Iron isotope fractionation between aqueous Fe(II) and goethite revisited: New insights based on a multi-direction approach to equilibrium and isotopic exchange rate modification

摘要

The Fe isotope compositions of naturally occurring Fe oxide minerals provide insights into biogeochemical processes that occur in modern and ancient environments. Key to understanding isotopic variations in such minerals is knowledge of the equilibrium Fe isotope fractionation factors between common minerals and aqueous Fe species. Because experimental measurements of isotopic fractionation may reflect a combination of kinetic and equilibrium fractionations during rapid dissolution and precipitation, even in experiments that employ the three-isotope method, assessment of the attainment of equilibrium is often difficult. Here, we re-examine Fe isotope exchange, via a ~(57)Fe tracer, and natural mass-dependent fractionation, through changes in initial ~(56)Fe/~(54)Fe ratios, between aqueous Fe(II) (Fe(II)_(aq)) and goethite. This approach uses the three-isotope method, but is distinct in its evaluation of kinetic isotope fractionation and the attainment of equilibrium by: (i) employing a multi-direction approach to equilibrium at 22 ℃ via reaction of three Fe(II)_(aq) solutions that had different initial ~(56)Fe/~(54)Fe ratios, (ii) conducting isotopic exchange experiments at elevated temperature (50 ℃), and (iii) modifying the rate of isotopic exchange through a combination of trace-element substitutions and particle coarsening to evaluate corresponding temporal changes in fractionation trajectories that may reflect changing instantaneous fractionation factors. We find that rapid isotopic exchange produces kinetic isotope effects between Fe(II)_(aq) and goethite, which shifts the ~(56)Fe/~(54)Fe ratios of Fe(II)_(aq) early in reactions toward that of goethite, indicating that the instantaneous Fe(II)_(aq)-goethite fractionation factor under kinetic conditions is small. Importantly, however, this kinetic fractionation is "erased" with continued reaction, and this is evident by the congruence for multiple-exchange trajectories of distinct initial Fe(II)_(aq) solutions toward the same final value over long reaction times. Experiments at higher temperature result in a smaller fractionation between Fe(II)_(aq) and goethite, consistent with a 1/T~2 temperature dependence. Coarsened and trace-element substituted goethites that had low surface areas produced much slower rates of isotopic exchange than the chemically pure forms or goethite of smaller crystal size, resulting in only partial isotopic mixing (10-40%). Fractionation-exchange trajectories produced during slow isotopic exchange are linear, however, and extrapolate to the same (within error) Fe(II)_(aq)-goethite fractionations at 100% isotopic mixing as that for reactions of pure goethite. We conclude that the equilibrium ~(56)Fe/~(54)Fe fractionation for Fe(II)_(aq)-goethite at 22 ℃ ranges between -1.04 ± 0.08% and -1.22 ± 0.11% (2σ), depending on particle size, where the more negative fractionation is influenced by surface Fe that has distinct isotopic properties; these results are consistent with earlier measurements by Beard et al. (2010). This work highlights the utility of using multiple exchange-fractionation trajectories to verify the attainment of equilibrium and resolving kinetic isotope effects, and the importance of isotopic exchange rate on disequilibrium mixing between components. We recommend that these techniques are essential for unambiguously demonstrating that measured fractionations during isotopic exchange experiments are, in fact, equilibrium fractionations.