An estimate of the Bulk Silicate Earth potassium isotopic composition based on MC-ICPMS measurements of basalts

We report a method for high-precision potassium isotope measurements that improves by an order of magnitude the precision compared to previous methods. The purification of K is achieved through ion exchange chromatography on AG50W-X8 cation exchange resins. The ⁴¹K/³⁹K ratios are analyzed with a GV Instruments IsoProbe P Multiple-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) equipped with a hexapole collision gas cell. With this technique, the argon-hydride interferences as well as the large ⁴⁰Ar⁺ peak from the Ar-plasma are eliminated. The ⁴¹K/³⁹K ratios can thus be obtained with a precision of 0.05 per mil. The precision and accuracy of this method is validated with gravimetrically determined ⁴¹K or ³⁹K enriched standards. New precise K isotopic compositions of three terrestrial basalts (BCR-2, BHVO-and a MORB) are also reported. These basalts have indistinguishable K isotopic compositions and are used to define a reference value of -0.479 ± 0.027 per mil for the ⁴¹K/³⁹K ratio of the Bulk Silicate Earth (BSE) relative to a Merck Suprapur potassium nitrate standard. Seawater and sylvites from evaporite deposits have similar ⁴¹K/³⁹K ratios, but higher by up to 0.227 per mil compared to the Merck Suprapur standard. Other commercially available K-salts/solutions also have ⁴¹K/³⁹K ratios similar to the Merck standard, while a SPEX K-chloride was higher by 0.384 per mil. This shows that K isotope variations will be useful as a tracer in low-temperature geochemistry processes.