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

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 Hf-182-W-182 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.