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

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P>Improvements in mass spectrometry have made it possible to identify naturally occurring K isotope (<SUP>39</SUP>K/<SUP>41</SUP>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 (Δ<SUP>41</SUP>K<SUB>min-sol</SUB>). Measured Δ<SUP>41</SUP>K<SUB>min-sol</SUB> are +0.50‰ for K<SUB>2</SUB>CO<SUB>3</SUB>·1.5H<SUB>2</SUB>O, +0.32‰ for K<SUB>2</SUB>SO<SUB>4</SUB>, +0.23‰ for KHCO<SUB>3</SUB>, +0.06‰ for K<SUB>2</SUB>C<SUB>2</SUB>O<SUB>4</SUB>·H<SUB>2</SUB>O, +0.02‰ for KCl, −0.03‰ for K<SUB>2</SUB>CrO<SUB>4</SUB>, −0.15‰ for KBr, and −0.52‰ for KI. Overall the Δ<SUP>41</SUP>K<SUB>min-sol</SUB> decreases with increasing <I>r</I> for K in crystals, where <I>r</I> 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 Δ<SUP>41</SUP>K<SUB>min-sol</SUB> - <I>r</I>. We applied <I>ab initio</I> 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 (Δ<SUP>41</SUP>K<SUB>min-KCl</SUB>) are highly consistent with experimentally derived Δ<SUP>41</SUP>K<SUB>min-KCl</SUB> 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 <I>r</I> are the principle controlling factors for K isotope fractionation, and the K isotope fractionation factors correlate with <I>r</I> following a 1/<I>r</I> <SUP>3</SUP> 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.</P>