Potassium isotope fractionation between K-salts and saturated aqueous solutions at room temperature: Laboratory experiments and theoretical calculations

Improvements in mass spectrometry have made it possible to identify naturally occurring K isotope (39K/41K) variability in terrestrial samples that can be used in a variety of geological and biological applications that involve cycling of K such as clay or evaporite formation. However, our ability to interpret K isotope variability is limited by a poor understanding of how K isotopes are fractionated at low temperatures. In this study, we conducted recrystallization experiments of eight K-salts in order to measure the K isotope fractionation factor between the salt and the saturated K solution (Δ41Kmin-sol). Measured Δ41Kmin-sol are +0.50‰ for K2CO3·1.5H2O, +0.32‰ for K2SO4, +0.23‰ for KHCO3, +0.06‰ for K2C2O4·H2O, +0.02‰ for KCl, −0.03‰ for K2CrO4, −0.15‰ for KBr, and −0.52‰ for KI. Overall the Δ41Kmin-sol decreases with increasing r for K in crystals, where r is the average distance between a K atom and its neighboring atoms of negative charge. Salts with monovalent anions and salts with divalent anion complexes define different linear trends with distinct slopes on a plot of Δ41Kmin-sol - r. We applied ab initio lattice dynamics and empirical crystal-chemistry models to calculation of K isotope fractionation factors between K salts; both methods showed that the calculated inter-mineral K isotope fractionation factors (Δ41Kmin-KCl) are highly consistent with experimentally derived Δ41Kmin-KCl under the assumption of consistent β factors for different saturated K solutions. Formulations for the crystal-chemistry model further indicate that both anion charge and bond length r are the principle controlling factors for K isotope fractionation, and the K isotope fractionation factors correlate with r following a 1/r3 relationship. Our experiment and theoretical study confirms the existence of significant equilibrium K isotope fractionation at ambient conditions, and the K isotope fractionation factors for halides and sulfate obtained in this study provide a basis for future K isotope studies on evaporites.