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An evaluation of diagenetic recycling as a source of iron for banded iron formations

By
R. Raiswell
R. Raiswell
1
School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
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Published:
January 01, 2006

REE and Nd isotope data indicate that most of the iron in banded iron formations is derived from hydrothermal sources but do not exclude a significant contribution from terrestrial sources, such as diagenetic recycling. A diagenetic model has been used to estimate the recycling of iron into overlying seawater, due to microbial reduction and dissolution at depth in anoxic sediment pore waters, followed by diffusion upward through a surface layer of sediment that contains oxygenated pore waters. Rates of iron recycling increase with higher pore-water dissolved iron concentrations, decreasing pH and temperature, and smaller thicknesses of the surface oxygenated layer. Iron can be recycled at rates of 1000–5000 µg cm−2 yr−1 from Proterozoic (pO2 = PAL) pore waters with dissolved Fe2+ = 1–5 µg cm−3, pH 6.5 (and T < 65 °C), or pH 7.0 (and T < 40 °C), or pH 7.5 (and T < 20 °C), provided the thickness of the surface oxygenated layer is less than 0.1 cm. Lower pO2 levels and more weakly oxygenated surface layers do not significantly increase the maximum recycling rates but enable these to be achieved at larger thicknesses of the surface layer, for all pH 6.5–7.5 and temperatures from 10 to 65 °C. Rates of iron supplied by diagenetic recycling can be substantially modified by the export efficiency (ϵ) from the source area and by the ratio (Source Area)/(Sink Area), which can either disperse or concentrate the recycling flux that is delivered to a sink area of banded iron formation. Banded iron formations that require maximum iron delivery rates of 22500 µg cm−2 yr−1 can be produced only by recycling rates of 5000 µg cm−2 yr−1 (and ϵ = 1) from a source area that is at least four times larger than the area of banded iron formation. Modern basins have ratios of shelf area (<200 m water depth) to deep basin area that commonly range from 0.2 to 4. Basins at either extreme have ratios of (Deep Basin Area)/(Shelf Area) or (Shelf Area)/(Deep Basin Area) that exceed 4 and are potentially able to concentrate iron either from a deep basin source area to banded iron formation on the shelf, or from a shelf source area to a banded iron formation depositing in the deep basin. However, these mass balance constraints require the existence of substantial areas of contemporaneous source sediments (or smaller areas of iron-enriched sediments) located either on the shelf or in the deep basin.

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GSA Memoirs

Evolution of Early Earth's Atmosphere, Hydrosphere, and Biosphere - Constraints from Ore Deposits

Stephen E. Kesler
Stephen E. Kesler
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Hiroshi Ohmoto
Hiroshi Ohmoto
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Geological Society of America
Volume
198
ISBN print:
9780813711980
Publication date:
January 01, 2006

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