Improved method for highly precise and accurate ¹⁸²W/¹⁸⁴W isotope measurements by multiple collector inductively coupled plasma mass spectrometry and application for terrestrial samples

Abstract

The extinct, relatively short-lived nuclide ¹⁸²Hf produced ¹⁸²W as a decay product. Fractionation of Hf-W in the very early Earth led to variations in the ¹⁸²W/¹⁸⁴W ratios of terrestrial rocks; however, because these variations are very small, quantifying ¹⁸²W/¹⁸⁴W ratios requires an extremely precise method. Here, we propose an improved method for highly precise and accurate method for measuring the ¹⁸²W/¹⁸⁴W ratios of terrestrial rocks. Samples were extracted with 4-methyl-2-pentanone and purified by cation and anion exchange chromatography prior to determination of the W isotope ratio by multiple collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) system coupled with a desolvating nebulizer. Sample preparation removed matrix elements (e.g., Hf, Ta, Os, and dimers of Nb and Mo) with masses similar to those of W isotopes, resulting in these elements having a negligible influence on the measured ¹⁸²W/¹⁸⁴W ratios. A W standard solution processed by ion exchange chromatography and/or solvent extraction showed a ¹⁸³W deficiency, even after mass fractionation correction of the measured isotope data. As reported previously, mass-independent fractionation increases the ¹⁸²W/¹⁸⁴W ratio if the ¹⁸³W/¹⁸⁴W ratio is used to correct for mass fractionation to for better precision in natural samples. However, accurate ¹⁸²W/¹⁸⁴W ratios for a basalt reference material (JB-2) were obtained, even if ¹⁸³W was used for mass fractionation correction. Our results show that it is also possible to correct for the effects of mass-independent fractionation on the ¹⁸³W/¹⁸⁴W ratio by sample-standard bracketing using a W standard solution subjected to the same preparation procedure used for the samples. A major advantage of the newly developed method is that it requires a smaller amount of sample (0.2–0.3 g; 50–80 ng W for JB-2) compared with that needed for other reported methods (typically 0.7–15 g; 500–1000 ng W). This decrease in sample amount was possible by removing matrix elements from the sample solutions, and cleaning the membrane of the desolvating nebulizer between analyses also contribute to enhancing the W ion beam intensity and to high precision. Analysis of different basalts from the Loihi, Kilauea islands and Ontong Java Plateau with various W isotopic compositions consistent with the previous studies demonstrated the reliability of the method.