NEW INSIGHT INTO DEPOSITION OF THE NEOPROTEROZOIC RAPITAN IRON FORMATION

Cryogenian iron formations, present on nine different continents, are of particular interest in determining paleo-ocean chemistry as Fe is sensitive to ocean redox. While rare earth element and yttrium patterns have been extensively studied and are now broadly understood, little work has been carried out on iron isotopes nor have such studies applied findings within a stratigraphic context. Currently, a single published iron isotope data set exists on Neoproterozoic iron formations [1]. Spanning 13m in section, the systematic upward trend in δ57Fe has been interpreted as a strong chemocline where the upward diffusion of iron results in an isotopic gradient with isotopically heavy iron enriching the deeper section. This model necessitates a single non-replenished Fe source in addition to a well stratified basin. Conservative calculation on the concentrations using current Rapitan grades and thicknesses would require concentration of roughly 240 g/L Fe (avg. grade 43% Fe2O3, σ = 4000 kg/m3, avg. basin depth 500 m, avg. deposit thickness 30m) . Such concentrations are unrealistically high and 3 orders of magnitude above current levels in Lake Nyos, Cameroon thus neccesitating an iron source during deposition complicating the Fe isotope story [1]. Herein we present a new geochemical data set for the Rapitan iron formation within a stratigraphic context including bulk rock REE and δ57Fe as well as δ13C on carbonate nodules. We attempt to provide new insights into the Rapitan iron formation and its connection to the “Snowball Earth” event.

[1] Halverson et al. 2011. Earth and Plan. Sci. Let. 309(1-2), 100-112.

[2] Kling et al. 1989. J. of Volc. and Geotherm. Res. 39, 151-165.