Ar/Ar Dating Independent of Monitor Standard Ages

Abstract:

Because the reported age of an analyzed sample is dependent on the age of the co-irradiated monitor standard(s), Ar/Ar dating is a relative dating technique. There is disagreement at the 1% scale in the age of commonly used monitor standards, and there is a great need to improve the inter-laboratory calibrations. Additionally, new approaches and insights are needed to meet the challenge of bringing the Ar/Ar chronometer to the highest possible precision and accuracy. In this spirit, we present a conceptual framework for Ar/Ar dating that does not depend on the age of monitor standards, but only on the K content of a solid standard. The concept is demonstrated by introducing a re-expressed irradiation parameter (J_K) that depends on the ratio of 39Ar_K to 40Ar^* rather than the 40Ar^*/39Ar_K ratio. J_K is equivalent to the traditional irradiation parameter J and is defined as J_K = (³⁹Ar/⁴⁰K) • (λ/λ_e). The ultimate precision and accuracy of the method will depend on how precisely and accurately the ³⁹Ar and ⁴⁰K can be estimated, and will require isotope dilution measurements of both from the same aliquot. We are testing the workability of our technique at the 1% level by measuring weighed and irradiated hornblende and biotite monitor standards using GLO-1 glauconite to define a calibration curve for argon signals versus abundance.