ABSTRACT

K-Ar biotite and fission-track (FT) zircon ages were determined on the Nisatai Dacite (ND), Iwate Prefecture, Japan, to assess its suitability for a geologic age standard. The concentration of potassium and argon isotope ratios were determined on a biotite sample twice and three times, respectively, and yielded reproducible results within analytical errors. A K-Ar age was given as 21.0±0.3 (2 sigma) Ma. FT zircon ages were determined on two samples from different localities as 21.8±1.4 (2 sigma) and 23.9±1.4 Ma. The K-Ar age was obtained from an outcrop adjacent to that for the former FT age, with a good agreement between the two methods. The concordance in age, in conjunction with previous zircon FT length data, suggests a potential applicability of the ND to both the K-Ar and FT system calibration in Cenozoic time.