The photochemistry of manganese and the origin of banded iron formations

The photochemical oxidation of Fe²⁺ -hydroxide complexes dissolved in anoxic Precambrian oceans has been suggested as a mechanism to explain the deposition of Banded Iron Formations (BIFs). Photochemical studies have not yet addressed the low levels of manganese in many of these deposits, which probably precipitated from solutions bearing similar concentrations of Fe²⁺ and Mn²⁺. Depositional models must also explain the stratigraphie separation of iron and manganese ores in manganiferous BIFs. In this study, solutions containing 0.56 M NaCl and ≈ 180 μM MnCl₂ with or without 3 to 200 μM FeCl₂ were irradiated with filtered and unfiltered UV light from a medium-pressure mercury-vapor lamp for up to 8 hours. The solutions were deaerated and buffered to pH ≈ 7, and all experiments were conducted under O₂-free (< 1 ppm) atmospheres. In experiments with NaCl + MnCl₂, approximately 20% of the Mn²⁺ was oxidized and precipitated as birnessite in 8 hours. Manganese precipitation was only observed when light with λ < 240 nm was used. In experiments with NaCl + MnCl₂ + FeCl₂, little manganese was lost from solution, while Fe²⁺ was rapidly oxidized to Fe³⁺ and precipitated as γ-FeOOH or as amorphous ferric hydroxide. The Mn:Fe ratio of these precipitates was ≈ 1:50, similar to the ratios observed in BIFs. A strong upper limit on the rate of manganese photo-oxidation during the Precambrian is estimated to be 0.1 mg cm^-2 yr^-1, a factor of 10³ slower than the rate of iron photo-oxidation considered reasonable in BIF depositional basins. Thus, a photochemical model for the origin of oxide facies BIFs is consistent with field observations, although models that invoke molecular O₂ as the oxidant of Fe²⁺ and Mn²⁺ are not precluded. Apparently, oxide facies BIFs could have formed under anoxic, as well as under mildly oxygenated atmospheres.