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

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

机构地区：[1]Department of Earth Science and Astronomy,The University of Tokyo,3-8-1 Komaba Meguro-ku,Tokyo 153-8902,Japan [2]Center for Northeast Asian Studies,Tohoku University,41 Kawauchi,Aoba-ku,Sendai 980-8576,Japan [3]Department of Geology,Universitédes Sciences et Techniques de Masuku,BP:901,90025 Franceville,Gabon [4]Department of Earth and Planetary Science,The University of Tokyo,7-3-1 Hongo,Bunkyo-ku,Tokyo 113-0033,Japan [5]Geochemical Research Center,The University of Tokyo,7-3-1,Hongo,Bunkyo-ku,Tokyo 113-0033,Japan [6]Graduate School of Human and Environmental Studies,Kyoto University,Yoshida-nihonmatsu,Sakyo-ku,Kyoto 606-8501,Japan [7]Center for Fundamental Education,Okayama University of Science,1-1,Ridai-cho,Kita-ku,Okayama City,Okayama 700-0005,Japan

出　　处：《Geoscience Frontiers》2024年第3期31-49,共19页地学前缘（英文版）

基　　金：Shoko Science Co.Ltd.This work was partly supported by a grant from the Ministry of Education,Culture,Sports,Science and Technology,Japan(No.17H04858,19H01081,19K14828,20H01991,23K13205);the Fukada Grant-in-Aid(2021)of Fukada Geological Institute(No.135).

摘　　要：The Paleoproterozoic Francevillian Group in the eastern Gabonese Republic has been recently attracting increasing attention because it includes distinctive macroscopic structures interpreted as eukaryotic fossils.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 sediments.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 characteristics 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 preferentially absorbed by

关 键 词：Carbonate diagenesis Francevillian Group LA-ICP-MS Seawater composition Y/Ho fractionation 

分 类 号：P597.3[天文地球—地球化学] P512.2[天文地球—地质学]