Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 210, Issue -, Pages 247-266Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2017.05.002
Keywords
Iron isotopes; Archean; Paleo-environment; Barite deposits; Anoxic ocean
Categories
Funding
- National Program of Planetology (PNP) of the Institut National des Sciences de l'Univers
- UnivEarths Labex program at Sorbonne Paris Cite [ANR-10-LABX-0023, ANR-11IDEX-0005-02]
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The occurrence of Early Archean barite deposits is intriguing since this type of sediment requires high availability of dissolved sulfate (SO42-), the oxidized form of sulfur, although most authors argued that the Archean eon was dominated by reducing conditions, with low oceanic sulfate concentration (< 10 mu M) relative to present day levels of 28,000 mu M. In order to better assess the redox state of the paleo-atmosphere and -oceans, we examined Fe and S isotope compositions in a sedimentary sequence from the 3.2 Ga-old Mendon and Mapepe formations (Kaapvaal craton, South Africa), recovered from the drill-core BBDP2 of the Barberton Barite Drilling Project. Major elements were also analyzed to constrain the respective imprints of detrital vs metasomatic processes, in particular using Al, Ti and K interrelations. Bulk rock Fe isotope compositions are linked to mineralogy, with delta(56) Fe values varying between - 2.04% in Fe sulfide-dominated barite beds, to + 2.14% in Fe oxide-bearing cherts. delta S-34 values of sulfides vary between -10.84 and +3.56%, with Delta S-33 in a range comprised between -0.35 and +2.55%, thus supporting an O-2-depleted atmosphere (< 10 - 5 PAL). Iron isotope variations together with major element correlations show that, although the sediments experienced a pervasive stage of hydrothermal alteration, the rocks preserved a primary/authigenic signature predating subsequent hydrothermal stage. Highly positive delta 56 Fe values recorded in primary Fe-oxides from ferruginous cherts support partial Fe oxidation in a reducing oceanic environment (O-2 < 10(-4) mu M), but are incompatible with a model of complete oxidation at the redox boundary of a stratified water column. Iron oxide precipitation under low O2 levels was likely mediated by anoxygenic photosynthesis, and/or abiotic photo-oxidation processes. Our results are consistent with global anoxic conditions in the 3.2 Ga-old sediments, implying that the barite deposits were most likely sourced by atmospheric photolysis of S gases produced by large subaerial volcanic events, and possibly SO42- produced by magmatic SO2 disproportionation in hydrothermal systems. (C) 2017 Elsevier Ltd. All rights reserved.
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