An experimental study of photo-oxidation of Fe(II): Implications for the formation of Fe(III) (hydro)oxides on early Mars and Earth

Abstract

Photo-oxidation of aqueous Fe(II) (Fe2+ and FeOH+) to Fe(III) (Fe3+) was likely involved in the formation of iron oxide deposits on early Mars and Earth. Previous studies have reported the photo-oxidation reaction rate (i.e., quantum yield, φ = the number of oxidized ferrous ions divided by the number of photons absorbed by ferrous ions) under acidic conditions (pH 0.4–3.0). However, the quantum yield has not been systematically investigated using chemical actinometry in the range of weakly acidic to neutral pH, where the photo-oxidation would have occurred on early Mars and Earth. We report quantum yields for the photo-oxidation of aqueous Fe(II) species over a pH range of 0.5–7.6 with Hg and Xe lamps (with and without optical filters) based on measured Fe(II) concentrations and photon fluxes. The quantum yield under continuous UV and visible light (>200 nm, Xe lamp) varies with pH: φ = 0.103 (±0.005) + 2.17 (±0.27) × [H+]0.5 at pH = 3.0–7.0. Our quantum yield is a few times higher than those reported by the previous studies that used a Hg lamp, indicating the wavelength dependence of the quantum yield. At higher pH (7.1–7.6), with a UV cutoff at ≤ 300 nm (filtered Xe lamp), photo-oxidation of Fe(II) is attributed to oxidation of FeOH+, with a quantum yield of 0.08 ± 0.01. Based on these quantum yields, we estimated Fe(III) (hydro)oxide precipitation rates in the early Gale lakes on Mars, and in Archean oceans on Earth. Results suggest that photo-oxidation may account for the amounts of Fe(III) (hydro)oxides in Gale sediments, assuming aqueous Fe(II) was supplied to the lakes through upwelling groundwater. Photo-oxidation of Fe(II) in Archean oceans on Earth could have been several times more intense than previously thought.