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 (K-39/(41) K) 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 (Delta K-41(min-sol)). Measured Delta K-41(min-sol) are + 0.50% for K2CO3 center dot 1.5H(2)O, + 0.32% for K2SO4, + 0.23% for KHCO3, + 0.06% for K2C2O4 center dot H2O, + 0.02% for KCl, -0.03% for K2CrO4, -0.15% for KBr, and -0.52% for KI. Overall the Delta K-41(min-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 Delta K-41(min-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 (Delta K-41(min-KCl)) are highly consistent with experimentally derived Delta K-41(min-KCl) under the a ssumption of consistent beta 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/r(3) 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. (C) 2017 Elsevier Ltd. All rights reserved.