[SVC47-P20] メルト包有物分析による阿蘇火山中岳2014，1989，1979年噴火マグマの揮発性成分濃度
キーワード：阿蘇火山、中岳、マグマ、メルト包有物、揮発性成分、脱ガス
Volatile content of magma is one of important controlling factors for magma ascent and volcanic eruption. Melt-inclusion analysis is a powerful method for estimating volatile content of melt in magma before eruption. At Naka-dake, Aso volcano, volcanic activity increased September 2013, and minor eruptions occurred January 2014. Since then, eruptions had intermittently occurred up to October 2016, together with intense volcanic gas emission. In this study, we carried out petrological observation and chemical analyses of melt inclusions of the scoria of the 2014, 1989 and 1979 eruptions of Naka-dake to know the eruption and degassing processes.
Whole-rock composition of the scoria was determined by a wave-dispersive X-ray fluorescence analysis (XRF). Observation of the cross sections, chemical analyses of the minerals, determination of major elements and sulfur contents of melt inclusions and groundmass were carried out by electron probe micro analyzer (EPMA). Water and CO2 contents of the melt inclusions and matrix glass were determined by secondary ion mass spectrometry (SIMS). Analytical errors of the volatile analyses were ±0.2 wt.% for H2O, ±0.0028 wt.% for CO2, ±0.007 wt.% for S (Saito et al., 2010).
Five scoria of the 26-27 November 2014 eruptions have andesite composition (SiO2 =54 wt.% and K2O=2.0 wt.%) and are identical to those of scoria of the November 1979 eruptions (Ono and Watanabe, 1985). The scoria contained 22-31 vol % plagioclase phenocrysts, 5-13 vol % clinopyroxene phenocrysts, a few vol % of olivine and FeTi-oxide phenocrysts. The plagioclase phenocrysts have core of An62-91 and rim of An60-79. The clinopyroxene phenocrysts have core of Wo36-41En43-47Fs15-20 and rim of Wo36-40En42-46Fs17-19. The olivine phenocrysts have core of Fo65-68 and rim of Fo59-68. Two-pyroxene thermometry (Lindsley, 1983) applied to an orthopyroxene inclusion contained by a clinopyroxene phenocryst gave magma temperature of 1113±51ºC. Melt inclusions in plagioclases, clinopyroxenes and olivines have andesite composition (SiO2=58-62 wt.%, K2O=3.1-4.7 wt.%), that is similar to chemical composition of the groundmass. The melt inclusions have volatile content of 0.6-0.8 wt.% H2O, 0.003-0.017wt.% CO2 and 0.008-0.036 wt.% S. The variation in CO2 and S contents of the melt inclusions is not related to the K2O content, suggesting magma degassing with pressure decrease. Gas saturation pressure estimated from the H2O and CO2 contents and solubility model (Papale et al., 2006) is 22-78 MPa, corresponding to 1-3km depths. Combining the melt-inclusion analysis with observation of volcanic gas, we can estimate degassed-magma volume. The amount of degassed magma (1-3 km3) was estimated, based on the sulfur contents of the melt inclusions and SO2 flux during a period of January 2014 to December 2016 (1000-3000 t/d; JMA, 2016), assuming that only SO2 existing as sulfur component in the volcanic gas and magma density of 2700 kg/m3.
Melt inclusions in plagioclases, clinopyroxenes and olivines from the 1979 and 1989 eruptions have andesite composition (SiO2=57-62 wt.% and K2O=2.3-3.8 wt.% for 1979 eruptions, SiO2=57-63 wt.% and K2O=3.2-5.4wt.% for 1989 eruptions). The melt inclusions of the 1979 eruptions have volatile content of 0.3-1.6 wt.% H2O, 0.007-0.034wt.% CO2 and 0.010-0.035wt.% S. The melt inclusions of the 1989 eruptions have 0.3-0.6 wt.% H2O, 0.003-0.009wt.% CO2 and 0.008-0.031wt.% S. Major elements and volatile contents of melt inclusions of the 2014 eruptions are similar to those of the 1979 and 1989 eruptions. The similarity of chemical composition of whole-rocks and melt inclusions among these eruptions suggest petrologic characteristics and volatile content of the magma in the magma chamber had not changed from 1979 to 2014.
Whole-rock composition of the scoria was determined by a wave-dispersive X-ray fluorescence analysis (XRF). Observation of the cross sections, chemical analyses of the minerals, determination of major elements and sulfur contents of melt inclusions and groundmass were carried out by electron probe micro analyzer (EPMA). Water and CO2 contents of the melt inclusions and matrix glass were determined by secondary ion mass spectrometry (SIMS). Analytical errors of the volatile analyses were ±0.2 wt.% for H2O, ±0.0028 wt.% for CO2, ±0.007 wt.% for S (Saito et al., 2010).
Five scoria of the 26-27 November 2014 eruptions have andesite composition (SiO2 =54 wt.% and K2O=2.0 wt.%) and are identical to those of scoria of the November 1979 eruptions (Ono and Watanabe, 1985). The scoria contained 22-31 vol % plagioclase phenocrysts, 5-13 vol % clinopyroxene phenocrysts, a few vol % of olivine and FeTi-oxide phenocrysts. The plagioclase phenocrysts have core of An62-91 and rim of An60-79. The clinopyroxene phenocrysts have core of Wo36-41En43-47Fs15-20 and rim of Wo36-40En42-46Fs17-19. The olivine phenocrysts have core of Fo65-68 and rim of Fo59-68. Two-pyroxene thermometry (Lindsley, 1983) applied to an orthopyroxene inclusion contained by a clinopyroxene phenocryst gave magma temperature of 1113±51ºC. Melt inclusions in plagioclases, clinopyroxenes and olivines have andesite composition (SiO2=58-62 wt.%, K2O=3.1-4.7 wt.%), that is similar to chemical composition of the groundmass. The melt inclusions have volatile content of 0.6-0.8 wt.% H2O, 0.003-0.017wt.% CO2 and 0.008-0.036 wt.% S. The variation in CO2 and S contents of the melt inclusions is not related to the K2O content, suggesting magma degassing with pressure decrease. Gas saturation pressure estimated from the H2O and CO2 contents and solubility model (Papale et al., 2006) is 22-78 MPa, corresponding to 1-3km depths. Combining the melt-inclusion analysis with observation of volcanic gas, we can estimate degassed-magma volume. The amount of degassed magma (1-3 km3) was estimated, based on the sulfur contents of the melt inclusions and SO2 flux during a period of January 2014 to December 2016 (1000-3000 t/d; JMA, 2016), assuming that only SO2 existing as sulfur component in the volcanic gas and magma density of 2700 kg/m3.
Melt inclusions in plagioclases, clinopyroxenes and olivines from the 1979 and 1989 eruptions have andesite composition (SiO2=57-62 wt.% and K2O=2.3-3.8 wt.% for 1979 eruptions, SiO2=57-63 wt.% and K2O=3.2-5.4wt.% for 1989 eruptions). The melt inclusions of the 1979 eruptions have volatile content of 0.3-1.6 wt.% H2O, 0.007-0.034wt.% CO2 and 0.010-0.035wt.% S. The melt inclusions of the 1989 eruptions have 0.3-0.6 wt.% H2O, 0.003-0.009wt.% CO2 and 0.008-0.031wt.% S. Major elements and volatile contents of melt inclusions of the 2014 eruptions are similar to those of the 1979 and 1989 eruptions. The similarity of chemical composition of whole-rocks and melt inclusions among these eruptions suggest petrologic characteristics and volatile content of the magma in the magma chamber had not changed from 1979 to 2014.