Iron isotopes constrain biogeochemical redox cycling of iron and manganese in a Palaeoproterozoic stratified basin

Iron Formations: Their Origins and Implications for Ancient Seawater Chemistry

“…Several conflicting interpretations exist for these values, such that independent geological and geochemical evidence should be used to discriminate these models (Johnson et al, 2008a;Tsikos et al, 2010). Further, light iron isotope values are not a silver bullet for DIR even if coupled with light carbonate carbon isotope values.…”

“…Most importantly, iron derived from hydrothermal sources has slightly negative or near-zero iron isotope values (e.g., Rouxel et al, 2008a,b), whereas a benthic flux (dissolved iron derived from iron reduction during early sediment diagenesis on continental shelves and supplied to deep euxinic part of the basin) is likely to have a pronounced negative iron isotope value (e.g., Severmann et al, 2008). This isotopically light benthic Fe source has also been found (Dauphas et al, 2004(Dauphas et al, , 2007Whitehouse and Fedo, 2007), 12 datapoints for the Neoproterozoic Rapitan IF (Halverson et al, 2011), 52 datapoints for IF and Mn formation from the Paleoproterozoic Hotazel Formation, Transvaal Supergroup, South Africa (Tsikos et al, 2010), and 45 datapoints for other Paleoproterozoic and Archean IFs (Planavsky et al, 2012). Gray horizontal bar represents the average d 56 Fe values for igneous rocks and hydrothermal sources (e.g., Beard et al, 2003;Rouxel et al, 2008a,b).…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…The significance of weak Ce anomalies remains contentious considering co-existing positive La anomalies (Tsikos and Moore, 1997). Whereas the Ongeluk volcanics cannot be discounted as a possible source of at least some of the Fe and Mn in the basin through low-temperature seafloor alteration, recent applications of combined iron isotope geochemistry and Mn/Fe ratios suggest that enrichment in the Hotazel palaeobasin in Mn and isotopically light Fe could have ensued progressively through prior protracted deposition of Mnpoor, high d 56 Fe BIF (Tsikos et al, 2010). Indeed, the new age constraints for the Hotazel 59 Formation (Gumsley et al, 2017), and the Fe isotope and Mn-enriched signature of the uppermost Griquatown and Koegas IFs in the Transvaal Supergroup (Kurzweil et al, 2016;Oonk, 2017), lend strong support to this interpretation.…”

8.1 The Great Oxidation Event

“…Whether or not this interpretation of the timing of the GOE is in conflict with the 2317 Ma minimum age cited above, depends on whether one accepts the relatively young whole-rock Pb-Pb age of the underlying Ongeluk lavas (2220 Ma as a maximum age; Cornell et al 1996) or the relatively old whole-rock Pb-Pb carbonate age of the overlying Mooidraai Formation (2390 Ma as a minimum age; Bau et al 1999). As discussed in Tsikos et al (2010), the large uncertainty in the age of the Hotazel Formation prevents us from placing it confidently within the sequence of events associated with the GOE.…”