Progress and Challenges in K-Ar and⁴⁰Ar/³⁹Ar Geochronology

K-Ar and more recently the 40 Ar/ 39 Ar variant are well established dating methods. The 40 Ar/ 39 Ar method requires irradiation with neutrons, posing some complications that are greatly outweighed by the benefits. The 40 Ar/ 39 Ar method is particularly powerful due to the availability of internal reliability criteria, the ability to analyze single crystals, and the amenability of the analyses to automation. 40 Ar/ 39 Ar dating has the capability for unsurpassed precision and is applicable to the broadest range of geologic environments and time scales of any radioisotope dating technique. For chronostratigraphic applications, 40 Ar/ 39 Ar is most important in the Cenozoic, becoming progressively less useful into the early Phanerozoic due to alteration and loss of radiogenic argon. Precision and accuracy of 40 Ar/ 39 Ar dating have been improved considerably in recent years, but an uncertainty of about 1% in the decay constant for 40 K, probably mainly in the electron capture decay branch, still limits accuracy at about this level. Inconsistent use of standards (neutron fluence monitors) and attribution of variable ages to standards is still a source of confusion, but straightforward recalculation procedures can overcome the underlying problems provided that appropriate standards are used.