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Li Y.,Chinese Academy of Geological Sciences | Li Y.,Key Laboratory of Isotope Geology | Hou K.,Chinese Academy of Geological Sciences | Hou K.,Key Laboratory of Isotope Geology | And 6 more authors.
Ore Geology Reviews

Banded iron formations (BIFs) are Precambrian chemical marine sedimentary formations that record major transitions of chemical composition, and oxidation-reduction state of oceans at the time of their deposition. In this paper, we report silicon and oxygen isotope compositions of a variety of BIFs from the North China Craton (NCC) in order to deduce the mechanism of their formation. Quartz in the various types of BIFs from the NCC are generally depleted in 30Si, where δ30SiNBS-28 values range from -2.0‰ to -0.3‰ (average, -0.8‰), similar to δ30SiNBS-28 values measured from modern submarine black chimneys and sinters. The δ18OV-SMOW values of quartz in the BIFs are relatively high (8.1‰-21.5‰; average, 13.1‰), similar to those of siliceous rock formed by hydrothermal activities. The δ30SiNBS-28 values of quartz in magnetite bands are commonly lower than those of quartz in adjacent siliceous bands within the same sample, whereas δ18OV-SMOW values are higher in the magnetite bands. A negative correlation is observed between δ30SiNBS-28 and δ18OV-SMOW values of quartz from siliceous and magnetite bands in BIF from Fuping, Hebei Province. The isotopic compositions of silicon and oxygen of quartz in BIFs provide insights for the formation mechanisms of silicon-iron cyclothems in BIFs. After the silicon- and iron-rich hydrothermal solution was injected onto the seabed, the abrupt temperature drop caused oversaturation of silicic acid, resulting in rapid precipitation of SiO2 and deposition of siliceous layers. Ferric hydroxide was precipitated later than SiO2 because of low free-oxygen concentration in the ocean bottom. Progressive mixing of hydrothermal solution with seawater caused a continuous drop in temperature and an increase in Eh values, resulting in gradual oxidation of hydrothermal Fe2+ and deposition of iron-rich layers. In summary, each silicon-iron cyclothem marks a large-scale submarine hydrothermal exhalation. The periodic nature of these exhalations resulted in the formation of regular silicon-iron cyclothems. The widespread distribution of BIFs indicates that volcanism and submarine hydrothermal exhalation were extensive; the low δ30SiNBS-28 and high δ18O V-SMOW values of the BIFs indicate that the temperature of seawater was relatively high at the time of BIF formation, and that concentrations of Fe2+ and H4SiO4 in seawater were saturated. © 2013 Elsevier B.V. Source

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