歴史溶岩のアルゴン同位体比

―若い火山岩のK-Ar年代測定における初生値補正の重要性―

抄録

Precise determination of argon isotopic ratios was made on twenty-seven historic lavas from Japan. Their isotopic ratios were mostly mass fractionated from that of the atmospheric argon toward the enrichment in lighter isotopes. This suggests that the correction for the mass fractionated initial ⁴⁰Ar/³⁶Ar ratio is very important for the K-Ar dating of volcanic rocks younger than 0.1 Ma. The limit of measurable K-Ar age is estimated to be 0.03 Ma for a normal andesitic lava having a fractionated initial argon.