Résumé
Iron formations (IFs) are iron- and silica-rich sedimentary rocks that formed in key periods of Earth's history. These chemical sediments are considered one of the best-preserved archives of Precambrian seawater. Despite a vast literature on the chemical and petrographic nature of IFs, many aspects regarding their formation are loosely constrained, in particular the crystallization pathways of iron oxides within iron rich layers are puzzling given the low ambient oxygen levels attending their deposition. Another question, that was seldom raised, is how and when these iron oxides acquired a magnetization and what paleomagnetic information could be derived from it. Here we present a detailed dataset of magnetic properties recorded along a 250 meters core section intercepting pristine Neoarchean (circa 2.74 Ga) IFs and associated Fe-rich carbonates from Carajas (Brazil). By combining standard bulk rock magnetic techniques (hysteresis, IRM acquisition, FORCs, AF demagnetization and thermomagnetic measurements) with modern magnetic field imaging (Scanning Magnetic Microscope) and high-resolution optical imaging (SEM and TEM) we were able to identify the specific ferromagnetic grains populations. Iron oxides represent typically 10-20% (and locally up to 50%) of the total mineral amount within individual petrographic thin sections. Magnetite and hematite are the main iron oxides identified. An inverse correlation between both oxides, in addition to textural evidence indicate partial alteration of hematite towards magnetite. The low Al (sub 2) O (sub 3) abundance (less than 0.2 wt.%) of the samples studied indicate that the oxides are non-detrital and formed through (bio)-chemical processes. In light of recently published trace element and Fe isotope data obtained on the same samples (Rego et al., 2021), we discuss a possible scenario where a fraction of the ambient, hydrothermally-derived, Fe(II) is fully oxidized to Fe(III) by anoxygenic photosynthetic micro-organisms and subsequently partially reduced to magnetite, with consequences for the remanence acquisition mechanisms in IFs.