Comparison of Conventional K–Ar and 40 Ar/ 39 Ar Dating of Young Mafic Volcanic Rocks

K–Ar and 40 Ar/ 39 Ar ages have been measured on nine mafic volcanic rocks younger than 1 myr from the Snake River Plain (Idaho), Mount Adams (Washington), and Crater Lake (Oregon). The K–Ar ages were calculated from Ar measurements made by isotope dilution and K 2 O measurements by flame photometry. The 40 Ar/ 39 Ar ages are incremental-heating experiments using a low-blank resistance-heated furnace. The results indicate that high-quality ages can be measured on young, mafic volcanic rocks using either the K–Ar or the 40 Ar/ 39 Ar technique. The precision of an 40 Ar/ 39 Ar plateau age generally is better than the precision of a K–Ar age because the plateau age is calculated by pooling the ages of several gas increments. The precision of a plateau age generally is better than the precision of an isotope correlation (isochron) age for the same sample. For one sample the intercept of the isochron yielded an 40 Ar/ 36 Ar value significantly different from the atmospheric value of 295.5. Recalculation of increment ages using the isochron intercept for the composition of nonradiogenic Ar in the sample resulted in much better agreement of ages for this sample. The results of this study also indicate that, given suitable material and modern equipment, precise K–Ar and 40 Ar/ 39 Ar ages can be measured on volcanic rocks as young as the latest Pleistocene, and perhaps even the Holocene.