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 (Fe 3+ ). The mechanism by which ferrous iron (Fe 2+ ) was oxidized into Fe 3+ in an atmosphere that was globally anoxic is highly debated. Of the three scenarios that have been proposed to explain BIF formation, photo-oxidatio...
| Published in: | Earth and Planetary Science Letters |
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| Main Authors: | , , |
| Format: | Article in Journal/Newspaper |
| Language: | unknown |
| Published: |
2017
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| Subjects: | |
| Online Access: | https://oro.open.ac.uk/48198/ https://doi.org/10.1016/j.epsl.2016.10.035 |
| Summary: | Banded iron formations (BIFs) contain appreciable amounts of ferric iron (Fe 3+ ). The mechanism by which ferrous iron (Fe 2+ ) was oxidized into Fe 3+ 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 Fe 2+ 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 Fe 3+ -bearing lepidocrocite and dissolved Fe 2+ follows a Rayleigh distillation with an instantaneous 56 Fe/ 54 Fe fractionation factor of +1.2‰. 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 δ 56 Fe–δ 57 Fe relationship). The experimental run products follow a mass-dependent law corresponding to the high-T equilibrium limit. The fact that a ∼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 (∼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 ... |
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