Iron isotope fractionation during proton- and ligand-promoted dissolution of primary phyllosilicates

We studied stable iron isotope fractionation during dissolution of a biotite and chlorite enriched mineral fraction from granite by HCl and 5 mM oxalic acid in a pH range of 4–5.9. Batch experiments covered a time period from 2 h to 100 days and were performed at initial potassium concentrations of 0, 0.5, and 5 mM to induce different levels of biotite exfoliation. All experiments were kept anoxic to investigate solely the dissolution step without the influence of oxidation and precipitation of secondary Fe oxyhydroxides. Oxalic acid increased the release of Fe by a factor of ∼15 compared with the HCl experiments. Addition of 0.5 mM K to initial solutions in proton-promoted dissolution decreased the release of Fe by 30–65% depending on the dissolution stage. In ligand-controlled dissolution, K reduced the Fe release only to a minor extent. All solutions of the early dissolution stages were enriched in light Fe isotopes by up to −1.4‰ in δ 56Fe compared with the isotopic composition of biotite and chlorite mineral separates, which we explained by a kinetic isotope effect. In proton-promoted dissolution, early released fractions of K-enriched experiments were significantly lighter (−0.7‰ to −0.9‰) than in the initially K-free experiments. The evolution of Fe isotope ratios in solution was modeled by a linear combination of kinetic isotope effects during two independent dissolution processes attacking different crystallographic sites. In ligand-controlled dissolution, K did not influence the kinetic isotope effect and the Fe isotope composition in solution in the late dissolution stages remained slightly lighter than the bulk composition of the biotite/chlorite enriched mineral fraction. This study demonstrates that the initial Fe weathering flux should be enriched in light Fe isotopes and that Fe isotope data in combination with dissolution kinetics and stoichiometry provide new insights into dissolution mechanisms.