Comparison of conventional K-Ar and Ar-40/Ar-39 dating of young mafic volcanic rocks

K-Ar and Ar-40/Ar-39 ages have been measured on nine mafic volcanic rocks y ounger than 1 myr from the Snake River Plain (Idaho), Mount Adams (Washingt on), and Crater Lake (Oregon). The K-Ar ages were calculated from Ar measur ements made by isotope dilution and K2O measurements by flame photometry. T he Ar-40/Ar-39 ages are incremental-heating experiments using a low-blank r esistance-heated furnace. The results indicate that high-quality ages can b e measured on young, mafic volcanic rocks using either the K-Ar or the Ar-4 0/Ar-39 technique. The precision of an (40)Arg/Ar-39 plateau age generally is better than the precision of a K-Ar age because the plateau age is calcu lated by pooling the ages of several gas increments. The precision of a pla teau age generally is better than the precision of an isotope correlation ( isochron) age for the same sample. For one sample the intercept of the isoc hron yielded an Ar-40/Ar-36 value significantly different from the atmosphe ric value of 295.5. Recalculation of increment ages using the isochron inte rcept for the composition of nonradiogenic Ar in the sample resulted in muc h better agreement of ages for this sample. The results of this study also indicate that, given suitable material and modern equipment, precise K-Ar a nd Ar-40/Ar-39 ages can be measured on volcanic rocks as young as the lates t Pleistocene, and perhaps even the Holocene.