The isotopic composition of atmospheric argon and 40Ar/39Ar geochronology: Time for a change?

A redetermination of the isotopic composition of atmospheric argon by Lee, J.-Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.-S., Lee, J.B., Kim, J.S. 2006. A redetermination of the isotopic abundances of atmospheric Ar. Geochimica et Cosmochimica Acta 70, 4507–4512] represents the first refinement since the work of Nier 1950. A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium. Physical Reviews 77, 789–793]. The new ⁴⁰Ar:³⁸Ar:³⁶Ar proportions imply <1% adjustments to ⁴⁰Ar/³⁹Ar ages in all but exceptional cases of very young and/or K-poor and/or Ca-rich samples, or cases in which samples are grossly under- or over-irradiated. Analytical protocols employing atmospheric argon to determine mass discrimination corrections are insensitive to the effects of revision on the air correction, but are subject to non-negligible adjustments arising from expanded heavy to light isotope ratios attending the increased mass discrimination correction. The competing effects of increased ⁴⁰Ar/³⁹Ar and ⁴⁰Ar/³⁷Ar ratios render the adjustments a function of sample chemistry and neutron irradiation parameters. The improved precision of atmospheric ⁴⁰Ar/³⁶Ar and ³⁸Ar/³⁶Ar permits increasingly sensitive detection of departures from atmospheric values. Non-atmospheric initial ⁴⁰Ar/³⁶Ar values are increasingly well-documented in volcanic materials, including subatmospheric values correlated with ³⁸Ar/³⁶Ar in a trend consistent with kinetic mass fractionation whereby incomplete equilibration between magma and atmosphere favors light isotope enrichment in the magma. The detailed mechanism(s) of such fractionation are unclear and must be clarified by further study. A detectable increase in atmospheric ⁴⁰Ar/³⁶Ar in the past 800 ka Bender, M.L., Barnett, B., Dreyfus, G., Jouzel, J., Porcelli, D., 2008. The contemporary degassing rate of ⁴⁰Ar from the Earth. Proceedings of the National Academy of Sciences 105, 8232–8237] suggests that ages of late Quaternary (e.g., <100 ka) materials incorporating large amounts of atmospheric argon such as biotite may be underestimated by as much as 100% if a modern atmospheric ⁴⁰Ar/³⁶Ar value is erroneously assumed, unless air argon is used to determine mass discrimination. Further evaluation of the evolution of paleoatmospheric ⁴⁰Ar/³⁶Ar, and the fidelity with which argon trapped in igneous materials reflects this, would be very productive. The use of isochrons rather than model (e.g., plateau) ages mitigates the vagaries associated with uncertain trapped argon isotope ratios, and the importance of strategies to derive statistically valid isochrons is underscored.