首页|Depositional condition of Paleoproterozoic Francevillian carbonate rocks revisited from rare earth element contents

Depositional condition of Paleoproterozoic Francevillian carbonate rocks revisited from rare earth element contents

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The Paleoproterozoic Francevillian Group in the eastern Gabonese Republic has been recently attracting increasing attention because it includes distinctive macroscopic structures interpreted as eukaryotic fos-sils.Therefore,its depositional setting and associated redox conditions need to be clearly understood.To clarify these,petrological studies and rare earth element(REE)abundances of carbonate rocks of the Francevillian Group in the Lastoursville basin were determined with ICP-MS coupled with a laser ablation sampling technique.Detailed microscopic observations indicate that dolostones in this basin underwent the complicated diagenetic history,including sparitization,decomposition of organic matter,reduction of ferromanganese oxides,silicification,and later calcification.The dolostones of lower Francevillian B(FB)Formation show heterogeneous textures formed by alterations during diagenesis.By the alterations,changes of REE abundances and degrees of positive Y anomalies appear to have occurred.This indicates that REE signatures in carbonate rocks can vary during diagenesis.Characteristically high Mn contents in the dolostones of upper FB Formation were derived from the reduction of ferromanganese oxides in sed-iments.The signatures of REE abundance in dolostones can be explained by the mixing of seawater and ferromanganese oxide components.In that sense,the dolostones inherited the trace element character-istics of precursor ferromanganese oxides even after the diagenesis.Y/Ho values of the most primary parts of each sample exceed 33,the newly determined threshold value for marine carbonate rocks based on our compilation.The magnitude of the La anomaly values also falls within the range of that of Holocene reefal microbialites.These indicate that a precursor of the dolostone of the Francevillian Group was deposited in a marine environment.In addition,presence of Ce anomalies in the upper FB Formation suggests that the Paleoproterozoic ocean was oxidized to such an extent as for Ce to be pref-erentially absorbed by ferromanganese oxides.

Carbonate diagenesisFrancevillian GroupLA-ICP-MSSeawater compositionY/Ho fractionation

Satoshi Yoshida、Karen Bakakas Mayika、Yuki Ishihara、Mathieu Moussavou、Hisashi Asanuma、Tomohiko Sato、Takafumi Hirata、Cédric Ligna、Yusuke Sawaki、Amboise Edou-Minko

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Department of Earth Science and Astronomy,The University of Tokyo,3-8-1 Komaba Meguro-ku,Tokyo 153-8902,Japan

Center for Northeast Asian Studies,Tohoku University,41 Kawauchi,Aoba-ku,Sendai 980-8576,Japan

Department of Geology,Université des Sciences et Techniques de Masuku,BP:901,90025 Franceville,Gabon

Department of Earth and Planetary Science,The University of Tokyo,7-3-1 Hongo,Bunkyo-ku,Tokyo 113-0033,Japan

Geochemical Research Center,The University of Tokyo,7-3-1,Hongo,Bunkyo-ku,Tokyo 113-0033,Japan

Graduate School of Human and Environmental Studies,Kyoto University,Yoshida-nihonmatsu,Sakyo-ku,Kyoto 606-8501,Japan

Center for Fundamental Education,Okayama University of Science,1-1,Ridai-cho,Kita-ku,Okayama City,Okayama 700-0005,Japan

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Ministry of Education,Culture,Sports,Science and Technology of JapanMinistry of Education,Culture,Sports,Science and Technology of JapanMinistry of Education,Culture,Sports,Science and Technology of JapanMinistry of Education,Culture,Sports,Science and Technology of JapanMinistry of Education,Culture,Sports,Science and Technology of JapanFukada Grantin-Aidof Fukada Geological Institute(2021)

17H0485819H0108119K1482820H0199123K13205135

2024

10.1016/j.gsf.2023.101771

地学前缘(英文版)
中国地质大学（北京） 北京大学

地学前缘(英文版)

CSTPCD
影响因子：0.576
ISSN：1674-9871
年,卷(期)：2024.15(3)
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