100万年より若い火山岩のK-Ar年代測定  [in Japanese] K-Ar age determination of volcanic rocks younger than 1 Ma  [in Japanese]

Advanced analytical technique of extremely low argon content has made it possible to date volcanic rocks with age to tens thousands years by the K-Ar method. Rare gas study in natural rocks with the technique has also presented an important problem which had direct relation to the validity of K-Ar age determination of young volcanic rocks. The problem is for the primary argon isotopic ratios in rocks different from atmospheric argon isotopics, which could be divided into the following three types :1) mass fractionation from atmospheric argon isotopes observed commonly in island are volcanic fields. 2) excess argon 40 in rim of pillow basalt lava from hot spot and ridge fields in Pacific ocean area. 3) excess argon 40 in coarse grained quartz and plagioclase phenocryts in andesite to dacite lavas in Japan. This paper presented a correction method of the age data for the mass fractionation type rocks (first type) with analysing of the primary argon 38/36 ratio by using a mass spectrometer for the rare gas study. For the rocks with the excess argon 40 (second and third type), the re is no plausible correction method. However, much higher reliability for the ages can be attained with careful treatment of the rocks to date, i.e., avoidance of dating of quick cooled samples and exclusion of coarse grained plagioclase and quartz phenocryst. In the K-Ar age determination technique, it is possible to date the rocks of the young age to tens thousands within ±50% for non-radiogenic argon=98%. In the application to volcano stratigraphy of each volcano, however, some more devices will be still required.