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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 (δ41K) 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 reproducibility 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 δ41K values ranging from −0.562‰ to −0.253‰ and the seawater standard has a much higher δ41K 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.

This study presents a method for high-precision stable potassium (K) isotope analysis using High-Resolution Multi-Collector Inductively Coupled Plasma Mass Spectrometry (HR-MC-ICPMS). Dry and cold plasma is used to suppress the formation of argon hydrides that are direct isobaric interferences on K isotopes. The residual 40Ar1H+ beam is resolved from 41K+ by using pseudo-high resolution mode, providing interference-free shoulders of 39K+ and 41K+ for isotopic measurements. Precision of better than 0.06‰ (95% confidence interval) has been routinely achieved based on multiple measurements of pure K standard solutions and rock standards over a six-month period. Mismatches in K concentration (up to 20%) and acid molarity (up to 67%) between samples and bracketing standards, as well as the presence of substantial amount of matrix element Rb, do not significantly affect the analytical precision and accuracy. Our study documents that high-precision K isotope analysis can be routinely achieved for a wide variety of natural samples by HR-MC-ICPMS, which makes it possible for using K isotopes to trace major geological processes.