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Rare earth elements in Precambrian banded iron formations: Secular changes of Ce and Eu anomalies and evolution of atmospheric oxygen

By
Yasuhiro Kato
Yasuhiro Kato
1
Department of Geosystem Engineering, University of Tokyo, Bunkyo-Ku, Tokyo 113-8656, Japan
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Kosei E. Yamaguchi
Kosei E. Yamaguchi
2
Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Kanagawa 237-0061, Japan
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Hiroshi Ohmoto
Hiroshi Ohmoto
3
Penn State Astrobiology Research Center of the NASA Astrobiology Institute and the Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
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Published:
January 01, 2006

Rare earth element (REE) analyses of Precambrian banded iron formations (BIFs) show that distinct negative Ce anomalies, although rather weak or moderate (Ce/Ce* = 0.5–0.9), are commonly present in Algoma-type BIFs of the Early and Middle Archean, and even in the 3.8–3.7 Ga Isua iron formation (IF). This indicates that the seawater columns from which the BIFs precipitated were not entirely anoxic and that Ce oxidation mechanisms already existed in the 3.8–3.7 Ga oceans. The presence of pronounced negative Ce anomalies (Ce/Ce* = 0.1–0.5) in Late Archean (2.9–2.7 Ga) Algoma-type BIFs suggests that strongly oxygenated oceanic conditions like today emerged by 2.9–2.7 Ga. This suggestion is consistent with geologic evidence that small but widespread Mn deposits formed during Late Archean time. The Hamersley and Transvaal IFs (2.7–2.4 Ga in age) have less noticeable Ce anomalies (Ce/Ce* = 0.7–1.0). These BIFs were deposited on an evolving rift in a land-locked ocean that became anoxic due to intense hydrothermal activity. The 2.2–2.1 Ga Superior-type IFs exhibit distinct negative Ce anomalies (Ce/Ce* = 0.2–0.7), but the ca. 1.9 Ga IFs and the ca. 0.7 Ga IFs have less distinct Ce anomalies. These variations in Ce/Ce* values of the post-2.7 Ga BIFs may reflect the episodicity in global mantle plume activity that created locally anoxic basins.

The Archean Algoma-type BIFs have distinctly positive but variable Eu anomalies (Eu/Eu* = 0.8–7). Their strong positive Eu anomalies suggest large contributions of hydrothermal fluids to the seawater involved in BIF precipitation. The large variation in Eu anomalies in Algoma-type BIFs reflects the large variation in mixing ratios of hydrothermal fluids and ambient seawaters at various depositional sites. However, the post-2.7 Ga Superior-type BIFs exhibit much lower and constant Eu/Eu* values (0.7–1.9). This implies that the REE chemistry of the basin water that hosted voluminous Superior-type BIFs was influenced by a riverine influx from the surrounding continent that grew rapidly due to global mantle plume activity, besides the intense hydrothermal influx.

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GSA Memoirs

Evolution of Early Earth's Atmosphere, Hydrosphere, and Biosphere - Constraints from Ore Deposits

Stephen E. Kesler
Stephen E. Kesler
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Hiroshi Ohmoto
Hiroshi Ohmoto
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Geological Society of America
Volume
198
ISBN print:
9780813711980
Publication date:
January 01, 2006

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