THE ORIGIN OF GREENALITE IN ARCHEAN AND PALEOPROTEROZOIC SEDIMENTS: NEW INSIGHT INTO THE CHEMISTRY OF FERRUGINOUS WATERS

Despite iron’s role in the chemical and biological evolution of the Precambrian Earth, its linkages with other biogeochemical cycles remain obscure. A variety of evidence suggests that persistent anoxia and the lack of a biological silica sink would have led to the intersection of Fe and dissolved silica (SiO₂(aq)) in a variety of ways. Here we present an experimental investigation of Fe-SiO₂(aq) interactions in anoxic seawater. We show that the presence of Fe²⁺ and SiO₂(aq) leads to the rapid nucleation of Fe-serpentine nanoparticles which then aggregate along preferred orientation to form greenalite at 25^oC. This non-classical crystal growth pathway is consistent with the unique crystal structure of greenalite and with its nearly primary origin in iron formation. Most importantly, a mechanistic underpinning for greenalite crystallization allows us to place new constraints on the chemistry of ferruginous Precambrian waters. We find that the pH required to initiate greenalite nucleation may have been as high as 7.7-8.3. This result implies that pH was a key trigger in coupling and decoupling Fe and Si during iron formation genesis, where the upwelling of alkaline water masses drove greenalite precipitation across several Archean and Paleoproterozoic basins. More broadly, this work suggests that previous estimates of the Fe concentration of Precambrian waters (based on siderite equilibrium) may underestimate maximum Fe levels by up to an order of magnitude. Because greenalite also occurs in some Al-bearing siliciclastic sediments but cannot precipitate in the presence of appreciable dissolved or particulate Al, we suggest that its presence probably reflects crystallization that took place in the water column. Thus a better understanding of greenalite distributions in chemical and siliciclastic sediments should reflect the evolution of both redox and acid-base chemistries in many Precambrian basins.