JpGU-AGU Joint Meeting 2020

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-GL Geology

[S-GL34] Geochronology and Isotope Geology

convener:Takahiro Tagami(Graduate School of Science, Kyoto University), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo)

[SGL34-02] K-Ar and 40Ar/39Ar geochronology of Kuju volcano, SW Japan

*Seiko Yamasaki1, Daniel P Miggins2, Anthony A.P. Koppers2, Hideo Hoshizumi1 (1.Geological Survey of Japan, Advanced Industrial Science and Technology, 2.Oregon State University)

Keywords:Kuju volcano, unspiked K-Ar dating, 40Ar/39Ar dating

We conducted an inter-method comparison study between unspiked K-Ar and 40Ar/39Ar method for five samples from Kuju volcano, SW Japan. The unspiked K-Ar method enables the correction for the mass fractionated initial 40Ar/36Ar ratios, and has been successfully applied to young samples, especially younger than 0.5 Ma. In the 40Ar/39Ar method, initial 40Ar/36Ar ratios can be assessed by the use of an inverse isochron plot, resulting in higher precisions. We also measured two unirradiated samples by Ar/Ar dating system to compare 38Ar/36Ar ratios which is used for mass fractionation correction in the unspiked K-Ar method.

The age results for groundmass samples are consistent with stratigraphic relationships, as well as concordant with reported conventional K-Ar, Thermo Luminescence, Fission Track and 14C ages. The uncorrected ages for the youngest sample show negative ages in both methods, but for all corrected age spectra we obtained positive ages. The calculated age differences between corrected and uncorrected ages are up to ~100 ka. We conclude that the initial argon correction is effective in dating younger samples, providing more robust ages for Quaternary samples.