A metrological approach to measuring Ar-40* concentrations in K-Ar and Ar-40/Ar-39 mineral standards

In geochronology, isotopic ages are determined from the ratio of parent and daughter nuclide concentrations in minerals. For dating of geological material using the K-Ar system, the simultaneous determination of Ar-40 and K-40 concentrations on the same aliquot is not possible. Therefore, a widely used variant, the Ar-40/Ar-39 technique, involves the production of Ar-39 from K-39 by neutron bombardment and the reliance on indirect natural calibrators of the neutron flux, referred to as "mineral standards." Many mineral standards still in use rely on decades-old determinations of Ar-40 concentrations; resulting uncertainties, both systematic and analytical, impede the determination of higher accuracy ages using the K-Ar decay system. We discuss the theoretical approach and technical design of a gas delivery system which emits metrologically traceable amounts of Ar-40 and will allow for the sensitivity calibration of noble gas mass spectrometers. The design of this system is based on a rigorous assessment of the uncertainty budget and detailed tests of a prototype system. A number of obstacles and proposed resolutions are discussed along with the selection of components and their integration into a pipette system.