09:45 〜 10:00
[BCG06-04] Origin of the Algoma-type banded iron formation in the Eoarchean Nuvvuagittuq supracrustal belt
キーワード：縞状鉄鉱層、ヌブアギツック、アルカリ熱水、グリーナライト
Deciphering the ancient oceanic environment is one of the key issues in unravelling the coevolutionary history of life and the surface environment. Banded Iron Formations are typical sedimentary rocks in the Archean era and provide a unique record of the Archean hydrosphere. They are classified into two types: Superior- and Algoma-types. Algoma-type banded iron formations occur in the Archean greenstone belts associated with metabasalt and are considered to have been formed under the influence of hydrothermal activity. However, its formation process is still controversial: oversaturation of Fe-hydroxide caused by acidic Fe2+-rich hydrothermal fluid like modern black-smoker (Dymek & Klein, 1988) or by dissolved silica-rich alkaline hydrothermal fluid (Shibuya et al., 2010), and Fe2+ oxidation by Fe-oxidizing bacteria (Widdel et al., 1993). In addition, recent petrographic and experimental studies have suggested that the precipitation of Fe-silicate (e.g., greenalite) played an important role in the formation (Rasmussen et al., 2021).
The Nuvvuagittuq supracrustal belt in Nunavik, Northern Quebec is a >3.8 Ga supracrustal belt, which hosts one of the oldest metasedimentary rocks: banded iron formations and cherts. The banded iron formation mainly consists of magnetite, cummingtonite, and quartz layers, with minor layers containing garnet and hornblende. The chert primarily consists of quartz with various amounts of cummingtonite. Some of these layers are distinct from each other, suggesting that they reflect the original differences in precipitation processes.
Major element compositions shows that the Fe2O3 contents of cummingtonite-rich rocks are strongly correlated with those of MgO and MnO, which suggest that the primary Fe-precipitates of cummingtonite was an Fe2+-bearing mineral, i.e., silicate. On the other hand, the Fe2O3 contents of magnetite-rich rocks are strongly correlated with the P2O5 contents, which suggests that the primary Fe precipitates of magnetite were Fe3+-bearing hydroxide. Also, the rare earth element patterns show that the magnetite-rich rocks are more strongly influenced by hydrothermal activity than the cummingtonite-rich rocks.
The petrographic and chemical characteristics suggest that silicate and hydroxide were precipitated under weaker and stronger hydrothermal influence, respectively, and then were transformed into the cummingtonite and magnetite during the diagenesis, respectively. None of the aforementioned processes can account for these observations. Assuming a ferruginous and Si-saturated seawater composition, the calculated Pourbaix diagram of the Fe species shows that alkaline fluids can cause both greenalite and Fe hydroxide precipitation. Thus, we propose a new mechanism for the formation of Algoma-type banded iron formations, which is characterized by the precipitation of silicate and hydroxide driven by alkaline hydrothermal fluids.
The Nuvvuagittuq supracrustal belt in Nunavik, Northern Quebec is a >3.8 Ga supracrustal belt, which hosts one of the oldest metasedimentary rocks: banded iron formations and cherts. The banded iron formation mainly consists of magnetite, cummingtonite, and quartz layers, with minor layers containing garnet and hornblende. The chert primarily consists of quartz with various amounts of cummingtonite. Some of these layers are distinct from each other, suggesting that they reflect the original differences in precipitation processes.
Major element compositions shows that the Fe2O3 contents of cummingtonite-rich rocks are strongly correlated with those of MgO and MnO, which suggest that the primary Fe-precipitates of cummingtonite was an Fe2+-bearing mineral, i.e., silicate. On the other hand, the Fe2O3 contents of magnetite-rich rocks are strongly correlated with the P2O5 contents, which suggests that the primary Fe precipitates of magnetite were Fe3+-bearing hydroxide. Also, the rare earth element patterns show that the magnetite-rich rocks are more strongly influenced by hydrothermal activity than the cummingtonite-rich rocks.
The petrographic and chemical characteristics suggest that silicate and hydroxide were precipitated under weaker and stronger hydrothermal influence, respectively, and then were transformed into the cummingtonite and magnetite during the diagenesis, respectively. None of the aforementioned processes can account for these observations. Assuming a ferruginous and Si-saturated seawater composition, the calculated Pourbaix diagram of the Fe species shows that alkaline fluids can cause both greenalite and Fe hydroxide precipitation. Thus, we propose a new mechanism for the formation of Algoma-type banded iron formations, which is characterized by the precipitation of silicate and hydroxide driven by alkaline hydrothermal fluids.