Post-depositional REE mobility in a Paleoarchean banded iron formation revealed by La-Ce geochronology: A cautionary tale for signals of ancient oxygenation

Type Article
Date 2020-10
Language English
Author(s) Bonnand P.1, Lalonde Stefan2, Boyet M.1, Heubeck C.3, Homann M.2, 4, Nonnotte Philippe2, Foster I.2, Konhauser K.O.5, Köhler I.3, 6
Affiliation(s) 1 : Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, IRD, OPGC, F-63000 Clermont-Ferrand, France
2 : CNRS-UMR 6538 Laboratoire Géosciences Océan, European Institute for Marine Studies, Technopôle Brest-Iroise, 29280 Plouzané, France
3 : Department of Geosciences, Friedrich-Schiller-University Jena, Jena 07749, Germany
4 : Department of Earth Sciences, University College London, 5 Gower Place, WC1E 6BS London, UK
5 : Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
6 : Geozentrum Nordbayern, Friedrich-Alexander University Erlangen-Nürnberg, 91054 Erlangen, Germany
Source Earth And Planetary Science Letters (0012-821X) (Elsevier BV), 2020-10 , Vol. 547 , P. 116452 (11p.)
DOI 10.1016/j.epsl.2020.116452
WOS© Times Cited 3
Keyword(s) trace element signatures, Ce anomalies, La-Ce geochronology, rare earth element mobility

Precambrian banded iron formations (BIF) are chemical sedimentary deposits whose trace element signatures have been widely used to interrogate the chemical composition and redox state of ancient seawater. Here we investigated trace element signatures in BIF of the 3.22 Ga Moodies Group, Barberton Greenstone Belt (South Africa), which are interbedded with near-shore siliciclastic sedimentary rocks and represent one of the oldest known shallow-water occurrences of BIF. Unusual rare earth element (REE) signatures, notably with pronounced negative Ce anomalies in shale-normalized spectra, have been previously reported for chemical sediments of the Moodies Group, which we confirm here through an expanded dataset for Moodies BIF spanning three different localities. We find negative Ce anomalies as low as 0.2 Ce/Ce⁎ that are associated with unusual enrichment of LREE relative to HREE in the sample set. While total REE abundances and certain REE features appear strongly related to the concentration of detrital indicators (e.g., Zr), and are likely primary, other features, notably LREE enrichment, cannot be explained as a primary feature of the sediment. This is better explained by later addition of REE from a LREE-enriched but Ce-depleted fluid that generated the significant negative Ce anomalies observed in surface samples of Moodies Group BIF. This REE addition event influenced both Sm-Nd and La-Ce isotope systematics, the latter yielding an isochron of 60 ± 32 Ma, thus constraining the timing of emplacement of the negative Ce anomalies to the past 100 Ma, possibly upon surface exposure of the Barberton Greenstone Belt to wetter conditions during the Cenozoic. Our findings constitute a cautionary tale in that even the most immobile elemental redox proxies may be more sensitive to post-depositional modification than previously thought, and demonstrate the clear advantage offered by paleoredox proxies coupled to radiometric geochronometers to enable the direct dating of ancient signals of Earth surface oxygenation.

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Publisher's official version 11 1013 KB Open access
MMC 1. The supplementary material contains the major, trace, isotopic and XRD data. 80 KB Open access
MMC 2. The supplementary material contains the detailed analytical methods and a field guide. 11 1 MB Open access
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Bonnand P., Lalonde Stefan, Boyet M., Heubeck C., Homann M., Nonnotte Philippe, Foster I., Konhauser K.O., Köhler I. (2020). Post-depositional REE mobility in a Paleoarchean banded iron formation revealed by La-Ce geochronology: A cautionary tale for signals of ancient oxygenation. Earth And Planetary Science Letters, 547, 116452 (11p.). Publisher's official version : , Open Access version :