Episodic hydrothermal supply and microbial anaerobic Fe(II) oxidation in early Archean ocean: Insights from precursor mineral compositions of the 3.46 Ga Marble Bar Chert, Pilbara Craton, Western Australia

The 3.46 Ga Marble Bar Chert (MBC) from the Pilbara Craton in Western Australia is known as the oldest chert on Earth. The origin of its Fe minerals was investigated to decipher the geochemistry and redox conditions of the early Archean ocean, as well as to explore the possible microbial contribution to the earliest sedimentary processes on Earth. However, the composition of the precursor minerals in the MBC remains poorly understood, giving rise to controversies on its genesis. Here we performed high-resolution petrographic, laser Raman, Mössbauer, X-ray diffraction, and Mn K-edge X-ray absorption near edge structure spectroscopic studies on two typical types of finely laminated MBC, grey-black chert and red chert. The grey-black chert contains considerable amounts of siderite, carbonaceous materials, and minor phyllosilicates and hematite. By contrast, the red chert contains high hematite and silicates, minor siderite and rare carbonaceous materials. Microcrystals in the cores of chert polyhedral are among the earliest mineral phases without signs of alterations, recrystallisation, erosion or replacement histories. Ferrihydrite, greenalite, siderite, and green rust are possible precursors of Fe-bearing minerals in the MBC. Their respective proportion in each lamina was regulated by pH, Fe(II)-oxidation rate, DIC, and Fe(II) content in the seawater. Approximately half Fe in the MBC primary minerals existed as Fe(III), indicating the existence of indigenous Fe(II)-oxidation in the Paleoarchean seawater. The element Mn in the MBC is primarily Mn(II) coordinated with O, suggesting a reduced depositional environment and hence implying the involvement of anaerobic microbial Fe(II)-oxidation in the formation of the MBC. Collectively, the grey-black laminae with abundant carbonaceous materials reflect limited Fe(II)-oxidation and increased dissolved inorganic carbon content likely due to enhanced hydrothermal CO₂ supply, while the red laminae with considerable hematite represent substantial Fe(III)-supply due to rapid indigenous anaerobic Fe(II)-oxidation.