title: Chromium geochemistry of the ca. 1.85 Ga Flin Flon paleosol.
abstract: Fractionation of stable Cr isotopes has been measured in Archaean paleosols and marine sedimentary rocks and interpreted to record the terrestrial oxidation of Cr(III) to Cr(VI), providing possible indirect evidence for the emergence of oxygenic photosynthesis. However, these fractionations occur amidst evidence from other geochemical proxies for a pervasively anoxic atmosphere. This study examined the Cr geochemistry of the ca. 1.85 Ga Flin Flon paleosol, which developed under an atmosphere unambiguously oxidising enough to quantitatively convert Fe(II) to Fe(III) during pedogenesis. The paleosol shows an extreme range in Cr isotope composition of 2.76 ‰ δ53/52Cr. The protolith greenstone (δ53/52Cr: -0.23 ‰), the deepest weathering horizon (δ53/52Cr: -0.15 to -0.23 ‰) and a residual corestone in the upper paleosol (δ53/52Cr: -0.01 ‰) all exhibit Cr isotopic compositions comparable to unaltered igneous rocks. The most significant isotopic fractionation is preserved in the areas influenced by oxidative subaerial weathering (i.e. increase in Fe(III)/Fe(II)) and the greatest loss of mobile elements. The uppermost paleosol horizon is both Cr and Mn depleted and offset to significantly53Cr-enriched compositions (δ53/52Cr values between +1.50 and +2.38 ‰), which is not easily modelled with the oxidation of Cr(III) and loss of isotopically heavy Cr(VI). Instead, the currently preferred model for these data invokes the open-system removal of isotopically light aqueous Cr(III) during either pedogenesis or subsequent hydrothermal/metamorphic alteration. The53Cr enrichment would then represent the preferential dissolution or complexation of isotopically light aqueous Cr(III) species (enhanced by lower pH conditions and possibly the presence of complexing ligands) and/or the residual signature from preferential adsorption of isotopically heavy Cr(III). Both scenarios would contradict the widely held assumption that only redox reactions of Cr can generate large magnitude isotopic fractionations and, if substantiated, non-redox isotope effects would complicate the conclusive fingerprinting of ancient atmospheric O2from Cr isotope data alone.
authors.affiliation: Department of Geosciences, University of Tübingen, Tübingen, Germany.|Department of Geology, Trinity College Dublin, Dublin 2, Ireland.
authors.forename: M G
authors.initials: MG
authors.lastname: Babechuk
authors.affiliation: Department of Geosciences, University of Tübingen, Tübingen, Germany.
authors.forename: I C
authors.initials: IC
authors.lastname: Kleinhanns
authors.affiliation: Department of Geosciences, University of Tübingen, Tübingen, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Schoenberg
mesh_terms: D002859:Chromium Isotopes / Q000032:analysis*; D019015:Geologic Sediments / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction
chemical_list: D002859:Chromium Isotopes