New high-precision measurements of the isotopic composition of atmospheric argon

Earth’s atmosphere is used as a standard reference gas for mass spectrometric determinations of argon (Ar) isotopes used principally in geochronology. There are three published independent determinations of the Ar isotope composition of modern atmosphere that differ subtly. We have made new high-precision measurements of Ar isotope ratios of five different sources of air using a high-sensitivity multi-collector noble gas mass spectrometer in order to distinguish between them. The isotope ratios, corrected only for backgrounds, reside on a inverse square-root mass law fractionation line that passes through the air value proposed by Lee et al. [Lee J. Y., Marti K., Severinghaus J. P., Kawamura K., Yoo H. S., Lee J. B. and Kim J. S. (2006) A redetermination of the isotopic abundances of atmospheric Ar. Geochim. Cosmochim. Acta70, 4507–4512]. They are distinct from both the other proposed compositions and provide the first independent confirmation of the atmospheric Ar isotope composition. We suggest that the revised values should now be in routine employment.