Origin of the Neoproterozoic Fulu iron formation, South China: Insights from iron isotopes and rare earth element patterns

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Busigny, Vincent, Planavsky, Noah J., Goldbaum, Elizabeth, Lechte, Maxwell A., Feng, Lianjun, Lyons, Timothy W.
• Other Authors: Institut de Physique du Globe de Paris (IPGP), Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2018
• Subjects: Iron isotopes; REE; Proterozoic; Cryogenian; Iron formation; [SDU]Sciences of the Universe [physics]; Sea ice
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-03589305; https://doi.org/10.1016/j.gca.2018.09.006

International audience In the Neoproterozoic Era there was widespread deposition of iron formations in close association with global or near glaciations. These 'Snowball Earth' glaciations likely played a key role in iron formation distribution and deposition. However, the environmental conditions, Fe sources, and formation mechanisms remain debated. Here we present the rare earth element geochemistry and Fe isotope composition of the synglacial iron formation within the Neoproterozoic Fulu Formation, South China. The Fulu iron formation consists of layers of authigenic minerals (mainly hematite) and detrital components (quartz, feldspars, Fe chlorite, and minor biotite). Positive Eu anomalies in one of the Fulu localities indicate a hydrothermal influence, suggesting that Fe was mainly sourced from distal hydrothermal systems. The bulk-rock Fe isotope composition of the Fulu iron formation shows a large range, with δ 56 Fe from -0.23 to +1.78‰. Correlation between bulk-rock δ 56 Fe values and Al/Fe ratios demonstrates that δ 56 Fe variability reflects, in part, varying proportions of authigenic versus detrital components. The Fe isotope composition of authigenic hematite is calculated by a linear regression and shows δ 56 Fe between +0.83 and +2.21‰, with an average at +1.54 ± 0.50‰ (2σ, n = 41). Using a dispersion-reaction model, the high δ 56 Fe values of hematite constrain local dissolved O 2 concentrations of the ocean to less than 0.4 nmol/L, even in the shallow part of the water column. This relationship is consistent with highly reducing conditions in the Neoproterozoic oceans favored by isolation from the atmosphere by a sea ice. We attribute the extremely positive values to partial iron oxidation in waters that were cold relative to modern surface oceans. The dominant occurrence of hematite supports an abiotic precipitation pathway, given that biological activity would have introduced organic matter to the sediments and led to partial reduction of Fe(III) oxides and subsequent formation of magnetite ...