Iron and oxygen isotope fractionation during iron UV photo-oxidation: Implications for early Earth and Mars

Banded iron formations (BIFs) contain appreciable amounts of ferric iron (Fe3+). The mechanism by which ferrous iron (Fe2+) was oxidized into Fe3+ in an atmosphere that was globally anoxic is highly debated. Of the three scenarios that have been proposed to explain BIF formation, photo-oxidation by UV photons is the only one that does not involve life (the other two are oxidation by O-2 produced by photosynthesis, and anoxygenic photosynthesis whereby Fe2+ is directly used as electron donor in place of water). We experimentally investigated iron and oxygen isotope fractionation imparted by iron photo oxidation at a pH of 7.3. The iron isotope fractionation between precipitated Fe3+-bearing lepidocrocite and dissolved Fe2+ follows a Rayleigh distillation with an instantaneous Fe-56/Fe-54 fractionation factor of +1.2 parts per thousand. Such enrichment in the heavy isotopes of iron is consistent with the values measured in BIFs. We also investigated the nature of the mass-fractionation law that governs iron isotope fractionation in the photo-oxidation experiments (i.e., the slope of the delta Fe-56-delta Fe-57 relationship). The experimental run products follow a mass-dependent law corresponding to the high-T equilibrium limit. The fact that a similar to 3.8 Gyr old BIF sample (IF-G) from Isua (Greenland) falls on the same fractionation line confirms that iron photo-oxidation in the surface layers of the oceans was a viable pathway to BIF formation in the Archean, when the atmosphere was largely transparent to UV photons. Our experiments allow us to estimate the quantum yield of the photo-oxidation process (similar to 0.07 iron atom oxidized per photon absorbed). This yield is used to model iron oxidation on early Mars. As the photo-oxidation proceeds, the aqueous medium becomes more acidic, which slows down the reaction by changing the speciation of iron to species that are less efficient at absorbing UV-photons. Iron photo oxidation in centimeter to meter-deep water ponds would take months to years to complete. Oxidation by O-2 in acidic conditions would be slower. Iron photo-oxidation is thus likely responsible for the formation of jarosite-hematite deposits on Mars, provided that shallow standing water bodies could persist for extended periods of time. The oxygen isotopic composition of lepidocrocite precipitated from the photo-oxidation experiment was measured and it is related to the composition of water by mass-dependent fractionation. The precipitate fluid O-18/O-16 isotope fractionation of similar to+6 parts per thousand is consistent with previous determinations of oxygen equilibrium fraction factors between iron oxyhydroxides and water. (C) 2016 Elsevier B.V. All rights reserved.