[SVC36-P03] Transition of magma plumbing system during the past 14 thousand years of Mashu volcano, eastern Hokkaido
Keywords:Mashu caldera, magma plumbing system, magma chamber
Mashu volcano is characterized by repeated explosive eruptions during 35 thousand years, and formed a caldera (6 x 7 km diameter) by successive plinian eruptions and following a large-scale pyroclastic flows at about 7500 years ago (Katsui et al., 1975;1986; Sumita, 1990; Hasegawa et al., 2009; Kishimoto et al., 2009). This study focus on the transition of the magma plumbing system during the past 14 thousand years from caldera formation stage to post-caldera stage.
Inferred from the cumulative change of the magmatic ejection volume of the Mashu volcano over the past 14000 years, three large-scale eruption events occurred at intervals of about 7000 years: Ma-l at 14000 years ago, caldera-formation of Ma-f, g, h, i, and j). After the formation of the caldera, the lava effusion and the explosive eruptions were repetitiously occurred every few hundreds of years to decades. After the formation of the Kamuinupuri central cone about 4000 years ago, intermittent eruption of Ma-c tephras occurred after the longest dormancy period (about 1500 years) in the post-caldera stage. Currently it has been 1000 years since the latest eruption (Ma-b) of 1000 years ago. Considering the characteristics of the eruption history through the post-caldera stage, it may be the time when the magma accumulation is ready and a new eruption event begins.
In the caldera-formation period, the possibility of simultaneous eruptions from several magma chambers was pointed out from the change of the lithic type and the change of phenocryst composition (Wada et al., 2017). In the TiO2 vs. K2O diagram of bulk rock composition, the magmas that caused the caldera formation is different from the post-caldera stage and the Ma-l period magmas, suggesting that the magma plumbing system was different only in caldera-forming period.
Estimated from the MELTS program and the mineral thermometers, the silicic magma chambers of Mashu volcano where phenocrysts (plagioclase-pyroxenes-iron oxides) can co-exist in equilibrium is 4-5 kb in pressure, 4-5% H2O, temperature of 930 ℃ to 970 ℃ are most suitable, which may have not changed significantly during the past 14000 years.
Inferred from the cumulative change of the magmatic ejection volume of the Mashu volcano over the past 14000 years, three large-scale eruption events occurred at intervals of about 7000 years: Ma-l at 14000 years ago, caldera-formation of Ma-f, g, h, i, and j). After the formation of the caldera, the lava effusion and the explosive eruptions were repetitiously occurred every few hundreds of years to decades. After the formation of the Kamuinupuri central cone about 4000 years ago, intermittent eruption of Ma-c tephras occurred after the longest dormancy period (about 1500 years) in the post-caldera stage. Currently it has been 1000 years since the latest eruption (Ma-b) of 1000 years ago. Considering the characteristics of the eruption history through the post-caldera stage, it may be the time when the magma accumulation is ready and a new eruption event begins.
In the caldera-formation period, the possibility of simultaneous eruptions from several magma chambers was pointed out from the change of the lithic type and the change of phenocryst composition (Wada et al., 2017). In the TiO2 vs. K2O diagram of bulk rock composition, the magmas that caused the caldera formation is different from the post-caldera stage and the Ma-l period magmas, suggesting that the magma plumbing system was different only in caldera-forming period.
Estimated from the MELTS program and the mineral thermometers, the silicic magma chambers of Mashu volcano where phenocrysts (plagioclase-pyroxenes-iron oxides) can co-exist in equilibrium is 4-5 kb in pressure, 4-5% H2O, temperature of 930 ℃ to 970 ℃ are most suitable, which may have not changed significantly during the past 14000 years.