DUST TO DUST: FORMATION OF HEMATITE “DUST” BY REPLACEMENT OF FERROUS SILICATE “DUST” IN LATE ARCHEAN BANDED IRON FORMATIONS

Conventional models for the deposition of banded iron formations (BIFs) envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide/hydroxide particles in surface waters that settled to form laterally extensive layers of iron-rich sediment. A key assumption is that fine-grained hematite (so-called hematite dust) represents relict ferric oxide/hydroxide precipitates. However, this premise has never been proven. In a study of well-preserved samples of BIFs from the late Archean Hamersley Group, Western Australia, we find BIFs preserve evidence for progressive stages of in situ alteration from green chert, containing nanometer-sized ferrous silicate dust particles, to red chert with abundant hematite dust. Transmission electron microscopy in the transition zone between green and red chert reveals that dusty hematite formed after partial dissolution of iron-silicate nanoparticles and precipitation of iron oxide particles in the resulting cavities. These observations suggest that hematite dust in these samples is not a direct dehydration product of ferric oxide/hydroxide precipitates but the end-product of post-depositional dissolution and precipitation of ferrous silicates. If these results are corroborated in BIFs elsewhere, it follows that hematite dust may not a reliable proxy for the chemistry or biological processes in Archean ocean. Our results suggest that the primary iron precipitate in the BIFs studied here was an iron-silicate mud that was silicified at or just below the sediment-water interface. If correct, it raises the possibility that the deposition of BIFs required neither dissolved oxygen nor photosynthetic life, but was essentially an inorganic, chemical process, reflecting anoxic oceans enriched in iron and silica.