Data of K-Ar dating of small samples using isotope dilution measurement of potassium

We present a detailed procedure for precise determination of potassium concentrations of small geological samples using isotope dilution (ID). To significantly reduce the Ar+ related interference, a “cold plasma” technique is used on a Nu plasma multicollector ICPMS, facilitating the precise measurement of 41K+/39K+ at high mass resolution mode. The K concentrations of USGS standards (BHVO-2, BCR-2, AGV-2 and SBC-1) were determined by ID, with overall analytical uncertainties better than 2.5%. The variance is most likely dominated by weighing errors for small samples, and chemical heterogeneities under 5 mg sub-sampling scale. Using this technique, we were able to determine the K contents of single clay pellets (~0.1mg) previously degassed under vacuum to measure Ar concentrations, thereby enabling precise measurement of K-Ar ages.