Depositional setting of the Late Archean Fe oxide- and sulfide-bearing chert and graphitic argillite in the Shaw Dome, Abitibi greenstone belt, Canada

Hiebert, R. S.; Bekker, Andrey; Houlé, M G; Rouxel, Olivier

doi:10.1016/j.precamres.2018.04.004

Cited by 14 publications

(5 citation statements)

References 104 publications

(184 reference statements)

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“…The latter is a widely used reference for normalizing sedimentary lithologies, allowing easy comparison with previous studies. The (Bolhar et al, 2004;Hiebert et al, 2018;Planavsky et al, 2010). Moreover, the REE + Y patterns reveal a moderately steep trend with light-REE enrichments, as shown by Pr/Yb(SN) ratios ranging between 1.45 and 3.54 (Fig.…”

Section: Provenance Characterizationmentioning

confidence: 82%

Trace element perspective into the ca. 2.1-billion-year-old shallow-marine microbial mats from the Francevillian Group, Gabon

Aubineau

Albani²,

Bekker³

et al. 2020

Chemical Geology

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The sedimentary fabrics of Precambrian mat-related structures (MRS) represent some of the oldest convincing evidence for early life on Earth. The ca. 2.1 billion-year (Ga) old MRS in the FB2 Member of the Francevillian basin in Gabon has received considerable attention not only because they contain remnants of microbial mats that colonized large areas in oxygenated, shallow-marine settings, but they also contain evidence for ancient multicellular organisms that thrived on these microbial mats using them as a food source. Despite these insights, what remains lacking is a full characterization of the geochemical composition of the MRS to test whether the bulk composition of fossilized MRS is distinct from the host sediments (sandstones and shales). Here, we show that the trace element (TE) content of microbial textures belonging to pyritized MRS, poorly pyritized MRS, and "elephant-skin" textures (EST) is highly variable and differs from that of the host sediments. The poorly pyritized MRS contain a unique matrix with embedded Ti-and Zr-rich minerals and syngenetically enriched in TE. The EST, some of which are developed along the same stratigraphic horizon as the poorly pyritized MRS, display a distinct distribution of TE-bearing heavy minerals, suggesting a local difference in physical conditions during sedimentation. Similarly, high chalcophile-element (CE) content in pyritized MRS relative to the host sediments of the FB2 Member further points to local bacterially influenced enrichments with high rates of microbial sulfate reduction during early diagenesis.The geochemical relationship between the MRS and the Francevillian sediments (e.g., FB, FC, and FD formations) indicates that specific biological pathways for CE enrichments (i.e., microbially controlled accumulation) are not apparent. Our findings highlight bulk-rock TE distinction between the 2.1-billion-year-old MRS and their host sediments, but also indicate that environmental conditions, such as hydrodynamic regime and water-column redox chemistry, may simply overwhelm any potential biological signal. Our data suggest that the microbial impact may have only passively influenced TE enrichment in the studied sediments, implying that TE concentrations in MRS are a poor biosignature. Importantly, this work indicates that bulk TE geochemistry does not unveil specific microbiological processes in the rock record, which is consistent with the observed patterns in modern analogues.

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Section: Provenance Characterizationmentioning

confidence: 82%

Trace element perspective into the ca. 2.1-billion-year-old shallow-marine microbial mats from the Francevillian Group, Gabon

Aubineau

Albani²,

Bekker³

et al. 2020

Chemical Geology

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“…Transient and spatially restricted oxygenation events have also been suggested to occur during this interval based on a wide range of geochemical proxies 10,50,51 . It also needs to be noted that sulfur-rich exhalites in the ~2.7 Ga Abitibi greenstone belt possess highly negative Fe isotope and higher Mn contents, suggesting the presence of O2 in the upper part of the water column 52 . However, the IF data presented here do not record evidence of oxygenation events, likely due to either sampling bias or the relatively deep-water depositional environments (below storm wave base) of these IFs.…”

Section: Tracking Iron Oxidation With Iron Isotopesmentioning

confidence: 99%

Archean to early Paleoproterozoic iron formations document a transition in iron oxidation mechanisms

Wang

Robbins

Planavsky

et al. 2023

Geochimica et Cosmochimica Acta

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“…In particular, experiments should be conducted with solutions containing both Fe 2+ and Mn 2+ . By coupling δ 56 Fe measurements with measurements of Mn/Fe in co-sampled solutions and greenalite precursor precipitates, the chemical evolution of solutions and precipitates could be compared to the well-established negative correlation seen in ln(Mn/Fe) vs δ 56 Fe plots of IFs of all ages. − This negative correlation is well-explained by preferential incorporation of Fe into IF precipitates relative to Mn, because the latter requires higher redox potentials to be oxidized to insoluble Mn 4+ hydroxides . Meanwhile, Fe 2+ and Mn 2+ have very similar geochemical behaviors, so the degree of incorporation of Mn in Fe 2+ -greenalite and mixed valence greenalite precursor material will offer a useful second variable to disentangle the surprisingly similar δ 56 Fe systematics of HFOs and Fe 3+ -bearing greenalite.…”

Section: Future Directionsmentioning

confidence: 99%

Isotopic Constraints on the Nature of Primary Precipitates in Archean–Early Paleoproterozoic Iron Formations from Determinations of the Iron Phonon Density of States of Greenalite and 2L- and 6L-Ferrihydrite

Heard

Dauphas

Hinz

et al. 2023

ACS Earth Space Chem.

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Iron formations (IFs) are chemical sedimentary rocks that were widely deposited before the Great Oxidation Event (GOE) around 2.4–2.2 Ga. It is generally thought that IFs precipitated as hydrated Fe3+ oxides (HFOs) such as ferrihydrite following surface oxidation of Fe2+-rich, anoxic deep waters. This model often implicates biological oxidation and underpins reconstructions of marine nutrient concentrations. However, nanoscale petrography indicates that an Fe2+ silicate, greenalite, is a common primary mineral in well-preserved IFs, motivating an alternative depositional model of anoxic ferrous silicate precipitation. It is unclear, however, if Fe2+-rich silicates can produce the Fe isotopic variations in IFs that are well explained by Fe2+ oxidation. To address this question, we constrain the equilibrium Fe isotopic (56Fe/54Fe) fractionation of greenalite and ferrihydrite by determining the iron phonon densities of states for those minerals. We use ab initio density functional theory (DFT + U) calculations and nuclear resonant inelastic X-ray scattering spectroscopy to show that ferrous greenalite should be isotopically lighter than ferrihydrite by ∼1–1.2‰ at equilibrium, and fractionation should scale linearly with increasing Fe3+ content in greenalite. By anchoring ferrihydrite–greenalite mineral pair fractionations to published experimental Fe isotopic fractionations between HFOs and aqueous Fe2+, we show that ferrous greenalite may produce all but the heaviest pre-GOE Fe isotopic compositions and mixed valence greenalites can produce the entire record. Our results suggest that heavy Fe isotope enrichments alone are not diagnostic of primary IF mineralogies, and ferrihydrite and partially oxidized or even purely ferrous greenalite are all viable primary IF mineralogies.

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Depositional setting of the Late Archean Fe oxide- and sulfide-bearing chert and graphitic argillite in the Shaw Dome, Abitibi greenstone belt, Canada

Abstract: Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Cited by 14 publications

References 104 publications

Trace element perspective into the ca. 2.1-billion-year-old shallow-marine microbial mats from the Francevillian Group, Gabon

Trace element perspective into the ca. 2.1-billion-year-old shallow-marine microbial mats from the Francevillian Group, Gabon

Archean to early Paleoproterozoic iron formations document a transition in iron oxidation mechanisms

Isotopic Constraints on the Nature of Primary Precipitates in Archean–Early Paleoproterozoic Iron Formations from Determinations of the Iron Phonon Density of States of Greenalite and 2L- and 6L-Ferrihydrite

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