火山と年代測定 : K-Ar, ⁴⁰Ar/³⁹Ar年代測定の現状と将来(＜特集＞火山学の『夢』を語る)

抄録

K-Ar and ⁴⁰Ar/³⁹Ar geochronology is useful to solve various problems on the genesis and evolution of magmas and volcanoes. In this review paper, the usefulness of K-Ar and ⁴⁰Ar/³⁹Ar dating in volcanology is discussed, and three different approaches are described to further expand its significance in volcanology. They are 1) voluminous analyses, 2) precise analyses, and 3) accurate analyses in dating very young rocks. Obtaining considerable amount of ages is significantly important in clarifying the detailed growth history of volcanoes or volcanic fields. Systematic dating of regional volcanism should also contribute to construct tectonic evolution of the area. Well established conventional K-Ar dating by isotopic dilution method is to be expanded to significantly increase the age information on volcanoes and/or regional volcanism. Precise age information is particularly important to doccument the duration of volcanism and the detailed history of volcanoes, especially of older ages. The older the target volcanism is, the better resolution ages should be obtained in order to detect the small time difference between different volcanic episodes. Flood basalt volcanism, which fed enormous amount of magmas (10⁵-10⁶ km³) is now revealed to be of short duration (1-2 million years) from the advance of precise ⁴⁰Ar/³⁹Ar dating especially using a laser-fusion technique. This technique would be most promising for the detailed anatomy of volcanism in the future. Increasing demand is arising to date very young volcanic rocks accurately from the viewpoint of future hazard reductions. It is also useful to increase the knowledge how the magma plumbing systems are evolving beneath the active volcanoes. Due to the small accumulation of radiogenic ⁴⁰Ar in very young rocks, it is important to accurately know the initial ⁴⁰Ar/³⁹Ar ratios of samples. ⁴⁰Ar/³⁹Ar ratios in historic lavas are not equal to the atmospheric argon value, which is the basic assumption for the conventional method, but rather are on its mass-fractionation line. A better assumption of initial ⁴⁰Ar/³⁹Ar for age unknown samples is to calculate from analyses of stable ³⁸Ar/³⁶Ar ratios, and the method is called "mass-fractionation correction procedure". This method is not compatible to the addition of artificially concentrated ³⁸Ar spikes in the isotopic dilution method. Three different analytical techniques require three mass-spectrometers designated specially for each method, and are most appropriate for corresponding three geochronological approaches to volcanology. In the future, geochronological information from three different aspects should be tied up more closely with other physical and chemical approaches to integrate the model of magma generation, transport and eruption mechanisms.