Origin of the Neoproterozoic Baijianshan Banded Iron Formation at the Southeastern Margin of the Tarim Block in NW China: Implication for an Extremely Reducing Ocean

The Neoproterozoic banded iron formations (BIFs) were closely associated with the "Snowball Earth" during the breakup of the Rodinia, thus they played an important role in our understanding of the atmospheric and oceanic oxygen levels during this period. In this contribution, the Neoproterozoic (ca. 737 Ma) Baijianshan BIF at Southeast Tarim, northwestern China was identified. Magnetite is the dominated iron-species, which occurs as the lamina interbedded with chert. The BIF contains low concentrations of trace elements, and is depleted in light rare earth elements (LREEs) based on comparison with the Post-Archean Australian Shale (PAAS). In addition, the BIF exhibits slightly positive La-Eu anomalies, negligible Ce anomalies, insignificant Y anomalies, chondritic Y/Ho ratios (23-32), and slightly chondritic initial SMALL ELEMENT OFNd (t = 737 Ma) values (-0.45 to 1.46, averaging 0.37). All these features indicate that the precipitation of Baijianshan BIF was closely related to the submarine low-T hydrothermal fluids with little detrital contribution. Moreover, the Baijianshan BIF is characterized by the significant enrichment of heavy Fe isotopes, with delta(57)Fe(IRMM-01)4 values ranging from 1.78 parts per thousand to 3.05 parts per thousand, revealing the partial oxidation of Fe2+ into Fe3+ during the precipitation of this BIF. Our data suggest that the formation of Baijianshan BIF was closely associated with a significantly reducing ocean, which most likely was isolated from the oxidized atmosphere by a local ice sheet. This Neoproterozoic Baijianshan ocean has the initial oxygen levels as low as, or even lower than that of Archean and Paleoproterozoic oceans.