火山岩斑晶中の微小ガラス包有物の分析によるマグマ中の水および二酸化炭素濃度の直接測定―レーザープロープ法の開発と応用―

Volatile materials dissolved in magmlas play an important role in the evolution of magmas and volcanic eruptions. During magma ascent, the magma may be saturated with the volatiles because of decrease in solubilities of the volatiles in silicate melts with pressure. Formation of a gas phase would lead to decrease in the bulk density of magma and thereby accelerate the magma ascent. Expansion of the resulting gas phase may induce a volcanic eruption. The major volatile components in a nlagma are H(2)0, C0(2), S and Cl. Glass inclusions in phenocrysts are the most suitable samples for measuring volatile concentrations in pre-eruptive magmas. However, it has been difficult to measure the H(2)0 and CO(2) concentrations in glass inclusions, mainly because the size of the glass inclusions is very small (50-200 μm) and the sensitivity of analytical methods is not　sufficient. In this study, we have developed a laser microprobe　technique for determining a minute amount of H(2)0 and C0(2) dissolved in glass inclusions. The analytical system consisting of a Nd-YAG laser for selective heating of glass inclusions and a gas chromatograph-mass spectrometer for measuring a micro quantity of H(2)0 and C0(2) dissolved in glass inclusions. The H(2)0 and C0(2) were extracted from glass inclusions by laser heating under a vacuum. The extracted gas was collected in a cold trap and then introduced to GCMS by carrier gas. Gas concentration was calculated from the absolute amount of the extracted gas and the mass of glass melted. The optimum working conditions (lamp current, pulse frequency and shooting duration) of laser extraction were decided using a basaltic glass sample. A glass sample ground to less than 100 μm thick was pierced by a laser beam and measured for diameter of the hole produced to obtain the accurate determination of the volume of the glass melted. The mass of the melted glass was calculated from its volume and density. Efforts were made for reduction of background level of C0(2) and H(2)0. High purity helium carrier gas was farther purified with cold traps to remove C0(2) and H(2)0 contained in the carrier gas. Analytical line was thoroughly baked out at 300℃ before the analyses. With these procedures, the blank C0(2) was reduced to less than 0.07 ng and the blank H(2)0 to less than 6 ng. The detection limit of the present system was found to be 0.15 ng C0(2) and 15 ng H(2)0, considering the confidence limit of the calibration. The repeated analyses of C0(2) in basaltic glass indicate that the present technique allows us to analyze C0(2) concentration of glass inclusions as small as 70 μm in diameter within an accuracy of ±60 ppm, assuming that the glass inclusion contains 300 ppm C0(2). The H(2)0 analyses of the glass by the present technique gave significantly lower H(2)0 concentration than the bulk analysis, suggesting incomplete degassing of H(2)0 from molten glass during laser irradiation due probably to its slow diffusion rate. The glass inclusion samples from Killauea volcano, Hawaii, and Izu-Oshima volcano, Japan, were analyzed to determine CO(2) concentrations of pre-eruptive magma. The C0(2) concentration of pre-eruptive magma in South-West Rift Zone of Kilauea volcano was 230 ppm. This suggests that the magma has been significantly degassed during its storage in the summit magma chamber and migration to the rift system. The depth of the magma chamber was estimated to be 2.8-4.0 km from the C0(2) concentration of the pre-eruptive magma, in good agreement with the geophysical estimation. Comparison of the volatile budget of Kilauea volcano based on the C0(2) analysis of glass inclusion with the observed C0(2) flux suggests that parental magma contains 3000 ppm C0(2). The bulk density of magma containing exsolved gases is not snlall enough to ascend to the surface by its own buoyancy. This is consistent with the present settings of the Kilauean magma; the sumrnit magma chamber and rift magma system extending from the summit caldera. The C0(2) concentration of pre-eruptive magma of Izu-Oshima volcano was found to be 1 70 ppm. The measured C0(2) concentration indicates that the magma chamber exists at a depth of about 2 km beneath the volcano. At this depth, the magma should be saturated with C0(2). The estimated bulk densities of the magma (d=2.4-2.6 g/c㎥) containing C0(2)-rich gas phase are consistent with the density structure of the volcano, if the magma before degassing contains C0(2) more than 1700 ppm. The rate of magma supply was estimated from the C0(2) concentration of pre-eruptive magma and C0(2) fluxes based on measurements on surface volcanic gases. Assuming the primary magma contains 2500 ppm C0(2), the estimated rate of magma supply(6-19x 10(3) tons/day) is compatible with the observed rate of magma supply based on the total mass of ejecta of major eruptions during the last 1500 years

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, following a quiet period during 2011–17. Subplinian eruptions preceded lava effusion in 2011; however, no subplinian eruption occurred during 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, 0.25–1.4 wt% CO2) ascended into the shallow felsic magma (1.9–3.7 wt% H2O, 0.025–0.048 wt% CO2) at depths of 5–6 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 from 2018 and 2011 were similar, 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 during 2011–18. Melt inclusion analyses and volcanic gas observation noted a lower bulk volatile content in the 2018 magma (2.1–3.0 wt% H2O, 0.087–0.10 wt% CO2) than that in 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 February 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. Graphical Abstrac

Caldera collapse thresholds correlate with magma chamber dimensions

Abstract Explosive caldera-forming eruptions eject voluminous magma during the gravitational collapse of the roof of the magma chamber. Caldera collapse is known to occur by rapid decompression of a magma chamber at shallow depth, however, the thresholds for magma chamber decompression that promotes caldera collapse have not been tested using examples from actual caldera-forming eruptions. Here, we investigated the processes of magma chamber decompression leading to caldera collapse using two natural examples from Aira and Kikai calderas in southwestern Japan. The analysis of water content in phenocryst glass embayments revealed that Aira experienced a large magmatic underpressure before the onset of caldera collapse, whereas caldera collapse occurred with a relatively small underpressure at Kikai. Our friction models for caldera faults show that the underpressure required for a magma chamber to collapse is proportional to the square of the depth to the magma chamber for calderas of the same horizontal size. This model explains why the relatively deep magma system of Aira required a larger underpressure for collapse when compared with the shallower magma chamber of Kikai. The distinct magma chamber underpressure thresholds can explain variations in the evolution of caldera-forming eruptions and the eruption sequences for catastrophic ignimbrites during caldera collapse

Petrological characteristics and volatile content of magma of the 1979, 1989, and 2014 eruptions of Nakadake, Aso volcano, Japan

Abstract Petrological observations and chemical analyses of melt inclusions in scoria were used to investigate the magma ascent and eruption processes of the 1979, 1989, and 2014 eruptions of Nakadake, Aso volcano, Japan. Major elements and sulfur contents of the melt inclusions were determined using an electron probe microanalyzer, and their water and CO2 contents were determined using secondary ion mass spectrometry. Five scoria specimens from the 2014 eruptions had an andesite composition identical to the scoria from the 1979 and 1989 eruptions. Thermometry using the chemical composition of the groundmass and the rims of the phenocrysts indicated that the temperature of the 2014 magma was 1042–1092 °C. Melt inclusions in plagioclases, clinopyroxenes, and olivines in the 2014 scoria had an andesite composition similar to that of the groundmass. The volatile content of the melt inclusions was 0.6–0.8 wt% H2O, 0.003–0.017 wt% CO2, and 0.008–0.036 wt% S. The variation in CO2 and S content of the melt inclusions was not correlated with the K2O content, suggesting that the magma degassed as pressure decreased. Melt inclusions in plagioclases, clinopyroxenes, and olivines from the 1979 and 1989 scoria had similar major elements and volatile content to the 2014 eruption specimens. The similarity in chemical composition of both the whole-rock and melt inclusions among all samples suggests that the magmas of these eruptions were derived from the same magma chamber. The gas saturation pressure estimated from the H2O and CO2 contents of the 1979, 1989, and 2014 scoria ranged from 18 to 118 MPa, corresponding to depths of 1–4 km. Comparison of this depth with geophysical observations suggests that the inclusion entrapments occurred in the upper part of the magma chamber and/or a conduit. By combining the melt inclusion analysis with volcanic gas observations, we estimated the bulk volatile content of the magma. Based on the bulk sulfur content of the magma and the SO2 flux between January 2014 and December 2017, the amount of degassed magma over that period was estimated to be the equivalent of 1–3 km3 of dense rock. The estimated volume was more than 600 times larger than that of products erupted during the same period. This suggests that magma degassing occurred at several depths in the magma chamber due to magma convection in a conduit

西之島2015年噴火マグマの岩石学的特徴

http://www.godac.jamstec.go.jp/darwin/cruise/natsushima/nt15-e02/