Potassium isotopic compositions of international geological reference materials

There is a renewed interest in K isotope geochemistry driven by the advances in analytical precision and it is emerging as a useful tracer in a variety of disciplines ranging from Earth sciences to biology. However, high-quality K isotopic data for reference materials are still limited but highly needed. Here, we report high-precision stable K isotopic compositions (delta K-41) 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 were measured on a Nu Plasma II high-resolution MC-ICPMS. The re-producibility of K isotopic analysis, based on over one year of measurements of pure K solutions and rock standards, was <= 0.06% (95% confidence interval). Synthetic solutions made by mixing single element standards to represent various rock matrices confirmed the accuracy of our methods. The 23 reference materials have delta K-41 values ranging from -0.562% to -0.253% and the seawater standard has a much higher delta K-41 value of 0.143%. The comprehensive dataset presented here provides a reference for quality control and inter-laboratory comparison of high-precision K isotopic analyses for future studies.