[S-VC25] International Volcanology

[SVC25-07] Geochemistry of magmas in the lead-up and recovery of the 7.6 ka caldera collapse of Mashū volcano, Hokkaido

★Invited Papers

*Simon Barker¹, Osamu Ishizuka², Chris Conway², Kyoko Kataoka³, Yusuke Minami², Graham Leonard⁴, Bruce Charlier¹ (1.School of Geography, Environment and Earth Science, Victoria University of Wellington, New Zealand, 2.Geological Survey of Japan/AIST, Tsukuba, Ibaraki, Japan, 3.Research Institute for Natural Hazards and Disaster Recovery, Niigata University, Japan, 4.GNS Science, Lower Hutt, New Zealand)

Keywords:Magma, Caldera, Geochemistry, Petrology

Mashū volcano in Hokkaido has been highly active through the Holocene and produced many eruptions of varying size and eruption style, including a large caldera-forming eruption at ~7.6 ka. To better understand the history of the volcano and magmatic processes occurring at Mashū we undertook field work to sample eruptive products (pumice, ash) for geochemical analysis. In total, 48 rock samples were collected from >11 eruptions and multiple carbon samples were obtained to refine eruption ages. Carbon dates provide new constraints on chronology and indicate that previously undated eruptions occurred between 1.1 and 2.2 kyr at Mashū and at ~4.6 ka. Pumice and lava samples were analysed for major, trace element and isotopic (Sr, Pb) compositions that highlight the relatively homogeneous composition of the dacite magmas erupted during each episode. Only minor temporal changes in magma chemistry are observed during and following the 7.6 ka caldera-forming eruption suggesting that the same silicic magmatic system continues to feed young eruptions. Ongoing work will assess changes in mineral zoning and compositions to assess magma temperature, eruption triggers and timing of magmatic processes.