Source heterogeneity for the major components of ∼ 3.7 Ga Banded Iron Formations (Isua Greenstone Belt,Western Greenland): Tracing the nature of interacting water masses in BIF formation |
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| Institution: | 1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;1. CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;2. School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Republic of Korea;3. No. 313 Geological Team, Bureau of Geology and Exploration of Anhui, Liu''an 236000, China;4. Key Laboratory for Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;1. Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada;2. Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA;3. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02542, USA;4. Mineral Exploration Research Centre, Harquail School of Earth Science, Laurentian University, Sudbury, Ontario P3E 2C6, Canada;5. UMR6538 Laboratoire Géosciences Océan, European Institute for Marine Studies, Technopôle Brest-Iroise, Plouzané 29280, France;6. Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada;7. Department of Geology, Rhodes University, Grahamstown 6140, South Africa;8. Department of Earth Sciences, University of California, Riverside, CA 92521, USA;9. Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706, USA;10. NASA Astrobiology Institute, USA |
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| Abstract: | We report trace element, samarium (Sm)–neodymium (Nd) and lead (Pb) isotopic data for individual micro-and mesobands of the Earth's oldest Banded Iron Formation (BIF) from the ∼ 3.7–3.8 Ga Isua Greenstone Belt (IGB, West Greenland) in an attempt to contribute to the characterization of the depositional environment and to the understanding of depositional mechanisms of these earliest chemical sediments. Rare earth element (REE)-yttrium (Y) patterns of the individual mesobands show features of modern seawater with diagnostic cerium (Ce/Ce⁎), presodymium (Pr/Pr⁎) and Y/holmium (Ho) anomalies. Very low high field strength elements (HFSE) concentrations indicate essentially detritus-free precipitation. Uranogenic Pb isotope data define a correlation line with a slope of 3691 ± 41 Ma, indicating that the uranium (U)–lead (Pb) system remained closed after the formation of this BIF. High 207Pb/204Pb relative to 206Pb/204Pb ratios compared to average mantle growth evolution models are a feature shared by BIF, penecontemporaneous basalts and clastic volcanogenic metasediments and are indicative of the ultimate high-μ (238U/204Pb) character of the source region, an essentially mafic Hadean protocrust. Sm–Nd isotopic relations on a layer-by-layer basis point to two REE sources controlling the back-arc basin depositional environment of the BIF, one being seafloor-vented hydrothermal fluids (εNd (3.7 Ga) ∼ + 3.1), the other being ambient surface seawater which reached its composition by erosion of parts of the protocrustal landmass (εNd(3.7 Ga) ∼ + 1.6). The validity of two different and periodically interacting water masses (an essentially two-component mixing system) in the deposition of alternating iron- and silica-rich layers is also reflected by systematic trends in germanium (Ge)/silicon (Si) ratios. These suggest that significant amounts of silica were derived from unexposed and/or destroyed mafic Hadean landmass, unlike iron which probably originated from oceanic crust following hydrothermal alteration by deep percolating seawater. Ge/Si distributional patterns in the early Archean Isua BIF are similar to those reported from the Paleoproterozoic Hamersley (Western Australia) BIF, but overall Ge concentrations are about one order of magnitude higher in the Archean BIF. This seems consistent with other lines of evidence that the ambient Archean seawater was enriched with iron relative to Proterozoic and recent seawater. |
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