Potassium Isotopic Compositions of International Geological Reference Materials

Renewed interest in K isotope geochemistry driven by the advances in analytical precision is leading to its emergence as a useful tracer in a variety of disciplines ranging from Earth sciences to biology. However, high-quality isotopic data for K reference materials are limited. We report high-precision stable K isotopic compositions (δ⁴¹ K) for 23 commercially available international reference materials, including igneous, sedimentary, and metamorphic rocks, as well as an in-house seawater standard. Potassium in digested samples was separated by cation-exchange chromatography with Bio-Rad AG50W-X8 (200-400 mesh) resin in 0.5 N HNO₃ media. Potassium isotopes are measured on a Nu Plasma II high-resolution MC-ICPMS. The reproducibility of K isotopic analysis, based on over one year of measurements of pure K solutions and rock standards, is ≤0.06‰ (95% confidence interval). Synthetic solutions that were made by mixing single element standards to represent various rock matrices confirmed the accuracy of our methods. Variations of K isotopes observed during column chemistry reveal that light isotopes were elutebefore the heavy ones; and therefore, ~100% recovery of the K eluted is needed for accurate measurement. The 23 reference materials have δ⁴¹ K values ranging from -0.562‰ to -0.253‰ and the seawater standard has a much higher δ⁴¹ K value of 0.143‰. The δ⁴¹ K of igneous rocks do not vary significantly, however, some rock standards do not have mantle value, e.g., one diorite (DR-N) has a lower δ⁴¹ K and two syenites (SY-1 and SY-2) have higher δ⁴¹ K relative to mantle values. This observation may indicate that both high-temperature and low-temperature processes could fractionate K isotopes. These updated isotopic values provides a reference for quality control and inter-laboratory comparisons of high-precision K isotopic analyses for future studies.