| dc.description.abstract | The precise nature of ocean redox conditions is among the most uncertain aspects of the early Earth’s surface zone. It is generally thought that Earth’s atmosphere was anoxic until after ca 2.4 Ga, the Great Oxygenation Event (GOE), but there is less agreement about the timing and extent of the oxygenation of the oceans. Chemical sediments such as (banded) iron formations ((B)IFs) may allow us to reconstruct the evolution of the Precambrian oceans, since they bear information on the redox state and chemical composition of the ambient seawater. The widespread deposition of IF prior to and around the GOE suggests a genetic link with the rise of atmospheric oxygen: they might directly record the oxidation of aqueous Fe(II) in near-surface environments. However, large quantities of Fe could also have accumulated due to an increased influx of hydrothermal Fe(II) into the oceans. Tsikos et al. (2010) reported very depleted bulk-rock Fe isotope values (δ56Fe of -1.6 ‰ down to -2.4 ‰) in the hematite- and Mn-rich samples of the ca 2.4 Ga Hotazel IF (Transvaal basin, South Africa), ascribing these data to deposition of Fe in a terminal, stratified basin that was depleted in heavy Fe isotopes due to earlier precipitation, evolved via Rayleigh fractionation. Here I present results from a unique drill-core (ca 3m) comprising the lowermost part of the Hotazel Fm. and its contact with the Ongeluk Fm. lava at the base. Overall textures, mineralogy and geochemistry of this hematite-rich rock suggest that in the lower part of the Hotazel ferric oxyhydroxides (hematite precursors) precipitated from seawater with a significant contribution of hydrothermal fluids directly sourced from the Ongeluk lava, whereas in the upper part of the drill-core precipitation took place from more open-marine seawater without a direct hydrothermal component. δ56Fe values increasing from -0.50 ‰ for the hematite samples closest to the hydrothermally-altered Ongeluk lava (δ56Fe of -0.7 ‰) to -0.26 ‰ for the uppermost hematite BIF part of the drill-core equally support this. Stratigraphically lower in the Transvaal Supergroup (Kuruman and Griquatown BIFs) near-zero to positive δ56Fe have been reported for hematite (Johnson et al., 2003). The δ56Fe of -0.26 of the most primary, seawater representative hematite BIF sample at the top of the drill-core may therefore represent the first position in the stratigraphy of the Transvaal basin where the isotopic signature becomes depleted and confirms the postulated link with a significant rise in the oxygen concentration of the basin. Moreover, the uniquely depleted δ56Fe signal of the Ongeluk lava hydrothermal fluids (δ56Fe of ≤ -0.7 ‰) illustrates the effect of progressive fluid-rock interactions under largely aerobic conditions in the shallowest parts of the Transvaal basin. | |
