Ohba et al. Earth, Planets and Space (2021) 73:81 https://doi.org/10.1186/s40623-021-01405-4

FULL PAPER Open Access Behavior of magmatic components in fumarolic gases related to the 2018 phreatic eruption at Ebinokogen Ioyama volcano, Kirishima Volcanic Group, Kyushu, Japan Takeshi Ohba1*, Muga Yaguchi2, Urumu Tsunogai3, Masanori Ito3 and Ryo Shingubara3

Abstract Direct sampling and analysis of fumarolic gas was conducted at Ebinokogen Ioyama volcano, Japan, between Decem- ber 2015 and July 2020. Notable changes in the chemical composition of gases related to volcanic activity included a sharp increase in ­SO2 and ­H2 concentrations in May 2017 and March 2018. The analyses in March 2018 immediately preceded the April 2018 eruption at Ioyama volcano. The isotopic ratios of ­H2O in fumarolic gas revealed the process of formation. Up to 49% high-enthalpy magmatic vapor mixed with 51% of cold local meteoric water to generate coexisting vapor and liquid phases at 100–160 °C. Portions of the vapor and liquid phases were discharged as fuma- rolic gases and hot spring water, respectively. The ­CO2/SO2 ratio of the fumarolic gas was higher than that estimated for magmatic vapor due to ­SO2 hydrolysis during the formation of the vapor phase. When the fux of the magmatic vapor was high, efects of hydrolysis were small resulting in low ­CO2/SO2 ratios in fumarolic gases. The high apparent equilibrium temperature defned for reactions involving ­SO2, ­H2S, ­H2 and H­ 2O, together with low ­CO2/SO2 and H­ 2S / SO2 ratios were regarded to be precursor signals to the phreatic eruption at Ioyama volcano. The apparent equilibrium temperature increased rapidly in May 2017 and March 2018 suggesting an increased fux of magmatic vapor. Between September 2017 and January 2018, the apparent equilibrium temperature was low suggesting the suppression of magmatic vapor fux. During this period, magmatic eruptions took place at Shinmoedake volcano 5 km away from Ioyama volcano. We conclude that magma sealing and transport to Shinmoedake volcano occurred simultaneously in the magma chamber beneath Ioyama volcano. Keywords: Phreatic eruption, Volcanic gas, Fumarole, Hydrothermal reservoir, Magmatic vapor

Introduction announcement of an evacuation advisory to tourists. A Although phreatic eruptions are generally small in skier was also killed by falling rocks ejected during the scale, they can nonetheless cause injuries and fatali- phreatic eruption of Moto-Shirane Volcano Japan in 2018 ties. Te phreatic eruption at Ontake Volcano Japan in (Japan Meteorological Agency 2020a). No precursory 2014 killed 58 tourists at the summit area (Oikawa et al. volcanic earthquake was observed prior to that eruption. 2016). Prior to the eruption, some volcanic earthquakes Te detection of volcanic earthquakes is in some cases had been observed by the Japan Meteorological Agency insufcient to mitigate disasters resulting from phreatic (JMA); however, the eruption occurred in advance of the eruptions (Barberi 1992; Stix and de Moor 2018), and it is, thus, necessary to evaluate the eruptive potential of such volcanoes on the medium and long term using *Correspondence: [email protected] 1 Tokai University, 4‑1‑1 Kitakaname Hiratsuka, Kanagawa 259‑1291, Japan methods in addition to seismic observation. Phreatic Full list of author information is available at the end of the article eruption involves the explosion of hydrothermal fuid

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reservoirs that develop in the relatively shallow crust. increased according to the frequency of earthquakes Tese reservoirs comprise groundwater from aquifers, (Ohba et al. 2019a). Such changes in fumarolic gas can be which absorbs high-enthalpy magmatic vapor (MV). SO­ 2 attributed to the injection of MV into the HR. Tis injec- and ­CO2 are the typical constituent chemical species of tion induces increases in fuid pressure in the HR and MV components. By understanding the behavior of MV, may initiate volcanic earthquakes (Ohba et al. 2019a). the potential of medium- to long-term phreatic eruptions From the viewpoint of general volcanic disaster pre- can be evaluated. Geochemical observations of fumarolic vention, it is important to investigate MV components in gas could improve our understanding of MV behavior, fumarolic gases prior to phreatic eruptions at volcanoes because such gases dominate the gas phase in hydro- such as Hakone volcano. A phreatic eruption took place thermal reservoirs that leak to the surface (e.g., Ossaka in April 2018 at Ebinokogen Ioyama volcano (hereafter et al., 1980; Taran et al. 2002; Tassi et al. 2003; Chiodini “Ioyama volcano”) (Tajima et al. 2020). Between Decem- et al. 2010; Fischer et al. 2015; Ohba et al. 2019a, b). In ber 2015 and July 2020, we repeatedly collected and addition to the MV component, fumarolic gases include analyzed fumarolic gases at Ioyama volcano to investi- components formed within the hydrothermal reservoir gate the behavior of MV components in fumarolic gases (HR). Here, HR components are composed of reducing before and after the eruption, and to extract geochemical chemical species such as H­ 2S and CH­ 4. Tose H­ 2S and parameters that could be used to denote precursor events ­CH4 are thought to be reduced from the magmatic SO­ 2 in advance of the main eruption. 2+ and ­CO2, respectively, by the Fe­ contained in the crus- tal rocks contacting with the fuid in hydrothermal reser- Geological settings voir (Giggenbach 1997). Te coexistence of MV and HR Te Kirishima volcanic group is located near the volcanic components in fumarolic gases was identifed at White front on Kyushu Island Japan (Fig. 1a). Ioyama volcano Island, New Zealand (Giggenbach 1987). Before the phre- (Io) is among the volcanoes in this group, and is sur- atic eruption at Hakone Volcano in 2015, volcanic earth- rounded by many volcanic craters and cones formed dur- quakes occurred, and the ratio of MV to HR components ing the Quaternary period (Fig. 1b). Te formation ages

Fig. 1 a Location of Ebinokogen Ioyama volcano (Io) on Kyushu Island (KI), Japan. The Philippine Sea Plate (PP) is subducting beneath the Eurasian Plate (EP) on which KI is situated. PP and EP are separated by the Nankai trough (NT). Circles and broken line indicate active volcanoes and the volcanic front, respectively. b Io surrounded by Ebinodake volcano (Ed), Karakunidake volcano (Kr), Shiratori-yama crater (Sy), Rokkannonmiike crater (Ro), and Fudoike crater (Fd). Kd is a debris avalanche deposit. P and W denote ponds and collapse walls, respectively. Eq denotes the region in which the volcanic earthquake hypocenters are distributed. Sm, Ss, Oy, Ks and On denote Shinmoedake volcano, Shishidodake volcano, Ohatayama volcano, Koshikidake volcano and Onamiike volcano, respectively. Bold red curves indicate the crater rim. c Location of fumaroles a, b, c and h. Phreatic eruptions took place at e1 and e2 on April 16 and 26, 2018, respectively. Hot spring water was discharged at K3 and Y1. R denotes roads Ohba et al. Earth, Planets and Space (2021) 73:81 Page 3 of 17

of Ebinodake volcano (Ed), Karakunidake volcano (Kr), chamber beneath Ioyama volcano to Shinmoedake Shiratori-yama crater (Sy), Rokkannonmiike crater (Ro) volcano. and Fudoike crater (Fd) were estimated to be 70–300 ka, To clarify the magma plumbing system of the Kirishima 15 ka, 18–25 ka, 25 ka, and 6.3 ka, respectively (Imura volcanoes, a magnetotelluric (MT) survey was conducted and Kobayashi 2001). A part of northwest fank of Kr has (Utada et al. 1994). Based on the one-dimensional (1D) collapsed and spread a debris avalanche deposit (Kd). analysis of these results, the emplacement of a magma Te andesitic Ioyama volcanic body was developed on chamber was suggested at − 10 km from the surface Kd during a magmatic eruption in 1768 (Imura and Kob- (Utada et al. 1994). A low-resistivity region was also ayashi 2001). During January 2011, subplinian eruptions detected approximately 5 km beneath Ioyama volcano, (VEI = 3) occurred at Shinmoedake volcano (Nakada which was interpreted to be a small magma chamber or et al. 2013), which is located 5 km southeast of Ioyama magmatic gas reservoir. Tree-dimensional (3D) analyses volcano. Following the 2011 events, magmatic eruptions of these MT survey data confrmed the depth of magma took place at Shinmoedake volcano in October 2017, chamber at − 10 km depth, located 10 km west of Ioy- March 2018 and April 2018. Prior to the 2017 erup- ama volcano as shown schematically in Fig. 2 (Aizawa tion, crustal deformation was observed in the area of the et al. 2014). Tese analyses also detected a vertical elec- Kirishima volcano group (Japan Meteorological Agency tric conductor beneath Ioyama volcano, which was inter- 2020b). Tis deformation was interpreted to result from preted to be a magmatic vapor reservoir zone (MVR in the combination of shrinking beneath Ioyama volcano Fig. 2) with temperatures greater than 400 °C. Tis ver- and infation beneath Shinmoedake volcano, suggesting tical conductor extends between − 5 and − 3 km, and that the magma transport had occurred from a magma was estimated to be a magma pathway that was partially

Fig. 2 Schematic illustration of the magma plumbing system revealed by a previous 3D-MT survey (Aizawa et al. 2014). MV and M refers to magmatic vapor and magma, respectively. Io and Sm denote the Ioyama and Shinmoedake volcanoes, respectively. MVR, SMC and DMC are the magmatic vapor reservoir, shallow magma chamber and deep magma chamber, respectively Ohba et al. Earth, Planets and Space (2021) 73:81 Page 4 of 17

formed before magmatic eruptions at Shinmoedake vol- Layer-5: Hydrothermal reservoir (at depths greater cano (Aizawa et al. 2014). than − 600 m from the surface and temperatures is An area of active fumarolic activity has been observed higher than 200 °C). around the summit of Ioyama volcano since the early Volcanic earthquakes at Ioyama volcano occurred of 1900s (Funasaki et al. 2017). During the 1970s, the mostly in Layer-5. Te point in Layer-5 just below the fumarolic activity was particularly intense, and the tem- summit crater was a pressure source of crustal defor- perature of fumarolic gases reached 247 °C (Kagiyama mation since July 2017 (Japan Meteorological Agency et al. 1979). In 1994, the temperature of fumarolic gas 2020b). decreased to 101 °C (Ohba et al. 1997), and after the 1990s, the fumarolic activity gradually decreased. After 2007 the discharge of fumarolic gas around the summit Sampling and analysis of fumarolic gases area of Ioyama volcano ceased completely (Funasaki et al. Fumarolic gases were repeatedly sampled at positions 2017), and the seismic activity has been continually low a, b and c (see Fig. 1c). Samplings were obtained at 17 (Japan Meteorological Agency 2020b). occasions between December 2015 and July 2020. In Since mid-2015, however, volcanic earthquakes have addition to the above three fumaroles, one fumarolic been observed sporadically. In December 2015, fumarolic gases was sampled at position h in July 2020 (Fig. 1c). gas discharge was confrmed to occur for the frst time Te outlet temperature of fumarolic gas was measured since 2007 (position a, Fig. 1c). Since December 2015, using a thermocouple with a K-type sensor (Yokogawa fumarolic gas emissions at Ioyama volcano have become Electric Corp., TX-10). Fumarolic gas was introduced increasingly intense. Tajima et al (2020) reported super- using a titanium pipe inserted directly into the fuma- fcial phenomena occurring at Ioyama volcano in details roles from which samples were obtained. Te end of the from 2015 to 2018. According to their report, the geo- titanium pipe was connected to a rubber tube, which thermal surface area with temperatures higher than 50 °C was in turn connected to a 120 ml pre-evacuated Pyrex was approximately 2000 ­m2 in May 2016. Tis geother- glass bottle with an airtight stop cock (Giggenbach mal area gradually increased to around 10,000 ­m2 in June 1975), where 20 ml of 5 M KOH solution was intro- 2017. In May 2017, a strong fumarole characterized by a duced. Water vapor and acidic gases ­(CO2, ­SO2, ­H2S, roaring sound developed (position h, Fig. 1c). Following etc.) were absorbed by the KOH solution whereas the the appearance of this fumarole, the geothermal surface residual gases (hereafter R-gas), e.g., ­N2, ­O2, Ar, He, ­H2, 2 area temporarily decreased to about 4000 m­ in Octo- and ­CH4 were maintained in the headspace of the glass ber 2017. Tis reduced geothermal area was maintained bottle. Te chemical species in the KOH solution were until around February 2018, at which time the number analyzed according to the methods of Ozawa (1968). of earthquakes increased sharply. On April 19 and 26, Te ­H2S/SO2 ratio in the fumarolic gas was deter- 2018, phreatic eruptions occurred (positions e1 and e2 mined using a KI–KIO3 solution (Lee et al. 2016) and in Fig. 1c, respectively). Te eruption on April 19 2018 the total molar amount of R-gas was determined by the began at 15:39 and lasted for about 15 h, and the height head space volume of the bottle and the inner pressure of the eruption column was about 500 m at maximum of the head space at room temperature. Based on the (Japan Meteorological Agency 2020b). Te total volume molar amount of ­H2O, CO­ 2, ­H2S, ­SO2 and R-gas, the of the ejecta was estimated to be 1500 ­m3, and no juvenile relative concentration (ppm) of each gas was calculated materials were detected in the ejecta (Tajima et al 2020). (Table 1). Te analysis of R-gas by gas chromatography Volcanic earthquakes on Ioyama volcano were followed the methods of Ohba et al. (2019a, b). observed to occur mostly below the summit crater and For isotopic analysis of water vapor in fumarolic gas, in the area extending to the south (Fig. 1b). Te depth this gas was cooled using a double-tube condenser of their epicenters was distributed in the range of − 3 made of Pyrex glass. Te isotopic ratio of the con- to − 1 km below sea level (Japan Meteorological Agency densed water was determined using an IR-laser cavity 2020b). According to Tajima et al (2020), the shallow ring down analyzer (Picarro Inc., L2120-i). Te analyti- crust beneath Ioyama volcano is estimated to have the cal precision of the analyzer was 0.12‰ and 0.05‰ 18 ± ± following layers. for δD and δ O, respectively. Te isotopic ratio of H­ 2 Layer-1: Vadose zone (− 50 m from the surface); in the R-gas was determined using a continuous fow Layer-2: Aquifer (the groundwater layer); system combined with a mass spectrometer (Termo Layer-3: Low-temperature hydrothermal layer (the Fischer Scientifc Delta V) (Tsunogai et al. 2011). Te groundwater mixed with hydrothermal fuid at tempera- analytical precision of the analyzer was ± 0.8‰ for δD tures less than 200 °C); of ­H2. Layer-4: Impermeable layer (caprock); Ohba et al. Earth, Planets and Space (2021) 73:81 Page 5 of 17 – 756 261 AETD, ℃ AETD, 437 385 256 726 110 437 340 252 262 549 817 926 208 416 584 804 891 292 301 1152 352 1310 288 1127 373 385 490 353 301 171 272 – 301 400 AETS, ℃ 283 232 314 313 258 277 745 296 353 338 347 752 380 376 300 374 579 416 361 361 403 611 291 571 314 366 681 385 295 363 305 SMOW , δD 2 – H − 242 ‰ − 355 − 400 − 468 − 229 − 611 − 348 − 399 − 464 − 457 − 284 − 199 − 173 − 506 − 354 − 271 − 204 − 178 − 436 − 430 − 141 − 391 − 120 − 446 − 141 − 398 − 385 − 321 − 402 − 426 − 484 − 565 − 471 SMOW O 18 O, δ 2 − 15.9 ‰ − 11.9 H − 9.2 − 14.1 − 7.5 − 6.3 − 4.6 − 6.6 − 4.0 − 5.3 − 1.9 − 3.6 − 2.6 − 2.2 − 3.4 − 3.7 − 1.9 − 3.5 − 1.3 − 3.7 − 3.8 − 3.7 − 2.6 − 2.2 − 6.0 − 2.4 − 11.2 − 8.1 − 6.8 − 6.7 − 2.7 − 14.3 − 14.6 − 11.7 SMOW O, δD O, 2 − 94 − 78 ‰ H − 67 − 91 − 56 − 52 − 46 − 57 − 46 − 41 − 44 − 43 − 45 − 42 − 47 − 48 − 44 − 48 − 40 − 49 − 51 − 48 − 46 − 46 − 60 − 45 − 78 − 69 − 57 − 64 − 44 − 89 − 93 − 84 – 1.58 Ar 1.92 2.65 0.82 1.09 0.17 1.91 0.34 0.47 0.37 0.21 0.31 0.26 0.84 0.42 0.59 0.35 0.21 0.49 0.47 0.55 0.31 0.36 0.96 0.21 0.39 0.62 0.42 1.02 0.37 2552 568 1.50 4 – − − 0.183 CH 0.261 0.401 0.277 0.211 0.054 0.206 0.157 0.165 0.074 0.060 0.044 0.088 0.106 0.077 0.049 0.055 0.072 0.076 0.109 0.045 0.045 0.064 0.046 0.020 0.066 0.122 0.140 0.046 0.066 0.034 with their corresponding apparent equilibrium temperatures (AETS equilibrium and AETD). temperatures apparent with their corresponding 2 2 275 – N 251 328 210 250 46 201 135 131 65 53 89 89 179 71 103 84 70 114 143 102 70 104 130 34 85 162 135 115 59 181 177,916 44,213 H 2 O and ­ 2 0.155 – O 0.092 0.145 0.116 0.091 0.040 0.050 0.000 0.093 0.052 0.022 0.066 0.000 0.076 0.012 0.059 0.475 0.056 0.044 0.328 0.075 0.166 0.369 0.034 0.048 0.179 0.070 0.000 2.97 0.019 0.110 29,985 1187 H 2 7.04 – 5.48 H 1.73 13.8 7.40 4.24 4.30 603 8.77 27.0 15.4 18.4 468 44.5 24.1 16.6 20.4 107 49.1 17.7 27.2 23.5 166 5.81 110 7.73 22.6 416 19.6 11.42 8.45 24.7 11.1 – 0.217 0.239 He 0.209 0.191 0.232 0.040 0.192 0.103 0.122 0.046 0.043 0.076 0.055 0.130 0.055 0.073 0.070 0.070 0.080 0.118 0.076 0.049 0.098 0.059 0.026 0.059 0.127 0.114 0.038 0.042 0.077 0.924 0.267 2 42.5 SO 50.6 – 26.8 55.0 84.7 18.8 58.5 680 48.9 42.6 67.3 45.3 2303 32.6 215 8.3 149 863 103 56.1 77.5 146 2901 29.0 421 56.9 42.2 367 103 15.2 28.9 889 419 S 2 960 1717 – 1192 H 4329 1037 1596 1459 1575 2977 2622 1756 1270 2177 2455 3077 2588 1418 565 1965 655 3227 540 2993 653 367 754 822 1078 508 2105 850 10,430 1276 2 15,845 24,918 – CO 14,163 20,773 16,532 5462 20,210 7887 14,237 4017 6493 8208 6601 11,939 5444 6373 8683 6145 6954 15,061 7798 6279 9104 4735 2867 7136 15,786 7988 4894 2997 5915 43,125 9299 O 2 982,869 973,056 – 984,285 H 974,618 982,088 992,873 978,065 989,119 982,596 993,226 991,615 990,369 988,363 985,348 991,168 990,910 989,645 992,249 990,814 984,067 988,768 992,940 984,731 994,446 996,200 991,959 983,165 990,014 994,356 994,811 993,015 735,078 943,027 96.2 95.2 88.1 97.2 95.2 97.2 97.0 95.3 96.6 96.9 96.4 96.4 96.0 96.5 96.0 96.4 96.0 97.7 95.9 96.8 96.8 95.0 96.2 96.0 96.7 95.8 95.6 95.2 96.1 96.1 96.0 93.6 94.1 Temp, ℃ Temp, 106.0 2/24/2016 5/6/2016 7/31/2020 12/22/2015 Date, mm/dd/yyyyDate, 8/30/2016 2/24/2016 1/17/2017 5/6/2016 5/15/2017 8/30/2016 9/15/2017 1/17/2017 10/18/2017 5/15/2017 11/28/2017 9/15/2017 1/26/2018 10/18/2017 3/28/2018 11/28/2017 10/19/2018 1/26/2018 1/25/2019 3/28/2018 5/31/2019 5/29/2018 2/14/2020 10/19/2018 5/15/2017 1/25/2019 9/15/2017 5/31/2019 2/14/2020 11/8/2019 Concentration of chemical species in fumarolic gases and isotopic ratio of ­ gases and isotopic of chemical species in fumarolic Concentration b b a a 1 Table Location The unit of concentration is in micro-mol/mol ( = ppm) The b a b a b a b a b a b a b a b a b a b a b a b a c a c a a a Ohba et al. Earth, Planets and Space (2021) 73:81 Page 6 of 17

Results Table 1 lists the data obtained on fumarolic gas, including AETD, ℃ AETD, 250 133 178 677 296 107 115 118 112 102 92 471 outlet temperature, chemical composition, isotopic ratios 18 (δD and δ O) of H­ 2O and the isotopic ratios (δD) of H­ 2. Te outlet temperatures were predominantly close to the AETS, ℃ 250 271 255 552 336 241 244 264 353 282 348 360 boiling point (95.8 °C) of water under standard air pres- sures at the altitude of the fumaroles (1300 m) (Fig. 3a). SMOW Te outlet temperature of fumarole “a” was 106 °C in , δD 2 H ‰ − 469 − 586 − 534 − 239 − 433 − 626 − 613 − 615 − 627 − 635 − 653 − 346 September 2017, 10 °C higher than the boiling tempera- ture. Te outlet temperatures of fumaroles a and c have dropped signifcantly below their boiling point since SMOW O

18 November 2019. Te histograms in Fig. 3a, b indicate the number of earthquakes observed monthly by the JMA. O, δ 2 ‰ H − 3.7 − 4.5 − 2.7 − 3.3 − 4.1 − 10.3 − 8.5 − 12.7 − 16.0 − 13.9 − 18.0 − 14.2 Vertical lines indicate the phreatic eruption of Ioyama volcano (April 19, 2018) and the magmatic eruptions at

SMOW Shinmoedake volcano (October 11, 2017, March 1, 2018, and April 5, 2018). Te fumarolic gases were mostly O, δD O, 2 ‰ H − 48 − 51 − 44 − 46 − 50 − 73 − 65 − 80 − 93 − 82 − 99 − 78 dominated by H­ 2O vapor, except the fumaroles a and c after November 2019 (Fig. 3b). Figure 4 shows temporal Ar 0.67 0.90 0.64 0.32 0.98 3.59 1.22 2.90 859 7.54 108 0.98 changes in the logarithmic concentrations (ppm) of CO­ 2, ­H2S, ­SO2, He, H­ 2, ­O2, ­N2, ­CH4, and Ar. After November 4 CH 0.057 0.144 0.039 0.101 0.069 0.674 0.248 0.307 3.41 1.65 8.66 0.00 2019, the concentrations of ­N2, ­O2, and Ar in fumaroles a and c increased signifcantly, indicating air contamina- tion. Te SO­ 2 and H­ 2 concentration of fumaroles a, b, 2 N 111 189 113 89 105 649 206 284 71,048 2240 10,337 109 and c increased signifcantly during May 2017 and March 2018 (Fig. 4c, e).

2 In the following, we divide the observation period O 0.026 0.076 0.063 0.139 0.034 0.380 0.103 0.035 14,915 2.32 32.6 8.08 based on the isotopic ratio of ­H2O (Fig. 5a, b).

2 Period- I: December 2015 to December 2016; H 3.14 5.16 4.52 108 16.0 3.23 3.52 5.46 21.2 6.71 13.2 6.20 Period- II: January 2017 to May 2018; Period- III: June 2018 to July 2020. He 0.075 0.132 0.079 0.081 0.062 0.545 0.164 0.169 1.53 1.81 5.44 0.034 During Period-1, δD increased from approximately 18 2 − 90‰ to the around of − 50 ~ − 40‰, while the δ O SO 19.8 39.7 19.6 420 41.4 18.3 17.3 23.9 251 69.8 201 357 increased from about − 14‰ to the range of − 4 ~ − 1‰ (Fig. 5a, b). Te number of earthquakes during this S 2 1305 H 2511 2557 609 1367 2874 2555 2840 19,146 6955 24,290 183 period was relatively small, and the activity of Ioyama volcano is regarded to be in its preparatory stage. 18 2 During Period-II, the δD and δ O maintained rela- 18 CO 8984 11,639 7152 7370 8215 52,031 23,782 12,479 160,775 128,932 542,741 3785 tively high values. Te highest values of δD and δ O were recorded at fumarole b in March 2018 (Fig. 5a, O

2 b). Troughout this period, the volcanic activity was 989,576 H 985,615 990,153 991,404 990,254 944,418 973,434 984,364 732,980 861,783 422,263 995,551 enhanced. Shinmoedake volcano exhibited three mag- matic eruptions, and phreatic eruptions occurred at Ioy- 96.2 95.8 95.7 97.0 96.1

94.8 ama volcano. More earthquakes occurred in Period-II 97.6 94.7 94.0 93.5 85.9 95.9 Temp, ℃ Temp, than in Period-I. In Period-III, the δD decreased to 100‰ and the 18 − δ O decreased to − 18‰. Te number of earthquakes was high at the beginning of this period but gradually decreased. No eruptions occurred in this period at Ioy- 10/18/2017 Date, mm/dd/yyyyDate, 11/28/2017 1/26/2018

3/28/2018 ama volcano. 5/29/2018 10/19/2018 1/25/2019 5/31/2019 11/8/2019 2/14/2020 7/31/2020 7/31/2020 (continued) For the δD of H­ 2, the highest values were recorded at fumarole “a” in March 2018 (Fig. 5c). In terms of the c 1 Table Location c c c c c c c c c c h δD of ­H2, the time variation at the three fumaroles did Ohba et al. Earth, Planets and Space (2021) 73:81 Page 7 of 17

not harmonize. For example, considering the changes of ­H2O in hydrothermal systems (Giggenbach 1987), between August 2016 and January 2017, the δD at fuma- the left-hand side (LHS) of Eq. (2) is calculated to be as role “a” increased signifcantly, while that at fumarole b follows: decreased. Te δD at fumarole c was low between Octo- = −1 + ber 2018 and July 2020, whereas the δD of fumarole b was LHS 8924 T 1.242 (3)  200‰ higher than that of fumarole c for the same period. Te three fumaroles are located close to one another where T is temperature in Kelvin. AETS is defned as the (Fig. 1c); thus, their distinct δD implies that the phenom- temperature that satisfes Eq. (3). ena controlling the δD of H­ 2 operated at a shallow depth. It is useful to consider AETS because it is the qualitative If δD of ­H2 was fxed at a large depth, the δD is expected parameter depending on conditions favoring the retro- to be similar for fumaroles located close each other. grade reaction in terms of reaction (1). During the trans- 18 Te δ O and δD values of ­H2O in fumarolic gas port of MV from the source to surface, the temperature exhibit a linear distribution (Fig. 5d). K3 and Y1 are hot decreases along the temperature profle of the passage. spring waters collected at Ioyama volcano (Tajima et al When the magmatic vapor rises rapidly, the chemical 2020); collection locations are shown in Fig. 1c. K3 rep- composition of magmatic vapor will be quenched at high resents local meteoric water (Tajima et al. 2020). Y1 is a temperature without a large change, then, a high AETS is hot spring water discharging near the eruptive point e1 calculated for the fumarolic gas composition. When the (Fig. 1c), which appeared just before the phreatic erup- magmatic vapor rises slowly, the chemical composition tion of Ioyama volcano, with a temperature on April of magmatic vapor will be quenched with a large change 16, 2018 of 93 °C (Tajima et al. 2020). AMW is general because an enough time is allowed to occur reactions isotopic composition of H­ 2O degassed from andesitic at lower temperature among gas species or reactions magma (Taran et al. 1989; Giggenbach 1992). Some between gas species and crustal rock, a low AETS is cal- fumarolic gas samples are distributed near the lines con- culated for the fumarolic gas composition (Giggenbach necting AMW and K3, but most are distributed in the 1987). Terefore, an increase in AETS is often observed area below these lines, suggesting that a simple mixing during periods of enhanced volcanic activity (e.g., Ohba model between the local meteoric water and AMW is et al 2011). Another apparent equilibrium temperature insufcient to explain the isotopic ratio of ­H2O in fuma- (AETD) is defned in terms of the following reaction: rolic gas. + = + HD H2O H2 HDO (4)

Discussion Assuming equilibrium in reaction (4), we obtain the Temporal change of apparent equilibrium temperatures equation of Richet et al (1977): 1 (AETs) − 2 AETD = 4.474 × 10−12x2 + 3.482 × 10−9x + 9.007 × 10−8 Te apparent equilibrium temperature based on sulfur  species in fumarolic gas (hereafter AETS) is defned in − 273.15 terms of the following reaction (e.g., Ohba et al 1994): (5) + = + where AETD is in degree C and x is given by SO2 3H2 H2S 2H2O (1) δD(H2O) + 1000 Assuming the equilibrium of reaction as follows: x = 1000 ln δD(H2) + 1000  (6) SO2 H2 K + fH O =− − log log 2 log 3 log AETS and AETD were high in Period-II relative to H2O H2O Periods-I and -II (Fig. 6a, b). For example, the AETS in H2S + log May 2017 were 752, 745, and 681 °C at fumaroles a, b, H O 2 and c, respectively and the AETD in May 2017 was 926, (2) 340, and 490 °C at fumaroles a, b, and c, respectively. where K and fH2O are, respectively, the equilibrium con- Although the AETS at fumaroles a, b and c often show stant of reaction (1) and the fugacity of ­H2O, both of close values, the AETD of fumaroles a, b and c occasion- which are functions of temperature. Te fractions in ally show large diferences due to the correspondingly parentheses on the right-hand side of Eq. (2) refer to the large diferences in the δD of H­ 2. Terefore, to estimate molar ratio of each gas species relative to H­ 2O. Based the qualitative velocity and fux of MV, changes in AETS on the temperature dependence of the standard equilib- are more reliable than AETD. rium constants of formation of elements and the fugacity Ohba et al. Earth, Planets and Space (2021) 73:81 Page 8 of 17

a :a :b :c :h

b

2015 2016 2017 2018 2019 2020 Fig. 3 a Changes in fumarolic gas outlet temperature. The horizontal broken line indicates the boiling temperature of water at fumarole altitude.

b Changes in the ­H2O concentration of fumarolic gas. The red histograms in (a) and (b) indicate the monthly number of volcanic earthquakes observed by the JMA. The blue vertical solid line indicates the eruption at Ioyama volcano and blue vertical broken lines indicate eruptions at Shinmoedake volcano

Behavior of MV decrease in MV fux (Fig. 7b). Te simultaneous observed Te behavior of MV during Period-II can be estimated reduction in geothermal area (Tajima et al. 2020) is con- by correlating changes in AETS with the eruptive events sistent with a decreased MV fux. During the period of occurring at Shinmoedake and Ioyama volcanoes (Fig. 7). low AETS, a magmatic eruption occurred in October AETS increased signifcantly during May 2017, when 2017 at Shinmoedake volcano, suggesting that magma a strong fumarole appeared at point h. No eruptions was transported from the SMC to Shinmoedake volcano. occurred at Shinmoedake volcano in May 2017. Conse- Te decreased fux of MV may be explained by magma quently, it is estimated that a stagnant magma in the shal- sealing (Fournier 1999). Te number of earthquakes low magma chamber (SMC) released a large fux of MV increased from January to March 2018 (Fig. 7c). Fur- (Fig. 7a). Tis increased fux shortened the residence time thermore, AETS rose in March 2018. Multiple eruptions of MV, increasing the value of AETS. Such an increase in occurred at Shinmoedake volcano during this period, and MV fux is supported by the enlarged geothermal area it is, thus, highly probable that SMC discharged magma observed during this time (Tajima et al. 2020), which toward Shinmoedake volcano, and the fux of MV to Ioy- reached its maximum in June 2017. Te AETS was low ama volcano increased simultaneously. Magma move- between September 2017 and January 2018, suggesting a ment and the increased MV fux likely occurred due to Ohba et al. Earth, Planets and Space (2021) 73:81 Page 9 of 17

1000 a b c 800

600

:a 400 :b :c 200 :h 0 1000 d e f 800

600

400

200

0 1000 g h i 800

600

400

200

0 201520162017 2018 2019 2020 2015 2016 2017 2018 2019 2020 2015 2016 2017 2018 2019 2020

Fig. 4 Changes in the concentration of CO­ 2 (a), ­H2S (b), ­SO2 (c), He (d), ­H2 (e), ­O2 (f), ­N2 (g), ­CH4 (h), and Ar (i). The vertical axis is calibrated as the logarithm of ppm (µmol/mol) concentration. Red histogram and vertical lines have the same signifcance as in Fig. 3

Fig. 5 a Changes in the hydrogen isotopic ratio (δD) of ­H2O. Red histogram and vertical lines have the same meaning as in Fig. 3. b Changes in 18 18 the oxygen isotopic ratio (δ O) of ­H2O. c Changes in the hydrogen isotopic ratio (δD) of ­H2. d δD and δ O of ­H2O in fumarolic gas. AMW denotes the general region of magmatic water emitted from andesitic magma. K3 denotes the isotope ratio of local meteoric water. Broken lines show the simple mixing relationship between AMW and K3. LW denotes the local meteoric water line Ohba et al. Earth, Planets and Space (2021) 73:81 Page 10 of 17

a volatile enriched magma supply from the deep magma (Lp), and the Ar distribution between Lp and Vp, the fol- chamber (DMC) to the SMC (Fig. 7c). lowing equations can be written: + − = + − Model for the formation of vapor and liquid phases HMV f HLW 1 f HVpg HLp 1 g (7) 18   Te correlation between δD and δ O of ­H2O in the fuma- + − = + − rolic gases at the Hakone and Kusatsu-Shirane volcanoes δMV f δLW 1 f δVpg δLp 1 g (8) has been explained by the mixing of high-temperature MV   and cold local meteoric water (LW), followed by a single- CMV f + CLW 1 − f = CVpg + CLp 1 − g step phase separation (Ohba et al 2019a, b). Te concept of (9)   this model is illustrated in Fig. 8. Herein, we aim to apply δ + 1000 the same model. Assuming enthalpy conservation, the con- α = Lp servation of isotopic ratios, the equilibrium distribution of δVp + 1000 (10) isotopes between the vapor phase (Vp) and liquid phase

Period-I Period-IIPeriod-III a :a :b :c :h

Period-I Period-IIPeriod-III b

2015 2016 2017 2018 2019 2020

Fig. 6 a Changes in the apparent equilibrium temperature (AETS) defned for the reaction involving SO­ 2, ­H2, ­H2S and ­H2O. b Changes in the apparent equilibrium temperature (AETD) defned for the deuterium exchange reaction between H­ 2O and H­ 2. Red histogram and vertical lines have the same meaning as in Fig. 3 Ohba et al. Earth, Planets and Space (2021) 73:81 Page 11 of 17

a b Sm Io Sm Io

MV MV

SZ SZ

SMC M SMC

M DMC M DMC

d

c :a Sm :b Io :c

MV

SZ

M SMC

AMJJASONDJFMAMJ M DMC 20172018 Fig. 7 Magma degassing and transport estimated from the correlation between changes in AETS and eruptions at Shinmoedake volcano (Sm). a Local situation in May 2017. A large fux of magmatic vapor (MV) was supplied to the aquifer beneath Ioyama volcano (Io). No magma was transported to Sm from the shallow magma chamber (SMC). The sealing zone (SZ) surrounding the SMC was open. b Local situation between September 2017 and January 2018. MV emission was prevented by SZ. Simultaneously, magma (M) was transported to Sm, causing a magmatic eruption. c Local situation between February and April 2018. A break in the SZ caused a large fux of MV to be supplied to Io. Magma was also transported to Sm. The fux of magma from the deep magma chamber (DMC) is likely to have increased from February to April 2018. d Changes in the apparent equilibrium temperature (AETS). Red histogram and vertical lines have the same meaning as in Fig. 3

C β = Lp and the amount of Ar, respectively. Equations 10 and 11 CVp (11) describe the equilibrium distribution between Vp and Lp in terms of stable isotopes and Ar, respectively. Using where H, δ, and C represent enthalpy, isotope ratio in Eqs. 7–11, the isotopic ratios and Ar/H2O ratios of Vp δ-notation, and the Ar/H2O molar ratio, respectively. and Lp were calculated. Te numerical values necessary Ar/H2O ratios are considered as a means to validate the for this calculation are listed in Table 2. model. In the above equations, f and g denote the mix- As shown in Fig. 9a, the calculated vapor phase lines at ing fraction of MV and the generating fraction of Vp, 100 °C and 160 °C (Vp-100 and Vp-160) are located near respectively. In general, g is not equal to f; their values the point of the observed fumarole. Furthermore, Y1 is are defned based on the amounts of ­H2O in MV, LW, located between the calculated liquid phase lines at 100 °C Vp, and Lp. Alpha (α) is the isotopic fractionation factor and 160 °C (Lp-100 and Lp-160). Tese agreements sug- between Lp and Vp in terms of D/H and 18O/16O ratios gest that MV and LW are mixed and that the phase-sepa- and beta (β) is the distribution coefcient between Lp rated vapor phase (Vp) corresponds to fumarolic gas. Te and Vp in terms of the Ar/H2O ratio. Equations 7, 8, and temperature of phase separation is estimated to be about 9 describe the conservation of enthalpy, isotopic ratio, 100–160 °C. To calculate the Vp and Lp lines in Fig. 9a, Ohba et al. Earth, Planets and Space (2021) 73:81 Page 12 of 17

Table 2 Parameters for the calculation of the vapor and liquid phases Term Symbol Value Unit

Temp. of MV 900 °C Temp. of LW 15 °C Enthalpy of MV HMV 4391 kJ/kg Enthalpy of LW HLW 64 kJ/kg Enthalpy of Vp ditto At 100 ℃ HVp 2676 kJ/kg At 160 ℃ HVp 2757 kJ/kg Enthalpy of Lp ditto At 100 ℃ HLp 417 kJ/kg At 160 ℃ HLp 677 kJ/kg δ18O of MV δMV 8.0 ‰ − δ18O of LW δLW 8.2 ‰ − δD of MV δMV 15.0 ‰ − δD of LW δLW 53.5 ‰ − Ar/H2O ratio of MV CMV 0 Ar/H O ratio of LW CLW 3.0 ­10–7 2 × 18O/16O fractionation factor α a D/H fractionation factor α a

Ar/H2O distribution coefcient β b a: Horita and Wesolowski (1994), b: Fermandez–Prini et al. (2003) Fig. 8 Schematic illustration of local meteoric water (LW) in the Enthalpy values are cited from the standard steam table (e.g., Japan Society of Mechanical Engineers 1999) shallow aquifer (Aq) beneath Ioyama volcano interacting with magmatic vapor (MV). MV is discharged from the shallow magma chamber (SMH). MV moves up through the sealing zone (SZ), the magmatic vapor reservoir (MVR), and caprock (CR). MV produces volcanic earthquakes (Eq) in the region between MVR and CR. isotope ratios is due to an increase in the mixing ratio of Interactions between MV and LW produce the vapor phase (Vp) and MV. Terefore, the increase in the isotope ratios of H­ 2O the liquid phase (Lp). A portion of both the Vp and Lp are discharged denotes an increased fux of MV (under the assumption at the surface as fumarolic gas (FG) and hot spring water (HS), 2– of a constant fux of LW). respectively. A portion of the ­SO2 in MV is converted to ­SO4 ion and Some fumarolic gases have isotopic ratios that are ­H S gas, which are preferentially distributed to Lp and Vp, respectively 2 much lower than the Vp line (Fig. 9a). When a part of ­H2O in Vp is condensed and lost, the isotopic ratio of the remaining water vapor decreases. Tis direction of “f”, which denotes the mixed fraction of MV, varied within change by condensation has the same direction as the the range of 0.10–0.6 at 100 °C. At 160 °C, “f” varied vector connecting L and V-100 in Fig. 9a, i.e., the iso- within the range of 0.15–0.6. If “f” has a value higher than tope ratios of liquid and vapor equilibrated at 100 °C. these ranges, Lp is not formed, i.e., only Vp forms. If “f” At the moment of condensation, the isotope ratio of has a value lower than these ranges, Vp is not formed; i.e., the vapor phase leaves the Vp line (Vp-100 or 160) and only Lp is formed. When “f” is fxed at its maximum, the moves toward the lower left. Some fumarolic gases with isotope ratios of Vp and Lp are located at the right end isotope ratios lower than the Vp line can be explained of the Vp and Lp lines, respectively. When “f” is fxed at by the partial condensation of water vapor (Ohba et al. a minimum value, the isotope ratios of Vp and Lp are 1997). Isotope ratios lower than the Vp line can be also located at the left end of the Vp and Lp lines, respectively. explained by the addition of water vapor generated Te highest δ18O values were observed at fumarole “b” from LW (Ohwada et al. 2003). In natural fumaroles, 18 the above two efects may occur simultaneously. In in March 2018. Te “f” values producing the same δ O 18 on the Vp line were 0.49 and 0.48, at 100 °C and 160 °C, Period-III, the low δD and δ O values can, therefore, respectively. Te fumarolic δD and δ18O increased rap- be explained by the condensation of water vapor and or idly during Period- I, and high values were maintained the infuence of LW. throughout Period- II (Fig. 5a, b). According to the above In Fig. 9b, the Ar/H2O molar ratio of fumarolic gas is model for the formation of Vp and Lp, the increase in Vp calibrated on the horizontal axis via conversion to the Ohba et al. Earth, Planets and Space (2021) 73:81 Page 13 of 17

meteoric water (vLW in Fig. 9b). Te Vp curve overlaps with the isotopic ratios of fumarolic gases, especially in the low mF(Ar) and high δ18O region, indicating the validity of the Vp and Lp formation model. In the above model, mixing between LW and MV occurs during a single stage, but, as shown in Fig. 8, mixing can occur at both the HR located under the cap- rock and in the aquifer. It is not possible to evaluate the relative contributions of these two mixing events based solely on the isotope ratio of fumarolic gases.

Correlation between concentrations of gaseous species Examining the correlation among the concentrations of species in fumarolic gases is useful for estimating the origin of each species, as well as for estimating the state of the hydrothermal system. In the He–N2–Ar ternary system (Fig. 10a), it has been reported that a mixing relationship holds between two end members, i.e., mag- matic and atmospheric components (Kita et al. 1993; Giggenbach 1997; Taran 2011; Ohba et al. 2019a, b). In this study, we also found that the composition of fuma- rolic gas was distributed between these two endmembers (Fig. 10a). Te magmatic endmember (broken circle) has a fxed ­N2/He ratio and is poor in Ar. Te atmospheric 18 Fig. 9 a δD and δ O of ­H2O in fumarolic gas with the calculated endmember is composed of air and the component dis- vapor phase (Vp) and liquid phase (Lp). Vp-100 and Vp-160 denote solved in water saturated with air (ASW). Vp of 100 °C and 160 °C, respectively. Lp-100 and Lp-160 denote Lp In the He–N –CO ternary system (Fig. 10b), the fuma- at the temperature of 100 °C and 160 °C, respectively. The broken 2 2 lines annotated as Cd indicate the isotopic ratio of remaining water rolic gases are concentrated almost into a single group vapor after partial condensation. V-100, 150 and 200 are the isotopic (broken circle in Fig. 10b). Since helium is a magmatic ratios of water vapor equilibrated with liquid L at 100 °C, 150 °C, and component, the grouping in the He–N2–CO2 ternary 200 °C, respectively. b Magnifed Ar fraction, mF(Ar), and δ18O of ­H O 2 system suggests that CO­ 2 is also a magmatic component. in fumarolic gases with Vp-100 and Vp-160. vLW indicates the vapor Some points are distributed in the direction of the angle of phase generated from local meteoric water (LW) ­N2 away from the group. Teir distribution indicates con- tamination by atmospheric components. In the He–N2–CH4 ternary system, fumarolic gases can be divided into two groups, although the distance magnifed Ar fraction in the Ar–H O binary system, 2 between these groups is small (Fig. 10c). Some points which is defned as follows: distributed near the ­N2 corner indicate the contamina- 106 Ar tion by atmospheric components. According to Fig. 11a, H2O mF(Ar) =  the ­CH4/He ratio is low during the latter half of Period-II, 6 Ar + (12) 10 H O 1 when eruptions occurred at the Shinmoedake and Ioy- 2  ama volcanoes. At Periods-I and -III, the CH­ 4/He ratio As shown in Fig. 9b, even for the relationship between is generally high. Te distribution of fumarolic gases in 18 mF(Ar) and δ O, the calculated Vp curves at 100 °C Fig. 10c suggests that CH­ 4 is essentially a magmatic com- or 160 °C (i.e., Vp-100 or Vp-160) are closely linked to ponent (broken circle). In addition to magmatic ­CH4, fumarolic gases. Te increased atmospheric contami- non-magmatic ­CH4 component was observed; this is nation drives the points to the right in the diagram. deemed to be contaminative. For non-magmatic CH­ 4, Fumarolic gases with δ18O higher than that of the Vp- a thermogenic origin is possible, i.e., generation via the 160 curve are interpreted to have sufered the contami- decomposition of organic matter in the crust. nation by an atmospheric component. Fumarolic gases Based on the SO­ 2–CO2–H2S ternary system, the type located beneath the Vp-100 curve are considered to of the hydrothermal system can be estimated. According have been afected by the partial condensation of water to Stix and de Moor (2018), a hydrothermal system with vapor and the addition of water vapor originating in deep-seated degassing magma can be distinguished from Ohba et al. Earth, Planets and Space (2021) 73:81 Page 14 of 17

a hydrothermal system with a shallowly intruded degas- gases have ­H2S/SO2 molar ratios greater than 4; such sing magma by the SO­ 2–CO2–H2S ternary composition gases are located in the region considered to be hydrother- of fumarolic gas. In addition to this distinguishment, the mally dominated (HD; Fig. 10d). As shown in Fig. 11b, all ­SO2–CO2–H2S ternary system enables the evaluation of fumarolic gases measured during Period-I are classifed as interaction between MV and shallow aquifer. Te observed HD-type. Some fumarolic gases with low ­H2S/SO2 ratios ­CO2/St ratio (where St refers to the molar summation of are classifed as hydrothermal magmatic types. Further- ­SO2 and H­ 2S) of fumarolic gases are distributed in a large more, the hydrothermal magmatic types can be divided range (between 1.4 and 22) (Fig. 10d). Most fumarolic into deep hydrothermal magmatic (DHM) and shallow

Fig. 10 a Fumarolic gas compositions in the He–N2–Ar ternary system; broken circle indicates the expected magmatic end member. ASW is the composition of air dissolved in water. Broken lines indicate the mixing between the magmatic and atmospheric end members. b Fumarolic gas

compositions in the He–N2–CO2 ternary system; broken circle indicates the expected magmatic end member. Broken lines indicate the mixing between the magmatic and atmospheric end members. c Fumarolic gas composition in the He–N2–CH4 ternary system; broken circle indicates the expected magmatic end member. d Fumarolic gas composition in the ­SO2–CO2–H2S ternary system classifed as a hydrothermal system (Stix and de Moor 2018). SLS, HD, DHM, SHM, DM, SM and SR indicate the types of volcanic gas with the following geneses: “S loss scrubbing”, “hydrothermally dominated”, “deep hydrothermal-magmatic”, “shallow hydrothermal-magmatic”, “deep magmatic”, “shallow magmatic” and “sulfur remobilization”,

respectively. PF indicates the estimated parental magmatic fuid in the Kirishima volcanic area (Ohba et al. 1997). The ­CO2/SO2 and ­H2S/SO2 molar ratios are calibrated on the left and bottom sides, respectively. The ­CO2/St isomolar ratio is indicated by broken lines where St is the summation of ­SO2 and ­H2S Ohba et al. Earth, Planets and Space (2021) 73:81 Page 15 of 17

+ = − + + + hydrothermal magmatic (SHM) based on their ­CO2/SO2 4SO2 4H2O 3HSO4 H2S 3H (13) ratios. Fumarolic gases of the DHM type were observed during Periods-II and -III (yellow circles in Fig. 11b), Trough the above reaction, some ­SO2 in MV is 2– whereas SHM gases were observed only observed during absorbed by LW in the form of the SO­ 4 ion. Indeed, Period-II (red circles in Fig. 11b). Te fumarolic gas “a” near the fumaroles of Ioyama volcano, hot spring waters 2– observed in May 2017 and March 2018 is classifed into rich in ­SO4 , such as K3 and Y1, were discharged the SHM feld, with a ­CO2/SO2 ratio less than 4. Te AETS (Tajima et al 2020). In addition, MV can be depleted in of these two fumarolic gases were calculated as 752 °C sulfur species through other reactions (e.g., Takano et al. and 611 °C (Fig. 6a), suggesting an increased MV fux in 2004), such as both May 2017 and March 2018. Terefore, the discharge SO2 + 3H2S = 3S + 2H2O of SHM-type fumarolic gases is regarded as an index for (14) enhanced volcanic activity at Ioyama volcano. Te absorption of ­SO2 by LW and the formation of PF in Fig. 10d shows the composition of the parental native sulfur can explain the depletion of St in fumarolic fuid released from the magma of Kirishima volcano. gases relative to PF (Fig. 10d). Unlike SO­ 2, ­CO2 in MV is Te ­CO2/St of PF was estimated at 0.25–0.44 (Ohba et al not readily dissolved in Lp; thus, most of the constitu- 1997). In Fig. 10d, the sulfur component in PF is con- ent ­CO2 is distributed to Vp. Consequently, the ­CO2/ sidered to be ­SO2. Te ­CO2/St ratio of fumarolic gases SO2 ratio of fumarolic gases refects the degree of SO­ 2 (1.4–22) is higher than the ­CO2/St ratio of PF. If the removal. essential ­CO2/St ratio of MV is similar to the CO­ 2/St ratio of PF, it means that MV loses sulfur-bearing gases during travel to the surface. As suggested in the previ- Conclusions ous section, fumarolic gases was comprised of the vapor Tis study has reported direct sampling and analysis phase formed during the interaction of MV and LW. Te of fumarolic gas at Ioyama volcano between Decem- following hydrolysis reaction is expected to occur during ber 2015 and July 2020. Notable changes in the chemi- this interaction (Kusakabe et al. 2000): cal composition related to volcanic activity included increased concentrations of ­SO2 and ­H2 in May 2017

Fig. 11 a Changes in the CH­ 4/He molar ratio of fumarolic gas. b Changes in the ­SO2/H2S molar ratio of fumarolic gas. The y-axis values of 0.5 and 4 for ­H2S/SO2 ratio were indicated by the two horizontal broken lines. c Changes in the ­SO2/CO2 molar ratio of fumarolic gas. Fumarolic gases surrounded by red and yellow circles are classifed as being of shallow hydrothermal magmatic (SHM) and deep hydrothermal magmatic (DHM) origins, respectively. Red histogram and vertical lines are the same as in Fig. 3 Ohba et al. Earth, Planets and Space (2021) 73:81 Page 16 of 17

and March 2018. Sampling in March 2018 took place Magnetotelluric; R-gas: Residual gas; SMOW: Standard mean ocean water; St: Molar summation of SO­ 2 and ­H2S; vLW: Vapor generated from LW; VEI: Volcanic immediately before the eruption at Ioyama volcano in Explosivity Index; Vp: Vapor phase. April 2018. Based on the isotope ratio of H­ 2O in fuma- rolic gas, the observation period was divided into three Acknowledgements We would like to thank Dr. M de Moor and one anonymous reviewer for their periods: Period-1 (December 2015 to December 2016), constructive comments to the manuscript, by which the content of this Period-II (January 2017 to May 2018), and Period-III paper was signifcantly improved. We also thank Dr. T Fischer for his editorial handling of the manuscript. (June 2018 to July 2020). Using their ­SO2–CO2–H2S ter- nary composition, most collected fumarolic gases were Authors’ contributions classifed as HD. In particular, for Period-I, all fumarolic TO drafted the manuscript. YM sampled fumarolic gases and analyzed them. gases were classifed as HD. During Period-II, SHM and UT, MI and RS analyzed the fumarolic gas samples. All the authors read and approved the fnal manuscript. DHM were observed in addition to HD gas. Period-III featured DHM- and HD-type gases. Te fumarolic gas Funding released in March 2018 was classifed as SHM-type, This work was supported by the Japanese Ministry of Education, Culture, Sorts, Science and Technology under grant of the Integrated Program for Next Gen- and furthermore, its AETS was high. Terefore, the eration Volcano Research and Human Resource Development in 2016–2020. shift of composition to the SHM-type as well as rise of This study was partially funded by Earthquake Res., Inst., the University of AETS would be good precursors of phreatic eruptions, Tokyo, Joint Research program 2020-KOBO11. This work was also partially supported by JSPS KAKENHI Grant Number because they are related to a large fux of MV. JP16777378, JP18058728, JP19209043, and JP20298572. Correlating changes in AETS with the occurrence of eruptions at Shinmoedake volcano, the occurrence of Availability of data and materials The csv fles for Table 1 will be stored in repositories. magma sealing and transport was estimated as follows. In May 2017 and in March 2018, the AETS of fumarolic Declarations gas increased, suggesting an increased MV fux. Between September 2017 and January 2018, AETS decreased, sug- Ethics approval and consent to participate gesting a reduced MV fux, probably due the magma seal- Not applicable. ing in SMC. Simultaneously with this decrease in AETS, Consent for publication magmatic eruptions occurred at Shinmoedake volcano, Not applicable. suggesting magma transport from the chamber to Shin- Competing interests moedake volcano. It is noticeable that magma sealing and The authors declare that they have no competing interests. displacement of magma happened simultaneously. Interactions between MV and LW in aquifers gener- Author details 1 Tokai University, 4‑1‑1 Kitakaname Hiratsuka, Kanagawa 259‑1291, Japan. ated a vapor phase (Vp) and coexisting liquid phase (Lp). 2 Meteorological Research Institute, 1‑1 Nagamine Tsukuba, Ibaraki 305‑0052, A portion of Vp was released as fumarolic gas, whereas a Japan. 3 Graduate School of Environmental Studies, Nagoya University, portion of Lp was discharged as acidic hot spring waters Furo‑cho, Chikusa‑ku, Nagoya 464‑8601, Japan. 2– enriched in the ­SO4 ion. Te temperatures of Vp and Received: 12 January 2021 Accepted: 24 March 2021 Lp were estimated to be 100–160 °C. Based on the iso- topic ratio of H­ 2O, the maximum mixing fraction of MV was estimated at 0.49. Comparing the CO­ 2/SO2 ratios of fumarolic gas with an estimated essential CO­ 2/SO2 ratio of References MV, the former was found to be signifcantly higher than Aizawa K, Koyama T, Hase H, Uyeshima M, Kanda W, Utsugi M, Yoshimura R, Yamaya Y, Hashimoto T, Yamazaki K, Komatsu S, Watanabe A, Miyakawa the latter. Te following phenomena are considered likely K, Ogawa Y (2014) Three-dimensional resistivity structure and magma cause for this observation. Although much ­SO2 in MV was plumbing system of the Kirishima Volcanoes as inferred from broadband 2– magnetotelluric data. J Geophys Res Solid Earth 119:198–215. https://​doi.​ converted to ­SO4 and ­H2S by interaction with LW, ­CO2 org/​10.​1002/​2013J​B0106​82 did not undergo a similar reaction and was distributed to Barberi F, Bertagnini A, Landi P, Principe C (1992) A review on phreatic erup- the vapor phase. Te low CO­ 2/SO2 ratio of fumarolic gas tions and their precursors. J Volcanol Geotherm Res 52:231–324 immediately prior to the eruption at Ioyama volcano sug- Chiodini G, Caliro S, Cardellini C, Granieri D, Avino R, Baldini A, Donnini M, Minopoli C (2010) Long-term variations of the Campi Flegrei, Italy, gests limited interaction between MV and LW, and thus, volcanic system as revealed by the monitoring of hydrothermal activity. J fumarolic gas sustained the magmatic features observed. Geophys Res 115:B03205. https://​doi.​org/​10.​1029/​2008J​B0062​58 Fernández-Prini R, Alvarez JL, Harvey AH (2003) Henry’s constants and vapor- liquid distribution constants for gaseous solutes in H­ 2O and D­ 2O at high Abbreviations temperatures. J Phys Chem Ref Data 32:903–916 AETS: Apparent equilibrium temperature defned for the reaction including Fischer TP, Ramírez C, Mora-Amador RA, Hilton DR, Barnes JD, Sharp ZD, Le sulfur species; AETD: Apparent equilibrium temperature defned for the deute- Brun M, de Moor JM, Barry PH, Füri E, Shaw AM (2015) Temporal varia- rium exchange reaction between ­H2O and H­ 2; AMW: Andesitic magma water; tions in fumarole gas chemistry at Poás volcano, Costa Rica. J Volcanol ASW: Atmospheric components saturated in water; JMA: Japan Meteorological Geotherm Res 294:56–70 Agency; Lp: Liquid phase; LW: Local meteoric water; MV: Magmatic vapor; MT: Ohba et al. Earth, Planets and Space (2021) 73:81 Page 17 of 17

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