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

International audiencePrecambrian 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