrite, greenalite, and/or siderite. These results were obtained with no constraints imposed by Archean iron formation, aside from the exclusion of minnesotaite, a demonstrably secondary mineral. The quantity of hydrothermally-derived Fe2+ partitioned into Fe(II)-bearing minerals upon mixing (96-99% of total aqueous Fe), the relative proportion of greenalite precipitated, and its further concentration via plume -driven transport (by virtue of its low density), together imply that the mixing between hydrothermal vent fluids and anoxic seawater may have served as a principal mechanism for generating a large proportion of the mineral mass now preserved as Archean iron formation. These results reconcile long-standing sedimentological and geochemical observations indicating that Archean banded iron formations record a strong hydrothermal influence, and suggest that these unique deposits have the potential to constrain the nature of seafloor-hosted hydrothermal systems on the ancient Earth, and their chemical interactions with seawater.