K-Ar法による年代測定  [in Japanese] K-Ar age determination  [in Japanese]

Two types of mass spectrometer are used for noble gas studies and K-Ar age determination. The details of these apparatus are shown. In the determination of radiogenic ^<40>Ar content in very young volcanic rocks, isotopic dilution technique with ^<38>Ar spike may not be necessary. Care must be taken in the determination of young volcanic rocks which may contain the primary Ar with non-atmospheric isotopic ratios such as excess ^<40>Ar and mass fractionated Ar. For such samples, the K-Ar ages obtained don't indicate the cruption ages. Equations for the analytical errors in determination of Ar content and the error of age are presented. In the case of young age dated with pure ^<38>Ar spike, these equations can be reduced to rather simple ones which are as same as those by COX & DALRYMPLE (1967).