抄録
Current situation and future possibility of the K-Ar and 40Ar/39Ar dating methods are reviewed focusing their utilization to the Tertiary volcanic rocks. As Tertiary volcanic rocks in general are not completely free from weathering and alteration, special attention should be made in choosing appropriate samples for age analyses. K-Ar and 40Ar/39Ar analytical results are shown for two contemporaneous rocks of Middle Miocene age, one is a porphyritic intrusive rock with holocrystalline groundmass and the other is an extrusive lava with glassy groundmass. The former gives concordant K-Ar and 40Ar/39Ar ages, while the latter shows considerably younger K-Ar and 40Ar/39Ar total gas ages with the obviously disturbed 40Ar/39Ar age spectra in the low temperature fractions. These results suggest that holocrystalline rocks are preferable to glassy rocks in K-Ar and 40Ar/39Ar dating, because glass tends to easily loose the accumulated radiogenic 40Ar not only by decomposition but also by hydration. Accurate and precise age determinations can be achieved by the 40Ar/39Ar incremental heating experiments along with the age spectrum and isochron interpretations. Total fusion 40Ar/39Ar dating for single to a few grains of fresh K-bearing phenocrysts like biotite, sanidine and plagioclase is also useful in knowing the ages for glassy and/or altered rocks. Multiple analyses on such mineral separates ensure the reproducibility of the analyses and reduce the analytical uncertainty by statistical treatments. Systematic 40Ar/39Ar studies are now underway to construct the accurate and precise tectonic history of the Japanese island arc.
