Comment on “Isotopic fractionation between Fe(III) and Fe(II) in aqueous solutions” by Clark Johnson et al., [Earth Planet. Sci. Lett. 195 (2002) 141–153

Abstract

In a recent contribution [1], Johnson et al. reported the equilibrium isotope fractionation factor between dissolved Fe(II) and Fe(III) in aqueous solutions at pH=2.5 and 5.5. They suggest that because the iron isotope fractionation observed in their experiments spans virtually the entire range observed in sedimentary rocks, Fe(II)^ Fe(III) aqueous speciation may play a major role in determining iron isotope variations in nature where Fe(II) and Fe(III) can become physically separated. They discounted earlier conclusions by us and others [2,3] that signi¢cant equilibrium fractionation between speci¢c coexisting Fe(II)or Fe(III)-aqueous complexes (e.g., between aqueous Fe(II)(OH)xðaqÞ and Fe(II)ðaqÞ ion) is capable of producing iron isotope contrasts that can be preserved in nature. This is an important contribution not only because the authors recognize the importance of abiotic equilibrium iron isotope fractionation in nature in contrast to previous assertions [4], but also because it will help to focus discussion on the development and evaluation of experimental approaches that can reveal abiotic fractionation mechanisms. However, in this Comment we propose that the experiments presented in this paper cannot be interpreted as straightforwardly as Johnson et al. contend. In particular, we show that in one of their critical experiments attainment of either isotope mass balance or equilibrium was not demonstrated, and thus the results of that experiment cannot be used to calculate an Fe(II)^Fe(III) equilibrium fractionation factor. Johnson et al. determined isotopic fractionation between aqueous Fe(II) and Fe(III) based on two experiments in which isotopically ‘normal’ Fe(II)and Fe(III)-chloride solutions were mixed and presumed to attain isotopic equilibrium at pH=2.5 and 5.5. The time allowed for attainment of isotopic equilibrium was constrained by a third experiment in which isotopically ‘enriched’ Fe(III)-chloride solution was mixed with isotopically ‘normal’ Fe(II)-chloride solution at pH=2.5, and a time series of Fe(II) and Fe(III) isotopic compositions approaching equilibrium were ¢t with a second order rate function. This latter experiment demonstrated the approach to isotopic equilibrium through electron transfer between coexisting Fe(II)and Fe(III)-aqueous complexes in the absence of oxidation through the ferrous hydroxyl pathway that is common under higher pH, aerobic conditions. We point out that the ¢tted rate function was for conditions where aqueous