Exploration of the age standard zircon for (U-Th)/He method; Chronometric investigations of the Nisatai Dacite

Fukuda, Shoma   ; Sueoka, Shigeru   ; Kanno, Mizuho; Kohn, B. P.*; Tagami, Takahiro*

In order to explore suitable zircon samples as standard minerals for the zircon (U-Th)/He (namely, ZHe) method, we have attempted to conduct ZHe analyses of the Nisatai dacite sampled in Iwate prefecture. Previous researches reported solid ages at 22-21 Ma and a rapid cooling pattern based on various geochronometric investigations, which inferred a potential of an age standard for ZHe method. Consequently, ZHe ages in this study yielded at 21.5 0.2 Ma having wide dispersion of grain ages from 40 Ma to 15 Ma, probably caused by random errors. As possible causes of age dispersion, the zoning of parent nuclei in a crystal and the heterogeneous rock pieces revealed by observations of thin-section are considerable. Therefore, the Nisatai dacite was concluded as unsuitable sample for the ZHe age standard. However, geochronologic data of other methods showed comparable ages such as zircon fission-track and U-Pb method, the Nisatai dacite may be used as these age standards. We will continue to explore another candidates for ZHe age standard, e.g., U-Pb age standards.