Magma ascent and degassing processes of the 2011 and 2017-18 eruptions of Shinmoedake in Kirishima volcano group, Japan, based on petrological characteristics and volatile content of magmas

Abstract The eruption activity of Shinmoedake in the Kirishima volcanic group of Japan resumed in 2017–18 after a quiet period that lasted between 2011 and 2017. Subplinian eruptions preceded lava effusion in 2011; however, no subplinian eruption occurred in 2017–18. Petrological studies and melt inclusion analyses were conducted to investigate the ascent and degassing of the magma to understand the cause of the different eruption styles. Chemical analysis of the melt inclusions from the 2011 eruption indicates that mafic magma with high volatile content (6.2 wt% H2O, 1.4–0.25 wt% CO2) ascended into the shallow felsic magma (3.7–1.9 wt% H2O, 0.048–0.025 wt% CO2) at depths of 4–5 km. Calculations indicate that the mafic magmas were of lower density (1717–1835 kg m-3) than the felsic magma (2264–2496 kg m-3) at 125 MPa and that the two magmas were mixed. The 2011 mixed magma with high volatile content (4.0 wt% H2O, 0.14–0.70 wt% CO2) had a bubble volume of approximately 50 vol% at 50 MPa, which is likely to have caused the subplinian eruption. The whole-rock and chemical compositions of the plagioclase-, clinopyroxene-, and orthopyroxene phenocryst cores were similar in 2018 and 2011, suggesting that the 2018 magma was a remnant of the 2011 magma. Chemical analyses of the groundmass from 2018 and the MELTS calculation indicate that the magma approached chemical equilibrium over the period 2011 to 2018. Melt inclusion analyses and volcanic gas observation gave a lower bulk volatile content for the 2018 magma (3.0–2.1 wt% H2O, 0.10–0.087 wt% CO2) compared to the 2011 magma. Comparison of the degassed-magma volumes estimated from the S and Cl contents of the melt inclusions, SO2 flux and volcanic gas composition, and erupted-magma volume indicates that excess degassing has been occurring in the magma due to convection since March 2011, which may have decreased the volatile content of the magma. The relatively low volatile content meant that the 2018 magma could not erupt explosively and lava was instead erupted via effusion.