Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 292, Issue -, Pages 1-23Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2020.09.017
Keywords
Iron formations; Fe isotopes; Nd isotopes; Ferruginous conditions; Anoxia; Submarine volcanism
Categories
Funding
- Natural Science Foundation of China [41972075, 41702063]
- China Postdoctoral Science Foundation [2017M613027, 2018T111004]
- Shaanxi Province Postdoctoral Research Funding Project [2017BSHEDZZ14]
- Fundamental Research Funds for the Central Universities, CHD [300102279202, 300102279401]
- Donnelley Postdoctoral Fellowship from the Yale Institute for Biospheric Studies
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Iron formations (IFs) are iron-rich chemical sediments widespread in the Precambrian, providing key evidence of anoxic and ferruginous marine conditions throughout Earth's history. The Shikebutai IF in northwest China's Western Tianshan region is predominantly composed of hematite, jasper, and siderite, with an authigenic origin indicated by its chemical composition dominated by Fe2O3 and SiO2.
Iron formations (IFs) are iron-rich chemical sediments that were widespread throughout the Precambrian. The deposition of IFs has provided key evidence about the prevalence of anoxic and ferruginous marine conditions throughout Earth's history. Here, we describe the Shikebutai IF, hosted in a Late Carboniferous volcano-sedimentary sequence in the Western Tianshan region of northwest China. The IF is largely composed of hematite, jasper, and siderite. The chemical composition of the Shikebutai IF is dominated by Fe2O3 and SiO2, and very low concentrations of Al2O3, TiO2, Th, Nb, and Sc were observed, indicative of an authigenic origin. High Fe/Ti, Fe/Al, Co/Zn, and Ni/Zn ratios, along with positive Eu anomalies and epsilon(Nd)(t) values point to a significant submarine volcanic and hydrothermal contribution. The absence of negative Ce anomalies and heterogeneous delta Fe-56 values suggest progressive oxidation of hydrothermally sourced Fe(II) and deposition in an anoxic to low oxygen basin. Low P/Fe ratios in the Shikebutai IF, comparable to those found in the Precambrian, are consistent with Si inhibition of P sorption and P drawdown with high hydrothermal inputs. The intimate association of this Late Carboniferous IF with submarine volcanic activity suggests an origin that is analogous to Precambrian Algoma-type IFs. Further, the Shikebutai IF provides evidence for an episode of basin-scale anoxic, ferruginous conditions coinciding with the generally well-oxygenated deep oceans of the Phanerozoic, with deposition driven by enhanced submarine volcanic and hydrothermal activity in the Western Tianshan region during the Late Carboniferous. This demonstrates how, even with near modern surface oxygen levels, enhanced hydrothermal activity can shape the marine redox landscape and cause the development of ferruginous conditions without quantitative sulfate drawdown. (c) 2020 Elsevier Ltd. All rights reserved.
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