Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

S (Solid Earth Sciences) » S-GL Geology

[S-GL30] Geochronology and Isotope Geology

Wed. May 23, 2018 3:30 PM - 5:00 PM 304 (3F International Conference Hall, Makuhari Messe)

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), Chairperson:Sano Yuji, Tagami Takahiro

4:30 PM - 4:45 PM

[SGL30-09] 40Ar/39Ar dating for the youngest lava flow from Hakusan volcano, Central Japan

*Seiko Yamasaki1, Shun Nakano1, Daniel P. Miggins2, Anthony A.P. Koppers2 (1.Geological Survey of Japan, Advanced Industrial Science and Technology, 2.College of Earth, Ocean, and Atmospheric Sciences, Oregon State University)

Keywords:Hakusan Volcano, 40Ar/39Ar dating

Hakusan volcano is an active volcano located in the central part of Ryohaku Mountains, central Japan. The volcanic activity began around 420 ka in the northeast region, and the last documented eruption occurred in the summit area in 1659. For the youngest lavas in the summit area, previous studies reported K-Ar ages of ca. 40-10 ka. However, half of the samples were younger than the applicable limit of the conventional K-Ar method and yielded negative K-Ar ages (Kitahara et al., 2000). A Thermoluminescence (TL) age of ca. 11 ka was also reported for a lava flow from the summit of Mt. Gozengamine (Hasebe et al., 2016).

We present the first direct dating results using 40Ar/39Ar for the youngest lava flow (Shiramizunotaki) that flowed over 7 km to the east from the Gozengamine summit area. The age for this lava flow is estimated to be 2200 yBP based on 14C age of charcoal contained within a pyroclastic flow deposit beneath the lava flow (Kitahara et al., 2000). We conducted incremental heating experiments of groundmass concentration for the youngest lava flow. We applied the initial 40Ar/36Ar ratio from the inverse isochron back into the age equation instead of the assumed 40Ar/36Ar ratio of atmospheric argon of 295.5. This resulted in a new form the age spectrum (plateau ages were generated). These groundmass analyses yielded negative ages where the inverse isochron suggested a lower initial 40Ar/36Ar ratio of 287.9-288.9. The stacked initial-corrected plateau ages of two measurements yielded a combined age of 10.9±1.6 ka. Although this age is older than the estimated eruption age from 14C measurement, it agrees well with the TL age for the lava flow from the summit of Mt. Gozengamine. Further geological mapping and geochronology analyses will be needed to better reconstruct the youngest volcanic activity from the summit area of Hakusan volcano.