[S-GL21] Geochronology and Isotope Geology

[SGL21-P04] ⁴⁰Ar/³⁹Ar geochronology of the Yonger Ontake Volcano, Japan

*Seiko Yamasaki¹, Teruki Oikawa¹, Daniel P. Miggins², Anthony A.P. Koppers² (1.Geological Survey of Japan, Advanced Industrial Science and Technology, 2.Oregon State University)

Keywords:40Ar/39Ar dating, Ontake Volcano

Ontake Volcano is a composite volcano located at the southern end of the Norikura volcanic chain. The eruptive products of the volcano are divided into two groups separated by an erosional gap: Older Ontake Desosits (ca. 780-420 ka) erupted before caldera and consists of lavas and volcaniclastic deposits, especially debris flow deposits. Younger Ontake Deposits (<110 ka) piled up in the caldera basin and some lava flows extended over dissected valleys on Older Ontake Desosits.
We conducted ⁴⁰Ar/³⁹Ar dating for nine andesite lavas from the Younger Ontake Volcano. Sample preparation to separate groundmass fraction was conducted at Geological Survey of Japan and incremental heating ⁴⁰Ar/³⁹Ar dating was measured using multicollector ARGUS-VI mass spectrometry at the Oregon State University Argon Geochronology Laboratory.
Two or three samples from three volcanic unit of the Younger Ontake (Marishiten VP, Yonnnoike-Mamakodake VP, Ichinoike VP) were measured in this study. All samples yielded good plateau ages and concordant with the stratigraphy and previously reported K–Ar ages. From the results of dating, there were some activity peaks to form large volcanic body in ca.70-50, 60-50 and 40-30 ka on Ontake Volcano and long lava flows occurred with high discharge rate during the period.