Quantifying the rate of Fe2+-catalyzed recrystallization based on a unifying model framework

Abstract

Iron oxides and oxyhydroxides (referred to as “iron oxides”) have been proposed as geochemical proxy archives for the reconstruction of paleoenvironmental conditions. Interpreting their isotopic and elemental compositions is potentially complicated by Fe2+-catalyzed recrystallization, a process by which iron oxides undergo substantial post-formational changes in their isotopic compositions in the presence of Fe2+ without exhibiting overt mineralogical or morphological changes. One major challenge in accounting for the effect of this process on iron oxide compositions is the ambiguity associated with quantifying recrystallization rates from isotopic data collected in laboratory experiments. Specifically, quantifying recrystallization rates has required an assumption regarding the further dissolution and reprecipitation that the recrystallized solid undergoes (referred to as “back-reaction”). Assumption of the fraction of back-reacting recrystallized solid may substantially affect the rates and extents of recrystallization calculated from bulk isotopic data. Recent experimental evidence indicates that back-reaction between recrystallized iron oxides and aqueous Fe2+ is gradual and partial. Here, we developed a model based on these recent experimental insights that simulated isotope dynamics during Fe2+-catalyzed recrystallization of goethite to quantify the rate of recrystallization. We estimated rate parameters using isotopic data collected by Joshi et al. (2017) and Frierdich et al. (2015a). We then simulated isotope dynamics using the estimated rate parameters and compared the simulations to published datasets from Frierdich et al. (2015a), Handler et al. (2009), Frierdich et al. (2014b), Joshi and Gorski (2016), Frierdich et al. (2019a), which were collected under similar experimental conditions of pH 7.5 and high solid Fe:fluid Fe molar ratios (20.2–22.0). We observed that a model that included partial back-reaction described all the datasets investigated here with a narrow range of rates: initial rates of 2.65–4.67μmol*m−2*d−1 and final rates of 4.72*10−3 - 0.178μmol*m−2*d−1. The resulting extents of goethite recrystallization were 14.1–19.8% over 28–60 days, substantially lower than previous estimates. The low rates and extents quantified in this work indicate that Fe2+-catalyzed recrystallization is likely less extensive than previously reported, and that this process has a limited impact on proxy archives in the rock record. The results of our work also suggest that back-reaction may have a major effect on isotopic fractionation factors interpreted from laboratory experiments and should be considered in future studies.