High-precision measurements of tungsten stable isotopes and application to earth sciences

Abstract

Mass-dependent isotope fractionation of tungsten (W) isotopes has not received much attention until recently. This is mainly due to the small fractionation expected – as tungsten has a relatively high atomic mass – combined with the insufficient precision that could be achieved with the existing techniques. Tungsten is used in the ¹⁸²Hf–¹⁸²W radio-chronometer. Hence, tungsten isotopes are currently mainly used for studying the first stages of the solar system history, as they are well suited to trace metal–silicate equilibration processes. At the same time, evaporation, condensation and diffusion are known to fractionate stable isotopes. A better understanding of W stable isotope behavior during terrestrial and asteroidal processes will thus potentially shed light on those events. We here present an improved separation procedure based on anion-exchange chromatography that allows achieving quantitative recovery of W. Taking advantage of the last generation of multi-collector inductively coupled plasma mass-spectrometers (MC-ICPMS), we also set up a method to analyze W mass-dependent isotope fractionation with an external reproducibility better than 80 ppm and an internal reproducibility of 30 ppm. This new analytical procedure has been applied to igneous and iron-rich samples, from granites to chondrites and iron meteorites. Isotope variations observed for natural samples are well resolvable and vary from −0.05 to +0.36 per mil per mass unit.