The Behavior of Rare Earth Elements during Green Clay Authigenesis on the Congo Continental Shelf

Type Article
Date 2023-08
Language English
Author(s) Bayon Germain1, Giresse PierreORCID2, Chen Hongjin3, 8, Rouget Marie-Laure8, Gueguen Bleuenn8, Moizinho Gabriel RibeiroORCID4, 5, 8, Barrat Jean-AlixORCID6, Beaufort Daniel7
Affiliation(s) 1 : Univ Brest, CNRS, Ifremer, Geo-Ocean, F-29280 Plouzané, France
2 : Centre of Education and Research on Mediterranean Environments (CEFREM), UMR CNRS 5110, Perpignan Via Domitia University, F-66860 Perpignan, France
3 : Key Laboratory of Marine Mineral Resources, Ministry of Natural Resources, Guangzhou Marine Geological Survey, China Geological Survey, Guangzhou 510760, China
4 : Géosciences-Environnement Toulouse, Université de Toulouse, UPS (SVT-OMP), CNRS, IRD, F-31400 Toulouse, France
5 : Instituto de Geociências, Universidade de Brasília, Campus Universitário Darcy Ribeiro, Ala Central, Brasília 70910-900, DF, Brazil
6 : Univ Brest, CNRS, LEMAR, Institut Universitaire Européen de la Mer (IUEM), F-29280 Plouzané, France
7 : Université de Poitiers, IC2MP-UMR 7285-CNRS, F-86073 Poitiers, France
8 : Univ Brest, CNRS, Ifremer, Geo-Ocean, F-29280 Plouzané, France
Source Minerals (2075-163X) (MDPI AG), 2023-08 , Vol. 13 , N. 8 , P. 1081 (23p.)
DOI 10.3390/min13081081
Note This article belongs to the Special Issue Formation and Evolution of Glauconite. New Scale Approach
Keyword(s) neodymium, hafnium, kaolinite, phosphate minerals, green marine clay authigenesis

Clay mineral authigenesis at continental margins plays an important role in global marine element cycles. However, despite being increasingly used as tracers for both modern and past oceanographic conditions, the behavior of the rare earth elements (REEs) and their isotopes during marine clay authigenesis still remains poorly known. In this study, we report on a detailed geochemical investigation of glauconite from the West African continental shelf, near the mouth of the Congo River. Elemental, neodymium, and hafnium isotope analyses were conducted on both acid leachate and separated clay-size fractions of glauconite pellets, in order to investigate the behavior of REE during the formation of authigenic clays. Our data indicate that kaolinite dissolution and subsequent Fe-bearing clay authigenesis act as a net source of REEs to seawater. We show that enhanced glauconitization, as inferred from increasing Fe and K contents, is accompanied by significant decoupling of the REE toward markedly LREE-enriched shale-normalized patterns in neoformed clay separates. Using both Nd and Hf isotopes and SEM observations, we rule out any seawater influence and argue that this shift primarily reflects the progressively overwhelming presence of insoluble nanocrystals of detrital LREE-rich phosphates, which are known to occur in close association with kaolinite in tropical soils. Due to their marked insolubility in surface environments, such nanocrystals can be preserved during kaolinite dissolution and subsequently incorporated into the aggregates of authigenic green clays forming the peloids. Most strikingly, we show that the combined influence of net REE loss (due to kaolinite dissolution) and decoupling (due to subsequent entrapment of inherited LREE-bearing accessory phases into neoformed clay minerals) is accompanied by preferential release of a dissolved REE fraction characterized by seawater-like distribution patterns. These findings reinforce the emerging view that clay mineral dissolution and authigenesis at continental margins possibly play a major role in marine REE cycling.

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Bayon Germain, Giresse Pierre, Chen Hongjin, Rouget Marie-Laure, Gueguen Bleuenn, Moizinho Gabriel Ribeiro, Barrat Jean-Alix, Beaufort Daniel (2023). The Behavior of Rare Earth Elements during Green Clay Authigenesis on the Congo Continental Shelf. Minerals, 13(8), 1081 (23p.). Publisher's official version : , Open Access version :