Abstract
A high resolution stratigraphically controlled rare earth element and carbon isotope study of the 2.75 Ga old Algoma-type Helen Iron Formation has provided new insights into the chemistry of late Archean seawater. The Helen Iron Formation passes from a sediment dominated by rhyolitic detrital material in the first 20 m of the section into a purely chemical sediment reflecting the chemistry of the seawater from which it precipitated. An overprint of a hydrothermal signature, seen in the development of a positive europium anomaly, increasing in magnitude upsection is in good accordance with field observations indicating a deepening of the Helen basin through time. The lack of a cerium anomaly along with the predominance of Fe2+ over Fe3+ indicates that oxygen played a minor role in the deposition of the Helen Iron Formation.Carbon isotope data are unusual for iron formation as they cluster around 0‰ (VPDB) and indicate direct precipitation from seawater. However, highly negative carbon isotope excursions suggest two episodes of dissimilatory iron reduction in sediments rich in organic matter, the negative carbon isotope signatures being inherited from the oxidation of this organic matter. These excursions have high magnitudes of 3 and 10.5‰, respectively.These results indicate that the Helen Iron Formation was deposited in a restricted basin with high input of Fe2+ from distal hydrothermal sources and high levels of CO2, explained by submarine volcanic activity in the Helen Basin. We suggest that CO2 and Fe oversaturation led to the direct precipitation of siderite and minor ankerite and that the negative carbon isotope excursions observed are the result of the oxidation of organic matter previously produced by bacterial dissimilatory iron reduction. The Helen Iron Formation thus is a direct precipitate from seawater in a basin increasing in water depth from moderately deep to deep water and presents a significant local hiatus in submarine volcanism.
Published Version
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