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Petrological characteristics and volatile content of magma of the 1979, 1989, and 2014 eruptions of Nakadake, Aso volcano, Japan
Earth, Planets and Space volume 70, Article number: 197 (2018)
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.
The volatile content of magma is an important controlling factor in magma ascent and eruption processes. Melt inclusion analysis is a powerful method for estimating the volatile content of melt in magma before eruption (e.g., Anderson 1973; Anderson et al. 1989; Johnson et al. 1994; Lowenstern 2003; Metrich and Wallace 2008). Moreover, by combining the melt inclusion analysis with geophysical and geochemical observations, we can estimate the depth of the magma and the degassed magma volume (e.g., Saito et al. 2001, 2005; Saito 2005).
Nakadake, Aso volcano, is one of the most active volcanoes in Japan. Since the 1990s, various geophysical observations have been used to investigate the subsurface structure and plumbing system of the magma, including gravimetric observations (Komazawa 1995), seismic observations (Yamamoto et al. 1999; Sudo and Kong 2001; Tsutsui and Sudo 2004; Abe et al. 2010), and magnetotelluric surveys (Kanda et al. 2008; Hata et al. 2016). These surveys suggested the existence of a magma chamber at a depth of less than 20 km. In addition, geological observations were carried out to estimate the volume of the tephra created by recent eruptions (e.g., Kyoto Univ 1980; Ono and Watanabe 1985; Kumamoto Univ 2015a, b; Kumamoto Univ et al. 2016). Geochemical observations of volcanic gas were also conducted to estimate the flux and chemical composition of the emitted gas (Geological Survey of Japan 2015a; Japan Meteorological Agency 2018b; Shinohara et al. 2018). Together, these observations allowed the advancement of our understanding of the magma ascent and eruption processes.
In this study, we carried out petrological observations and chemical analyses of melt inclusions of the scoria from recent eruptions at Nakadake in 1979, 1989, and 2014. Our objectives were (1) to determine the petrologic characteristics of the magmas of the eruptions and their changes over time, (2) to investigate the volatile content of the magmas, the depth of the magma chamber, and the amount of degassed magma, and (3) to combine these parameters with geological and geophysical observations to model the magma ascent processes at Nakadake, Aso volcano.
Overview of Nakadake, Aso volcano and the 1979, 1989, and 2014 eruptions
Aso volcano is located in central Kyushu, western Japan (Fig. 1). This volcano became active about 300 ka, and four caldera-forming eruptions occurred by 90 ka (e.g., Ono and Watanabe 1985; Machida and Arai 2003). The post-caldera eruptions started soon after the caldera-forming eruptions, and the total volume of tephra and edifices of the post-caldera central cones was about 130 km3 dense rock equivalent (DRE) (Miyabuchi et al. 2004; Miyabuchi 2009, 2011). Volcanic activities of Nakadake, one of the post-caldera volcanoes, started around 22–21 ka (Miyabuchi 2009) and continued through the present (JMA 2013). The recent activity in 1976–1979, 1988–1990, and 2014–2016 at Nakadake has been characterized by a cycle of drying-up of the crater lake water followed by mud eruptions, then phreato- and phreato-magmatic eruptions, and finally Strombolian eruptions (e.g., Asosan Weather Station 1980; Kyoto Univ 1980; JMA 1990; Ono et al. 1995; Ikebe et al. 2008). The emission rate of SO2 from the crater also increased as the activity increased: less than 2 kg s−1 (160 t d−1) in 1976–1977 to 12 kg s−1 (1000 t d−1) in September 1979 and then 30 kg s−1 (2600 t d−1) in November 1979 (Kyushu Univ 1980). The total tephra produced by the 1979 eruptions was about 9.5 × 109 kg (3.5 × 106 m3 DRE; Kyushu Univ 1980; Ono and Watanabe 1985). Similarly, from 1988 to 1989, the SO2 emission rate from the crater increased from less than 12 kg s−1 (1000 t d−1) in 1988, prior to the eruptions, to 44 kg s−1 (3800 t d−1) during the climactic period (October to November 1989) and then returned to less than 12 kg s−1 (1000 t d−1) in 1990 (Kyushu Univ 1990). The most recent eruption series started with minor eruptions in January 2014 after an increase in earthquakes in 2013, followed by Strombolian eruptions on November 26–27, 2014 (JMA 2015), and more intermittent eruptions until October 2016 (JMA 2018a). The total tephra produced by the 2014 eruptions (November 25–29 and December 9–11) and the most recent large eruption, which occurred on October 8, 2016, was estimated to be about 0.2 × 109 kg (7 × 104 m3 DRE; Kumamoto Univ 2015a, b) and 0.6–0.7 × 109 kg (2–3 × 105 m3 DRE; Kumamoto Univ et al. 2016), respectively. SO2 emission rates exceeded 12 kg s−1 (1000 t d−1) starting in January 2014, peaked at 174 kg s−1 (15,000 t d−1) at the time of the explosive eruption on October 8, 2016, and continued until December 2017 (JMA 2018b). Geochemical observations of the volcanic gas composition by multi-GAS (Aiuppa et al. 2005; Shinohara 2013) on January 12, 2015, just after the November 25–26, 2014, eruption, showed two types of gas being emitted: One was associated with ash eruptions (type A; molar ratios CO2/H2O = 0.04, CO2/SO2 = 1 and H2O/SO2 = 25) and the other was fumarole, a gas emitted from the crater side wall (type B; CO2/ H2O = 0.2, CO2/ SO2 = 10 and H2O/SO2 = 45 in mol; GSJ 2015a). Apparent equilibrium temperatures (AETs) of the type A and B gases were calculated from H2O, SO2, H2S, and H2 contents in the gases, with an assumption of equilibrium among these gas components. The AETs were high (975 °C and 680 °C, GSJ 2015a), suggesting that the gases were magmatic in origin. The observed chemical composition of the gas emitted from 2010 to 2015 could be explained by a mixture of the type A and B gases (GSJ 2015a).
The major-element composition of the five 2014 scoria specimens was determined (Table 1) using wavelength-dispersive X-ray fluorescence analysis (XRF; Togashi 1989). Petrological observation on scoria from the Strombolian eruption on June 6, 1979 (sample ID AS790616S, called “the 1979 scoria” in this paper), scoria from the eruption on November 24, 1989 (sample ID AS891124 called “the 1989 scoria” in this paper), and scoria from the eruption on November 26–27, 2014 (called “the 2014 scoria” in this paper), were carried out as follows. The specimens were mounted in epoxy resin, ground with sand paper, and then polished with diamond powder (1 µm). The mode compositions and porosities of the specimens were determined (Table 2) using backscattered electron (BSE) images obtained by scanning electron microscopy (SEM) of cross sections of the scoria. Chemical analyses of phenocrysts, melt inclusions, microlites (< 0.1 mm), glass in the groundmass, and the bulk composition of groundmass (called “groundmass bulk” in this paper) were conducted using an electron probe microanalyzer (EPMA; JEOL JXA-8900 at GSJ, AIST). The experimental conditions used for the analysis of minerals and groundmass bulk were identical to those reported by Saito et al. (2002) and Saito et al. (2001). The major-element contents and the minor S and Cl contents of melt inclusions and glass in the groundmass were determined with the EPMA using an accelerating voltage of 15 keV, a beam current of 12 nA, and a defocused 5-µm-diameter beam (Tables 3 and 4). The details of the experimental conditions for the EPMA analysis are described in Saito et al. (2010). Analytical errors on the S and Cl contents were ± 0.007 wt% and ± 0.004 wt%, respectively (Saito et al. 2010). We also estimated the H2O content of the inclusions and the glass in the groundmass with the EPMA by assuming that the H2O content was equal to the difference between 100% and the sum of all other measured oxide contents. The error of this method is ± 1 wt% of the obtained measurements based on repeated analyses of standard glass samples.
We measured the H2O and CO2 contents of melt inclusions and glass in the groundmass of the 1979, 1989, and 2014 scoria by secondary ion mass spectrometry (SIMS) (Tables 3 and 4). Prior to the SIMS analyses, the mounted samples were polished with Al2O3 powder (1 µm). All mounted samples were coated with gold for the SIMS analyses. We used a Cameca IMS-1270 SIMS (installed at GSJ, AIST) to measure the H2O and CO2 contents of the melt inclusions. Cs+ ions were used as the primary beam and negatively charged secondary ions of 1H, 12C, and 30Si were collected (Hauri et al. 2002). The defocused Cs+ primary beam was restricted to 25 µm in diameter by a circular aperture to obtain a homogeneous primary beam of about 1 nA. Negative ions of 1H, 12C, and 30Si were detected using a total impact energy of 20 kV. The analyzed area of the sample surface was limited to a central square measuring 10 × 10 µm to avoid the crater edge effect (Saito et al. 2010). A normal-incidence electron gun was used for charge compensation on the sample based on Kita et al. (2004). We made SIMS calibration lines for H2O and CO2 on each measurement day using the reference glasses. A more detailed description of the SIMS analyses is provided in Saito et al. (2010).
Chemical compositions of the scoria
The five 2014 scoria specimens have identical andesite whole-rock composition (54 wt% SiO2 and 2.0 wt% K2O as recalculated to 100% on a water-free basis; Table 1; Fig. 2). In addition, the whole-rock compositions are identical to those of the 1979 and 1989 scoria (54–55 wt% SiO2 and 1.9–2.0 wt% K2O and 55 wt% SiO2 and 2.0–2.1 wt% K2O, respectively). The products of the 1929, 1933, 1958, and 1974 eruptions have slightly less evolved compositions and display greater variability than those of the 1979–2014 eruptions (53–54 wt% SiO2 and 1.5–1.9 wt% K2O; GSJ and Kumamoto Univ 1990; Fig. 2).
Three of the 2014 scoria specimens have a phenocryst content of 35–39 vol%, composed of 22–31 vol% plagioclase phenocrysts, 5–13 vol% clinopyroxene phenocrysts, and a small percentage by volume of olivine and FeTi oxide phenocrysts (Table 2). The groundmass of the 2014 scoria consists of 50–77 vol% of bubbles, with the remaining vol% made up of glass, plagioclases, clinopyroxenes, olivines, and FeTi oxides, with a large variation in glass content in the groundmass (3–90 vol%). Previous studies (Ono and Watanabe 1985; GSJ and Kumamoto Univ 1990) reported that the 1979 and 1989 scoria contained phenocrysts of plagioclase, clinopyroxene, olivine, and FeTi oxide. The 2014 scoria contain the same kinds of phenocrysts as in 1979 and 1989 scoria, consistent with the whole-rock composition of these specimens being identical.
The plagioclase phenocrysts of the three 2014 scoria specimens have cores of An62–91 and rims of An60–79 (Fig. 3). The plagioclase phenocrysts of the 1979 and 1989 scoria have cores of An60–76 and rims of An60–70, similar to the 2014 scoria, although the cores of the 2014 scoria have a larger An range (Fig. 3). Groundmass plagioclase cores of the 2014 scoria contain An50–65, except a single groundmass plagioclase, which has a slightly lower An than the rim of the plagioclase phenocrysts. The cores and rims of the olivine phenocrysts in the 2014 scoria specimens fall within a narrow range of core Fo65–68 and rim Fo59–68. The cores and rims of the olivine phenocrysts in the 1979 scoria have chemical compositions similar to those of the 2014 scoria. The clinopyroxene phenocrysts of the 2014 scoria have cores of Wo36–41En43–47Fs15–20 and rims of Wo36–40En42–46Fs17–19. One clinopyroxene phenocryst has an orthopyroxene inclusion with a core chemical composition of W4En66Fs30 (Additional file 1: Fig. S1). The cores and rims of the clinopyroxene phenocrysts in the 1979 and 1989 scoria have similar chemical compositions to those of the 2014 scoria. Similarity between the chemical compositions of the plagioclases, olivines, and clinopyroxenes among the 1979, 1989, and 2014 scoria indicates that they crystallized under the same physical and chemical magma conditions.
The groundmass bulk of the three 2014 scoria specimens has an andesitic composition: 59 wt% SiO2 and 3.2–3.4 wt% K2O (Table 4; Fig. 2). The groundmass glass in the two 2014 scoria specimens has a similar andesitic composition of 59 wt% SiO2 and 3.4–3.7 wt% K2O (Table 4; Fig. 2). The groundmass glass of the 1979 and 1989 scoria also has a similar composition of 59 wt% SiO2 and 3.7 wt% K2O and 58 wt% SiO2 and 3.3–3.5 wt% K2O, respectively (Table 4; Fig. 2). Furthermore, the 2003 ash particles have a similar composition of 58 wt% SiO2 and 3.5 wt% K2O (Table 4; Fig. 2; GSJ et al. 2004). The similarity in the chemical composition of groundmass glass of the 1979, 1989, and 2014 scoria and the 2003 ash particles is consistent with the hypothesis that these materials were derived from the same magma.
Melt inclusions in the 1979, 1989, and 2014 scoria are glassy and round to elliptical in shape and range in size from 0.01 to 0.2 mm (Fig. 4; Table 3). Melt inclusions in plagioclases, clinopyroxenes, and olivines in the 1979 scoria have an andesite composition of 57–62 wt% SiO2 and 3.2–4.5 wt% K2O (Fig. 2). Melt inclusions in plagioclases and clinopyroxenes in the 1989 scoria have a similar chemical composition of 57–63 wt% SiO2 and 3.2–5.4 wt% K2O (Fig. 2), as do those in the 2014 scoria, which have a composition of 58–62 wt% SiO2 and 3.1–4.7 wt% K2O. The similarity in the major-element compositions of the melt inclusions in the 1979, 1989, and 2014 scoria suggests that magma chamber conditions remained unchanged from 1979 to 2014. The chemical compositions of the inclusions in the 2014 scoria are similar to those of the groundmass bulk (Table 3; Fig. 2), indicating their inclusion entrapment immediately prior to the eruption.
The H2O content of each inclusion determined by SIMS analysis was consistent with the content determined by EPMA and was within the large experimental error of the EPMA (± 1 wt%). The melt inclusions in the 1979 scoria had volatile contents of 0.3–1.6 wt% H2O (analyzed by SIMS), 0.007–0.034 wt% CO2, and 0.010–0.035 wt% S (Table 3; Figs. 5 and 6). The melt inclusions in the 1989 scoria had 0.3–0.6 wt% H2O (by SIMS), 0.003–0.009 wt% CO2, and 0.008–0.031 wt% S, which agree with the volatile contents of the inclusions in the 1979 scoria, within experimental error (± 0.2 wt% H2O by SIMS, ± 0.0028 wt% CO2, and ± 0.007 wt% S). The melt inclusions in the 2014 scoria have volatile contents of 0.6–0.8 wt% H2O (by SIMS), 0.003–0.017 wt% CO2, and 0.008–0.036 wt% S (Table 3; Figs. 5 and 6), which are similar to those of the 1979 and 1989 scoria. The similarity of the volatile content of the melt inclusions among these eruptions suggests that the volatile content of the magma in the magma chamber did not change from 1979 to 2014. Tamura et al. (2015) reported similar volatile contents of the melt inclusions in bombs of Nakadake (0.4–1.6 wt% H2O and 0.01–0.02 wt% S), although the eruption time of the bomb was not determined.
The large variability in CO2 and S content of the melt inclusions in the 1979, 1989, and 2014 scoria is not related to the K2O content (Fig. 5), suggesting either the addition of volatiles or magma degassing with pressure decrease. The distribution of H2O and CO2 content of the inclusions (Fig. 6a) suggests magma degassing with pressure decrease (Fig. 6e). The mass ratios of CO2/H2O, CO2/S, and S/H2O in the type A and B gases emitted on January 12, 2015, were calculated from the known molar ratios of type A and B gases, assuming that SO2 is the only S species in the gas (solid and dashed lines in Fig. 6). The CO2/H2O and CO2/S ratios of the melt inclusions in the 1979, 1989, and 2014 scoria were lower than those in the volcanic gases, except for three inclusions in the 1979 scoria that had high CO2/S ratios (Fig. 6a, b). In addition, most of the melt inclusions analyzed for H2O by SIMS had lower S/H2O ratios than the volcanic gases, except for two inclusions in the 1979 and 1989 scoria (Fig. 6c). These disagreements between the CO2/H2O, CO2/S, and S/H2O ratios of the inclusions and those of the volcanic gas reflect the high variability of CO2 and S contents of the melt inclusions (Fig. 6). The large variation in CO2 and S content of the melt inclusions with almost constant K2O content (Fig. 5) indicates that crystallization of melt did not cause the variations. This indicates that the inconsistencies are likely a result of exsolution of CO2 and S from the melt prior to inclusion entrapment.
Temperature and pressure of magma
We estimated the magma temperature using a glass (liquid) thermometer for a melt in equilibrium with olivine + plagioclase + clinopyroxene [Eq. (16) in Putirka 2008]. Based on the chemical compositions of the groundmass bulk in the three 2014 scoria, temperatures of 1092 °C, 1069 °C, and 1077 °C were obtained, assuming a pressure of 100 MPa (Table 4). We also applied the olivine–liquid and plagioclase–liquid thermometers and the clinopyroxene–liquid thermobarometer (Putirka 2008) to chemical compositions of the rims of phenocrysts and the groundmass bulk in the 2014 scoria, resulting in the temperatures ranging from 1042 to 1090 °C and the pressures ranging from 250 to 570 MPa (Additional file 2: Table S1). These temperature estimates were nearly identical to those obtained by the glass thermometer (1069–1092 °C), considering the standard errors of the estimate determined by Putirka (2008; ± 26 °C, ± 29 °C, and ± 36 °C for the glass, olivine–liquid, and plagioclase–liquid thermometers, respectively, and ± 42 °C for the clinopyroxene–liquid thermobarometer). Taking this evidence into account, we concluded that the magma from the 2014 eruption likely had a temperature of 1042–1092 °C.
Similarly, we applied the thermometers and thermobarometers to the chemical compositions of the melt and their host phenocrysts of olivine-, plagioclase-, and clinopyroxene-hosted melt inclusions from the 1979, 1989, and 2014 scoria and obtained temperatures of 1034–1074 °C (average of 1059 °C with standard deviation of ± 15 °C) and pressures of 390–560 MPa (average of 450 MPa with standard deviation of 76 MPa) for the 1979 scoria, 1031–1071 °C (average of 1057 °C with standard deviation of ± 16 °C) and 280–560 MPa (average of 442 MPa with standard deviation of 102 MPa) for the 1989 scoria, and 1027–1081 °C (average of 1056 °C with standard deviation of ± 19 °C) and 420–460 MPa (average of 440 MPa with a range of 20 MPa) for the 2014 scoria (Table 3). Theses temperature and pressure estimates for the melt inclusions in the 2014 scoria were similar to those of the groundmass bulk and the phenocryst rims (1042–1092 °C and 250–570 MPa). In addition, the temperature and pressure estimates of the melt inclusions of 1979, 1989, and 2014 scoria are consistent within their standard deviations. Therefore, in the discussion that follows, we will use the average temperature estimate from the glass thermometry for the 2014 scoria (1079 °C with standard deviation of 12 °C) for the temperature of the magma erupted in 1979–2014. In addition, we will use 600 MPa as a maximum pressure of the magma in our MELTS calculation (Table 5; Additional file 3: Table S2).
The variation in CO2 and S content of the inclusions in conjunction with the major-element compositions staying consistent (Fig. 5b, c), and the distribution of H2O and CO2 content in the inclusions (Fig. 6a), suggests that the cause of the variability in CO2 and S content is a pressure decrease in the magma (Fig. 6e). Gas saturation pressures ranging from 51 to 118 MPa were obtained using the H2O and CO2 content of four inclusions in the 1979 scoria in the solubility model proposed by Papale et al. (2006) for andesite magmas (Table 3; Fig. 6a). This pressure range corresponds to a depth of 2–4 km under a lithostatic pressure gradient. The H2O and CO2 content of five inclusions in the 1989 scoria yielded a lower gas saturation pressure range of 18–50 MPa (Table 3; Fig. 6a), which corresponds to a depth of 1–2 km. The H2O and CO2 content of five inclusions in the 2014 scoria yielded a gas saturation pressure range of 22–80 MPa (Table 3; Fig. 6a), which corresponds to a depth of 1–3 km.
The tomographic inversions for P- and S-velocity structure (Sudo and Kong 2001) and the three-dimensional modeling of the electrical resistivity structure (Hata et al. 2016) beneath the Aso caldera indicate the presence of a magma chamber at 2–10 km depth (bsl; Fig. 7). In addition, the spatial pattern of the observed long-period tremors (Yamamoto et al. 1999) and the three-dimensional modeling of the electrical resistivity structure (Hata et al. 2016) suggest the existence of a conduit from 4 km depth (bsl) to 0 km depth (bsl; Fig. 7). Four inclusions of the 1979 scoria and one inclusion of the 2014 scoria that yielded gas saturation pressures of 80–118 MPa (corresponding to depths of 3–4 km) were trapped in their host phenocrysts at the upper part of the magma chamber or the lower part of the conduit at depths of 2–3 km (bsl). Two inclusions of the 1979 scoria, six inclusions of the 1989 scoria, and four inclusions of the 2014 scoria that yielded gas saturation pressures of 18–61 MPa (corresponds to depth of 1–2 km) were trapped in their host phenocrysts at the conduit at depth of 0–1 km (bsl). These results suggest that the melt inclusion entrapments occurred in a conduit during the magma ascent. Such inclusion entrapment during a magma ascent was also proposed for the 2000 eruption at Miyakejima volcano (Saito et al. 2005).
Bulk volatile content and density of magma
The CO2/H2O and S/H2O mass ratios of the inclusions were lower than those of the volcanic gas (Fig. 6a, c). Two possible explanations for the disagreement should be considered: (1) different origins of the volcanic gas and the melt inclusions and (2) supersaturation of CO2 and S at the time of inclusion entrapment. However, the former explanation is unlikely because the scoria specimens that erupted from November 26–27, 2014, to March 26, 2015 (Fig. 2 a, b; GSJ 2015b; NIED et al. 2015a, b), have identical whole-rock compositions, supporting the idea that the volatiles released by the magma on January 12, 2015, were somehow similar to those that erupted on November 26–27, 2014. Therefore, the disagreement of the mass ratios of CO2/H2O and S/H2O between the inclusions and the volcanic gas (Fig. 6) is most likely due to the supersaturation of CO2 and S at the time of inclusion entrapment.
The above discussion indicates that only measurements of the melt inclusions might cause underestimation of total volatile content of the magmas, especially regarding less dissolved volatile species such as CO2 (Papale 2005). In order to estimate the total volatile content of the magmas, we calculated the bulk CO2 and S content of the magma, assuming that entrapment of the inclusions occurred under supersaturation of CO2 and S and that the magma emitted volcanic gases of types A and B. The H2O content of the degassed magma after volcanic gas emission was assumed to be 0.12 wt% based on the average H2O content of groundmass glass (0.2 wt%; Table 4) and assuming a glass content of 60 wt%. The CO2 content of the degassed magma was assumed to be 0 wt% and the S content was assumed to be 0.006 wt% based on the average S content of the groundmass bulk of three 2014 scoria specimens (Table 4). A bulk H2O content of 1 wt% for the pre-eruptive magma was obtained, given the maximum possible H2O content of the inclusion (1.6 wt%; value obtained for inclusion mts10120608-2-p1i1 by SIMS; Table 3) and groundmass content in the scoria (60 wt%; Table 2). On the basis of the above assumptions, we calculated bulk CO2 and S content in the magma to be 0.09 and 0.07 wt% from the mass ratios of CO2/H2O and S/H2O in the type A gas (“A1 magma” in Table 6). If type B gas was emitted from the magma, the bulk CO2 and S content in the magma were calculated to be 0.5 and 0.04 wt% (“B1 magma”). We also performed the calculation for 2 wt% H2O content, as that represents the maximum estimated H2O content. The calculation yielded bulk CO2 and S content in the magma of 0.18 wt% CO2 and 0.08 wt% S for the type A gas (“A2 magma” in Table 6) and 1 wt% CO2 and 0.08 wt% S for the type B gas (“B2 magma” in Table 6). Finally, we performed the calculation for a bulk H2O content of 0.5 wt%, matching the H2O content of the inclusion of the 2014 scoria (0.8 wt%; determined by SIMS for inclusion mts16041403-2-p1i1; Table 3). The calculation yielded bulk CO2 and S content in the magma of 0.04 wt% CO2 and 0.03 wt% S for the type A gas (“A3 magma” in Table 6) and 0.2 wt% CO2 and 0.02 wt.% S for the type B gas (“B3 magma” in Table 6).
We applied the MELTS calculation using the whole-rock composition of the scoria (No. I141129S01 in Table 1), with a temperature of 1079 °C and NNO buffer to investigate whether the estimated bulk H2O content (A1–A3, B1–B3) could reconstruct the mode composition of the 2014 scoria and the chemical composition of the melt inclusions (Table 5; Additional file 3: Table S2). The MELTS calculation for A1, B1, A3, and B3 magmas yielded similar mode composition of the scoria and similar chemical composition of the melt inclusions (see Additional file 3: Table S2; Table 5; Fig. 2), indicating that the bulk H2O content of the magma was likely between 0.5 and 1 wt%. We also calculated the H2O and CO2 content of the melt in the magmas at each pressure using the solubility model of Papale et al. (2006) (Figs. 5 and 6). The relationships of H2O and CO2 content with K2O content predicted by the MELTS calculation for A1 and B1 magmas deviated slightly toward lower K2O contents, while those for A3 and B3 magmas were consistent with the observations (Fig. 5). This calculation suggests that a possible cause of the variations in the major-element composition of the inclusions is differences in bulk H2O content; i.e., the inclusions with SiO2 contents of 57–58 wt% were entrapped in magma with a bulk H2O content of 1 wt% (A1 and B1 magmas) and the inclusions with an SiO2 content of 59 wt% were entrapped in magma with a bulk H2O content of 0.5 wt% (A3 and B3 magmas). This means that the magma is not homogeneous with regard to bulk H2O content throughout the plumbing system. This inhomogeneity could be caused by a magma degassing processes such as magma convection in a conduit (e.g., Shinohara et al. 2002; Shinohara 2008).
The density contrast between magma and crust can control the ascent of magma, causing the magma to be trapped in a magma chamber if the density contrast becomes negligible (e.g., Walker 1989). The bulk density of magma is highly dependent on volatile content and bubble content. With the assumption that gas bubbles that formed from the exsolution of H2O and CO2 from the melt do not separate from the magma during its ascent, we calculated bulk densities of the magmas at different depths using the bulk H2O and CO2 contents of the magma (Table 5; Fig. 7; Additional file 3: Table S2 and Additional file 4: Table S3; Saito et al. 2005). Comparison between the bulk densities of the A1, B1, A3, and B3 magmas and that of the crust at different depths indicated that the magmas had lower bulk densities than the crust at pressures of 50–200 MPa (1–6 km depth (bsl); Table 5; Additional file 3: Table S2; Fig. 7), which would allow the magmas to ascend by buoyancy. After the magmas had ascended to 0 km depth (bsl; pressure of 25 MPa), two magma ascent processes were possible, depending on the bulk volatile content of the magma. One possibility is that the B1 magma, whose bulk density was less than that of the crust (2007 kg m−3 for B1 magma versus 2200–2400 kg m−3 for the crust at depths of < 1 km (bsl)), ascended to the surface by buoyancy and thereby caused an eruption. The other possibility is that the A1, A3, and B3 magmas, having relatively low volatile contents, encountered a density barrier at 0 km depth (bsl) because their bulk density (2400–2594 kg m−3) became higher than or similar to that of the crust (Fig. 7), making it difficult for these magmas to reach the surface.
Degassed magma volume
By combining melt inclusion analysis with observations of volcanic gas, we can estimate the degassed magma volume (e.g., Kazahaya et al. 2002). We calculated the mass of degassed magma based on the estimated bulk volatile content of the magma, the measured SO2 flux, and the chemical composition of the magmatic gas emitted from the Nakadake crater, using the following equation:
where MD is the mass of the degassed magma (kg), CM is the volatile content of the magma (kg kg−1), CD is the volatile content of the degassed magma (kg kg−1), and MV is the mass of the volatile material emitted from the crater (kg). We used the sulfur contents of A1, A3, B1, and B3 magmas (0.02–0.07 wt% S, Table 6) as the CM, because the MELTS calculation in the previous section indicated that the A1, A3, B1, and B3 magmas, with bulk H2O contents of 0.5–1 wt%, were more realistic. The sulfur content of the degassed magma (CD) was estimated from the sulfur content of the groundmass of the 2014 scoria (Table 4), assuming that groundmass content of the scoria was 60 wt%. We calculated the amount of magma degassed over a period from January 2014 to December 2017, because of intense volcanic gas emissions combined with intermittent eruptions during that period. An average sulfur emission rate of 8.7 kg s−1 was calculated from the SO2 flux observations over 157 days (1500 ± 1200 t d−1 SO2; Table 6; JMA 2018b), assuming that SO2 was the only sulfur species in the volcanic gas.
The degassing rate of the magmas over the 4-year period from 2014 to 2017 ranged from 13 × 103 kg s−1 to 54 × 103 kg s−1 (Table 6). The degassing rate of the A1 magma (13 × 103 kg s−1) was similar to that of basalt–andesite volcanoes (7–16 × 103 kg s−1 for Etna, Izu-Oshima, Sakurajima, and Asama volcanoes; Kazahaya and Shinohara 1996; Ohwada et al. 2013). However, the degassing rates of the A3, B1, and B3 magmas are higher than those of basalt–andesite volcanoes. The higher estimated degassing rates at Aso volcano could be caused by (1) underestimation of bulk sulfur content of the magma and/or overestimation of SO2 flux or (2) more effective degassing and gas separation from magma at Nakadake volcano compared with the other volcanoes. More detailed studies on volcanic gas and melt inclusions will be required to evaluate these possibilities.
The volume of degassed magma estimated for 2014–2017 (0.62–2.5 × 109 m3 DRE) is more than 600 times larger than that of tephra over the same period (0.0010 × 109 m3 DRE; Table 6), which is equal to the maximum estimate of erupted magma. If the lower limit for the emission rate of sulfur of 1.5 kg s−1 (= average of 8.7 kg s−1—standard deviation of 7.2 kg s−1) is used for the above calculation, the volume of the degassing magmas from 2014 to 2017 is 0.12–0.51 × 109 m3 DRE. Even this minimum estimate is more than 100 times larger than the maximum volume of erupted magma. A degassed magma volume so much larger than the tephra volume indicates that the volcanic gas was derived from non-erupted magma located in a deeper part of the system. Because the magma chamber is located at a depth of more than 2 km (bsl) (e.g., Sudo and Kong 2001; Hata et al. 2016), the extensive degassing could be due to magma convection in a conduit (e.g., Kazahaya and Shinohara 1996; Shinohara et al. 2002; Kazahaya et al. 2002; Shinohara 2008).
Applying the degassing rate from the 2014 eruptions, we also performed the same calculation for the periods from June 1989 to February 1991, from December 1992 to February 1993, and from July 2003 to January 2004, when a large SO2 emission rate of more than 12 kg s−1 (1000 t d−1) was observed (Kyushu Univ 2004). The total combined volume of degassed magma for the above periods was 1.1–4.5 × 109 m3 DRE for the A1, A3, B1, and B3 magmas (Table 6). These estimates represent a lower limit, because the above calculation did not incorporate the emission of SO2 during the quiescent degassing period since 1979. This estimated volume of degassing magma requires a magma chamber with a volume of at least of 1 km3 at that depth. Geophysical observations indicate the existence of a large magma chamber at a depth of less than 10 km. Sudo and Kong (2001) proposed that this magma chamber is spherical in shape with a radius of about 3 km, corresponding to a volume of 110 × 109 m3. The low resistivity region (C1), which has less than 10 O m (reported by Hata et al. (2016)), seems to have a major axis of about 6 km and a minor axis of about 3 km. Assuming an ellipsoid shape, this implies a volume of 28 × 109 m3. The large magma chamber could form an important portion of the total volcanic gases emitted from the volcano from 1979 to 2017.
Summary and conclusions
The whole-rock analyses of the scoria produced by the November 26–27, 2014, eruption indicated that they are andesite in composition and identical to those of the 1979 and 1989 eruptions (Fig. 2), suggesting that these magmas were derived from the same magma chamber. Combining melt inclusion analyses of the 1979, 1989, and 2014 scoria with MELTS calculations and volcanic gas observations, we estimated the bulk volatile content of the magmas to be 0.5–1 wt% H2O, 0.04–0.5 wt% CO2, and 0.02–0.07 wt% S. We also estimated the amount of magma necessary to supply the SO2 emitted from 1979 to 2017 to be 1.1–4.5 × 109 m3 DRE (Table 6). This suggests the existence of a deep-seated andesite magma chamber with a volume larger than a few km3. We speculate that this andesite magma chamber is the same as the one at a depth of 4–6 km (bsl), proposed by tomographic results for P- and S-wave velocity structure (Sudo and Kong 2001) and three-dimensional electrical resistivity structure (Hata et al. 2016; Fig. 7). Considering the similarity of the whole-rock composition between the 1929–1974 eruptions and the 1979–2014 eruptions (Fig. 2), this magma chamber might have existed since at least 1929.
Comparison of the densities of the andesite magma and the crust at different depths (Fig. 7) suggests that prior to each eruption in 1979, 1989, and 2014, the andesite magma ascended by buoyancy up to a depth of a few km. The saturation pressure of H2O and CO2 in the melt inclusions ranged from 18 to 118 MPa, corresponding to depths of 1–4 km under a lithostatic pressure gradient. Comparison of the pressure estimates with the depth of the magma plumbing system estimated by geophysical observations suggested that the inclusion entrapments occurred in an upper part of the magma chamber and/or a conduit. After the magmas had ascended to 0 km depth (bsl), the magmas with low volatile contents could have encountered a crustal density barrier, because the density contrast becomes small at that depth (Fig. 7). These magma heads that stayed at a shallow depth could supply heat and gas to the upper part of the system, which could have caused the precursor events like those that characterized the recent activity at Nakadake crater (drying-up of the crater lake water, mud eruptions, and intense volcanic gas emissions; e.g., Ono et al. 1995; Ikebe et al. 2008). The degassed magma at the head could descend in the conduit due to increases in its density, and non-degassed magma could ascend in its place by magma convection in the conduit (e.g., Kazahaya and Shinohara 1996; Shinohara et al. 2002; Kazahaya et al. 2002; Shinohara 2008). In addition, contact of these magmas with underground water or hydrothermal fluid beneath the Nakadake crater (Kanda et al. 2008) could have led to phreato- and phreato-magmatic eruptions that blew away the subsurface rocks overlying the magmas, allowing H2O exsolution in the magmas due to pressure decrease. This caused further ascent of the magmas up to the surface and finally Strombolian eruptions. The estimated volume of degassed magma over the period from 2014 to 2017, based on bulk sulfur contents of melt inclusions, was more than 600 times larger than that of eruptive products during the same period. This suggests degassing of magma in the chamber due to magma convection in a conduit.
below sea level
volatile content of the degassed magma
volatile content of the magma
dense rock equivalent
electron probe microanalyzer
mass of degassed magma
mass of volatile material emitted from the crater
scanning electron microscopy
secondary ion mass spectrometry
X-ray fluorescence analysis
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GS carried out the SEM, EPMA, and SIMS analyses of the eruptive products and the melts calculation and drafted the manuscript. OI carried out the whole-rock analyses of the eruptive products. YI performed the geological survey of the 2014 eruption at Nakadake, Aso volcano, and collected the samples. HH performed the geological survey of the 1989 and 2014 eruptions at Nakadake, Aso volcano, and collected the samples. IM participated in the SIMS analyses and performed the preliminary melts calculation. All authors read and approved the final manuscript.
We thank Drs. Maki Hata and Nobuo Matsushima for providing information on the electrical resistivity structure beneath the Aso volcano. This manuscript was greatly improved with critical and constructive comments from Drs. Corrado Cigolini and Katsuya Kaneko. This work was partly supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP15K05274. We would like to thank Editage (www.editage.jp) for English language editing.
The authors declare that they have no competing interests.
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