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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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Deserpentinization and high-pressure(eclogite-facies) metamorphic features in the Eoarchean ultramafic body from isua,Greenland

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Geoscience Frontiers 2022年第1期13卷 297-309页

作者： Juan Miguel Guotana Tomoaki Morishita Ikuya Nishio Akihiro Tamura Tomoyuki Mizukami Kenichiro Tani Yumiko Harigane Kristoffer Szilas D.Graham Pearson Graduate School of Natural Science&Technology Kanazawa UniversityJapan School of Environmental Science and Management University of the Philippines Los BañosLagunaPhilippines Lamont-Doherty Earth Observatory Columbia UniversityPalisadesNY 10964USA Volcanoes and Earth’s Interior Research Center Japan Agency for Marine-Earth Science and Technology2-15 NatsushimaKanagawa 237-0061Japan National Museum of Nature and Science 4-1-1AmakuboTsukuba-shiIbaraki 305-0005Japan Institute of Geology and Geoinformation Geological Survey of Japan/AIST Central 71-1-1 HigashiTsukubaIbaraki 305-8567Japan Department of Geosciences and Natural Resource Management University of CopenhagenØster Voldgade 101350 CopenhagenDenmark Department of Earth&Atmospheric Sciences University of AlbertaEdmontonABCanada

Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma isua supracrustal belt(ISB),hosted in the Itsaq Gneiss Complex of southwestern *** bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic *** rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral ***-rich humite-group minerals such as titanian chondrodite(Ti-Chn)and titanian clinohumite(Ti-Chu)often occur as accessory phases in the metamorphosed ultramafic *** Ti-rich humite minerals are associated with metamorphic *** host olivine is highly forsteritic(Fo96-98)with variable Mn O and Ni O *** concentrations of the rare-earth elements(REE)and high-field strength elements(HFSE)of the metamorphic olivine are higher than typical mantle *** textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial *** close association of olivine,antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between$1.3–2.6 GPa within the antigorite stability field(<700°C).The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels.

关键词： Titanian chondrodite Titanian clinohumite isua supracrustal belt UHP Deserpentinization

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