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Influence of contamination on banded iron formations in the Isua supracrustal belt, West Greenland: Reevaluation of the Eoarchean seawater compositions

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Geoscience Frontiers 2018年第4期9卷 1049-1072页

作者： Shogo Aoki chiho kabashima Yasuhiro Kato Takafumi Hirata Tsuyoshi Komiya Department of Earth Science and Astronomy The University of Tokyo Frontier Research Center for Energy and Resources School of EngineeringThe University of Tokyo Department of Systems Innovation School of EngineeringThe University of Tokyo Ocean Resources Research Center for Next Generation Chiba Institute of Technology Research and Development Center for Submarine Resources Japan Agency for Marine-Earth Science and Technology(JAMSTEC) Division of Earth and Planetary Sciences Kyoto University

Banded Iron Formations（BIFs） are chemical sediments, ubiquitously distributed in the Precambrian supracrustal belts; thus their trace element compositions are helpful for deciphering geochemical evolution on the Earth through time. However, it is necessary to elucidate factors controlling the whole-rock compositions in order to decode the ancient seawater compositions because their compositions are highly variable. We analyzed major and trace element contents of the BIFs in the 3.8-3.7 Ga Isua supracrustal belt（ISB）, southern West Greenland. The BIFs are petrographically classified into four types：Black-,Gray-, Green-and White-types, respectively. The Green-type BIFs contain more amphiboles, and are significantly enriched in Co, Ni, Cu, Zn, Y, heavy rare earth element（HREE） and U contents. However,their bulk compositions are not suitable for estimate of seawater composition because the enrichment was caused by secondary mobility of metamorphic Mg, Ca and Si-rich fluid, involvement of carbonate minerals and silicate minerals of olivine and pyroxene and/or later silicification or contamination of volcanic and clastic materials. The White-type BIFs are predominant in quartz, and have lower transition element and REE contents. The Gray-type BIFs contain both quartz and magnetite. The Black-type BIFs are dominated by magnetite, and contain moderate to high transition element and REE contents. But,positive correlations of V, Ni, Zn and U contents with Zr contents suggest that involvement of detrital,volcanic and exhalative materials influences on their contents. The evidence for significant influence of the materials on the transition element contents such as Ni in the BIFs indicates the transition element contents in the Archean ocean were much lower than previously estimated. We reconstructed secular variations of V,Co, Zn and U contents of BIFs through time, which show Ni and Co contents decreased whereas V, Zn and U contents increased through time. Especially, the Ni

关键词： Banded iron formations Eoarchean Isua supracrustal belt Bioessential elements Seawater compositions

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