Siderite precipitation in Paleoarchean oceans during hydrothermal venting.

Siderite [iron(II) carbonate] is a major constituent of most iron formations. Its origin, which is currently disputed, affects interpretations about the composition of the ocean, atmosphere, and biosphere on early Earth. Direct precipitation from anoxic seawater is unlikely because of its prohibitively slow nucleation rate even in supersaturated solutions. However, recent modeling suggests that siderite may have precipitated at higher temperatures during hydrothermal fluid-seawater mixing in ancient anoxic oceans. Here, we report the presence of iron-rich siderite microparticles (<1.0 micrometers) in 3.49- to 3.25-Ga exhalative iron cherts from the Pilbara Craton, Australia, which we interpret to have formed during venting of iron(II)-rich hydrothermal fluids into anoxic water columns. Geochemical modeling suggests that coexisting magnesium-rich siderite formed in solutions dominated by seawater. The lack of hydrothermal iron siderite and predominance of magnesium siderite in younger analogs indicate that the bulk of siderite in iron formations is diagenetic and formed from marine pore fluids.