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

• Published in: Geoscience Frontiers
• Main Authors: Shogo Aoki, Chiho Kabashima, Yasuhiro Kato, Takafumi Hirata, Tsuyoshi Komiya
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2018
• Subjects: Geology; QE1-996.5; Greenland
• Online Access: https://doi.org/10.1016/j.gsf.2016.11.016; https://doaj.org/article/33b430c7727747dfb80dc9c42b37ac97

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 and Co contents drastically decreased ...