Origin of the analytical 183W effect and its implications for tungsten isotope analyses

Budde, Gerrit; Archer, Gregory; Tissot, François; Tappe, Sebastian; Kleine, Thorsten

Zusammenfassung

Mass-independent tungsten isotope variations provide critical insights into the timing and nature of processes that occurred in the early Solar System and during planetary differentiation. However, W isotope analyses are often compromised by an analytical artifact manifesting itself as an apparent deficit in 183W, whose origin and nature have remained enigmatic. Here, by evaluating previously published high-precision W isotope data for a large and diverse set of terrestrial samples, we demonstrate that this artifact occurs independent of the type of mass spectrometer and confirm that it can be attributed to mass-independent fractionation of 183W. Contrary to previous proposals, we find that this ‘analytical 183W effect’ cannot be explained by a nuclear field shift, but may instead reflect a magnetic isotope effect. Regardless of its exact origin, our investigation reveals that this artifact is induced during the chemical separation of W, and that the specific combination of chromatographic purification and dry-down procedure determines its overall magnitude. Within a given analytical protocol, however, its size is strongly controlled by the amount of W that is processed, where the 183W effect increases with decreasing amount of W. Therefore, this work resolves apparent inconsistencies between previous studies regarding the occurrence and magnitude of the 183W effect, and provides directions for its mitigation and reliable correction. This in turn is crucial for the accurate interpretation of W isotope data with respect to radiogenic and nucleosynthetic anomalies for both terrestrial and meteoritic materials.