Passive degassing of magmatic volatiles from Iwate volcano, NE Japan, based on three-dimensional measurement of helium isotopes in groundwater

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2011jb008532, 2012 Passive degassing of magmatic volatiles from Iwate volcano, NE Japan, based on three-dimensional measurement of helium isotopes in groundwater Michiko Ohwada, 1 Kohei Kazahaya, 1 Jun ich Itoh, 1 Noritoshi Morikawa, 1 Masaaki Takahashi, 1 Hiroshi A. Takahashi, 1 Akihiko Inamura, 1 Masaya Yasuhara, 1 and Hitoshi Tsukamoto 1 Received 17 May 2011; revised 24 November 2011; accepted 28 November 2011; published 4 February [1] Magmatic volatile supply to the groundwater around Iwate volcano, which is an active volcano with weak fumarolic activity, is investigated using chemical and isotopic compositions of 180 groundwater samples from various depths. Magmatic 3 He flux is estimated by an applied method using helium isotopes. The three-dimensional distribution of 3 He flux can be obtained as well as that of the concentrations. High 3 He flux with high concentration is found in shallow groundwaters at N and NE flanks, and in shallow and deep groundwater around the fault zone. Low 3 He flux with high concentration in deep groundwater suggests the slow and long-term accumulation of magmatic volatiles. High 3 He fluxes and concentrations around the fault zone indicate that the fault acts as an ascending path of magmatic volatiles. Distribution of flux indicates the three-dimensional anisotropy of magmatic volatile supply, which is due to the groundwater flow variation restricted by the structure of the volcanic body. Supply rates of other magmatic volatiles can be also obtained using 3 He flux, and the total supply rates are estimated to be ( 3 He), (magmatic C), (Cl), and (S) mol/y. Comparison of supply rate to groundwater with volcanic gas emission rate reveals that the magmatic volatiles are largely supplied to the groundwater. However, the total emission combining the groundwater and volcanic gas is not prominently large compared with other active volcanoes. We infer that the method combining the magmatic volatile concentration and flux to groundwater is available for the evaluation of passive activity of volcano. Citation: Ohwada, M., K. Kazahaya, J. Itoh, N. Morikawa, M. Takahashi, H. A. Takahashi, A. Inamura, M. Yasuhara, and H. Tsukamoto (2012), Passive degassing of magmatic volatiles from Iwate volcano, NE Japan, based on three-dimensional measurement of helium isotopes in groundwater, J. Geophys. Res., 117,, doi: /2011jb Introduction [2] Magmatic volatiles degassed from magma are mainly released to the surface of the Earth through volcanic gas. The evaluation of magmatic activity at a degassing volcano is performed by measuring the SO 2 flux (e.g., Miyakejima volcano [Kazahaya et al., 2004], Sakurajima volcano [Mori et al., 2004]). However, there are many active volcanoes in which the volcanic gas emissions are few or passive. In this case, to evaluate the magmatic activity, we should investigate not only using the emission rate but also evaluating other paths of magmatic volatile released from the volcano. The groundwater flow system in volcano plays a role as a pathway of released magmatic volatiles, since the magmas and volcanic gases come into contact with the groundwater 1 Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan. Copyright 2012 by the American Geophysical Union /12/2011JB aquifers during their ascent, and tends to be restricted (or redirected) by geologic and tectonic structures of volcano [e.g., Allard et al., 1997; Kazahaya, 2000; Brusca et al., 2001; Federico et al., 2002; Morikawa et al., 2008a]. Moreover, whether or not an eruption will become explosive may be dependent on whether the degassed magmatic volatiles can escape through the wall of the conduit during the upwelling process of magma [Woods and Koyaguchi, 1994]. Therefore, quantitative estimation of the flux of magmatic volatiles such as C, S, Cl and noble gas components into the groundwater is also important for evaluating the temporal and spatial change of the volcanic activity. [3] Helium isotopes are useful tracers which can precisely track the interaction of magmatic volatiles with groundwater at volcano sites, since helium has the highest mobility among the noble gases and some of the helium derived from the juvenile mantle still exists. Studies of the distribution of magmatic He to the groundwater around volcano have indicated a significant contribution of magmatic He may occur into the groundwater system [e.g., Allard et al., 1997; 1of16

2 Federico et al., 2002; Saar et al., 2005; Morikawa et al., 2008a]. Helium concentration, on the other hand, is useful for estimating aspects of the groundwater flow regime and residence times because of continuously accumulating radiogenic 4 He produced within the aquifer and/or external flux from deeper regions [e.g., Andrews and Lee, 1979; Torgersen and Clarke, 1985]. The flux of each helium component can be obtained by various approaches such as using helium transport simulations [e.g., Stute et al., 1992; Castro et al., 1998; Castro, 2004; Patriarche et al., 2004], using the relationship between He concentration and residence time [e.g., Takahata and Sano, 2000; Morikawa et al., 2005, 2008a]. An estimation of magmatic volatile flux into groundwater, including 3 He and CO 2, was conducted on various volcanoes [Allard et al., 1997; Sorey et al., 1998; Morikawa et al., 2008a]. However, there have been few studies on the distribution of the magmatic 3 He flux into the groundwater covering the whole area of a volcano. [4] Iwate volcano is an active volcano located in northeastern Japan. The latest magmatic activity which caused the intrusion of a magmatic fluid occurred in Fumarolic activity and the emission of volcanic gas have been activated [Ohba et al., 2011], however, they are still less active when compared to other active volcanoes. Weak fumarolic activity in Iwate volcano suggests the existence of missing magmatic volatiles which are not emitted as volcanic gas but have possibly escaped through the groundwater. In Iwate volcano, shallow and deep groundwater systems are both developing, and studies on the shallow groundwater flow systems based on the chemical and isotopic features of springs around the volcano have already been reported [Kazahaya and Yasuhara, 1999; Kazahaya, 2000; Sato et al., 2000]. Many shallow and deep groundwaters exist over the whole area of the volcano as well as at the springs, and we can therefore obtain many groundwater samples at various depths. [5] In this study, the magmatic volatile supply to the groundwater around Iwate volcano is investigated using the chemical and isotopic compositions of 180 groundwater samples which were collected from various depths. First, we calculate the magmatic 3 He flux into groundwater by transforming the formula of helium accumulation of Morikawa et al. [2005]. Then, the quantification of magmatic volatile degassing from magma is attempted by the combination of the magmatic volatiles/ 3 He ratio and magmatic 3 He flux into the groundwater. The magmatic volatile supply to the groundwater system in Iwate volcano is revealed threedimensionally, which provides us with more precise and quantitative discussion on passive degassing activity. 2. Geological and Hydrological Settings [6] Iwate volcano is a typical polygenetic volcano located in northeastern Japan (Figure 1). The activity of Iwate volcano started at about 0.3 Ma [Nakagawa, 1987]. Sector collapse and subsequent reconstruction of the volcano were repeated. Iwate volcano consists of two volcanic bodies: the western part (Nishi-Iwate) and the eastern part (Higashi- Iwate). Nishi-Iwate has a 2.5 km by 1.5 km wide caldera with a fumarolic and geothermal area, named Oojigokudani, and has some other fumarolic areas (Figure 1). An erosional valley develops on older south flank. Higashi- Iwate consists of a younger basaltic stratocone named Yakushidake (2038 m) with a less-active fumarolic and geothermal area (Figure 1). Nishi-Iwate and Higashi-Iwate have an individual magma system with different depths; the magma of Nishi-Iwate is basalt to andesite and that of Higashi-Iwate is basalt [Sugawara and Matsubaya, 1997]. [7] In historic time, magmatic eruptions at Yakushidake occurred in 1686 and 1732, and phreatic eruption occurred at Oojigokudani in Most recent volcanic activation without magma extrusion was observed from This event is characterized by remarkable seismic activity and by crustal deformation caused by the intrusion of magmatic fluid and dilation of a pressure source [Sato and Hamaguchi, 1999; Ueki et al., 1999; Miura et al., 2000; Tanaka et al., 2002; Nakamichi et al., 2003]. The latest intrusion of magmatic fluid moderately activated the fumarolic and geothermal activity at the summit and the caldera area. [8] A simplified geological map and cross-section around Iwate volcano are shown in Figure 1. In this study, we roughly classified the geology into three units; the Quaternary volcanic rocks forming Iwate volcanic body, the Tertiary volcanic and sedimentary rocks, and the pre-tertiary basement rocks. The Quaternary volcanic rocks are composed of basaltic to andesitic lava and pyroclastic rocks, which are grouped into Iwate volcano (Higashi-Iwate and Nishi-Iwate), Amihari and other volcanic products. The youngest stratovolcano of Higashi-Iwate grows on the last collapse caldera, whose surface descends down to the north and east. The unit of Nishi-Iwate volcanic products overlies the unit of Amihari volcanic products (Figure 1, cross section A-A ). The Tertiary volcanic and sedimentary rocks are composed of silicic pyroclastic rocks and sediments, and the pre-tertiary basement rocks are composed of granite and sediments. [9] Active faults develop around Iwate volcano with a N-S strike (Figure 1). The faults on the west side are called the western marginal fault zone of the Shizukuishi basin (hereafter referred to as the Shizukuishi fault zone ), which lies through the volcanic body. [10] Shallow and deep groundwater flow systems develop around Iwate volcano (Figures 2 and 3). The springs on the N and NE flanks are absent at elevations higher than 500 m, but discharge with a great flow rate of m 3 /d at m elevation (Figure 2). Two large springs with a high flow rate of m 3 /d, named the Kanazawa- Shimizu (No. 10) and the Oide-yusui (No. 44), exist in the N and the NE flanks, respectively (Figures 1 and 2). In contrast, many springs were found at the elevation range of m in the S and SE flanks (Figure 2). The flow rates of springs at low elevation were relatively low at m 3 /d. The recharge elevations of these springs, which were determined by Kazahaya and Yasuhara [1999] using ddandd 18 O values, are m in the N and NE flanks and m in the S and SE flanks, respectively (Figure 2). The groundwater recharged at the summit area preferentially flows down in the N and NE direction and forms a large shallow groundwater flow. In contrast, the shallow groundwater flow system on the S flank is a relatively small. Beneath the NE flank of Iwate volcano, the distinct negative self-potential (SP) anomaly and the resistive layer representing the unsaturated zone are observed [Aizawa, 2008; Aizawa et al., 2009]. These results suggest the existence of a groundwater-saturated zone 2of16

3 Figure 1. Simplified geological maps around Iwate volcano, NE Japan, showing active faults (solid line) and concealed faults (dashed line). (a) Geological map (revised from Itoh and Doi [2005] and Geological Survey of Japan [2010]), (b) geological profiles at cross-section line A-A [Itoh and Matsumoto, 2006], and (c) topographic map around summit and caldera area of Iwate volcano. Meshed area shows a fumarolic area. beneath the unsaturated zone and are consistent with the geochemical and geological evidences mentioned above. 3. Samples and Analytical Methods [11] In this study, 180 water samples were collected from springs and boreholes of various depths in and around Iwate volcano (Figure 3 and Table S1 of the auxiliary material). 1 Volcanic hot springs with temperatures higher than 80 C are located within the caldera. The groundwater samples from boreholes were classified into shallow groundwater (depth 200 m) and deep groundwater (depth > 200 m). The aquifer of shallow groundwater mostly exists in the geologic unit of the Quaternary volcanic rocks, whereas the aquifer of deep groundwater mainly exists in the unit of the Tertiary 1 Auxiliary materials are available in the HTML. doi: / 2011JB of16 volcanic and sedimentary rocks and of the pre-tertiary basement rocks. The study area was sectioned to 4 areas, N (north flank), NE (northeast flank), SE (southeast flank) and S (southwest flank), taking both volcanological division and hydrological division such as water divide given by Kazahaya [2000] into consideration (Figure 3). [12] Water samples were collected in 500 ml plastic bottles for chemical analysis. Concentrations of Cl and SO 4 2 of water samples were determined with an ion chromatograph (DXi-500; Dionex). The alkalinity (HCO 3 ) was determined by titrating with acid up to a ph of 4.8. Water samples for dissolved inorganic carbon (DIC) isotope measurements were collected in 100 ml gas-tight plastic bottles to which diluted NaOH solution was added in the field to prevent CO 2 gas loss during its storage. The water samples for the measurement of d 13 C value of DIC were injected into a glass vial which was filled with helium gas and contained phosphoric acid. The generated CO 2 gas in the vial was

4 of the auxiliary material. The 3He/4He and 4He/20Ne ratios of the shallow groundwater and the springs ranged from 0.3 to 3.5 Ra (1 Ra = ) and 0.25 to 1.48, and those of the deep groundwater and volcanic hot springs ranged from 0.1 to 3.3 Ra and 0.51 to 72.2, respectively. Generally, groundwater acquires atmospheric noble gas in recharge areas. Especially, Ne in groundwater is mostly derived from the atmosphere. However, some groundwaters have lower 20 Ne concentrations than the solubility equilibrium value. When the bubbles go through groundwater, the noble gas in the water easily migrates into the bubbles resulting in Ne depletion in the water phase. In this case, the originally dissolved amounts of He including both the solubility equilibrium and the subsurface component will be depleted by suffering phase separation. Original He concentrations are calculated from the depletion factor of 20Ne, which is the ratio of the observed 20Ne and solubility 20Ne concentrations, assuming that the He removal also occurred under equilibrium. [15] For the discussion of magmatic volatiles, it is necessary to eliminate the atmospheric components from the Figure 2. Recharge area for each spring and river of Iwate volcano determined from the isotopic recharge elevation (H) and flow rate of each spring and river (all data are taken from Kazahaya [2000]). Squares show the recharge areas which are required to fill each flow rate. Solid line indicates the water divide given by Kazahaya [2000]. introduced to a stable isotopic mass spectrometer with a continuous flow GC system installed at the Geological Survey of Japan, AIST (Delta-Plus and Gas Bench II: Thermo-Finnigan) and the d13c values were measured. The analytical accuracy was 0.1 for d13c. [13] For the analysis of dissolved noble gas compositions, water samples were collected in a copper tube. The noble gas concentration in groundwater is low compared with that in the atmosphere, and thus it is easily contaminated by the atmosphere. For example, in cases where the groundwater was stored at open space such as a tank, the noble gas in the groundwater is equilibrated with the atmosphere and the original noble gas composition would be lost. Therefore, water sample collection for noble gas analysis was performed at a site where contact with the atmosphere would be small. Helium and neon concentrations and the helium isotope ratios were measured with a static noble gas mass spectrometer, model MM5400 (Micromass), which was installed at the Geological Survey of Japan, AIST. Technical details of the extraction of dissolved noble gases and the mass spectrometry, including the purification procedures, are described by Morikawa et al. [2008b]. The reproducibility of the 3 He/4He ratio and the helium and neon concentrations obtained from the replicated measurement of air saturated water (ASW) were about 2%, 3% and 3%, respectively. 4. Helium Isotopic Compositions in the Groundwater 4 [14] The measured 3He/4He and 4He/20Ne ratios and 3He, He and 20Ne concentrations in water are listed in Table S1 Figure 3. Topographic map of Iwate volcano, showing the locations of sampling sites. The numbers correspond to the sample numbers give in Table S1 of the auxiliary material and Table 1. Study area is sectioned to four geographic sectors (N, NE, SE, and S) considering both the sectors as volcanologically divided into two volcanoes (Nishi-Iwate and Higashi-Iwate) and hydrologically divided into areas by a water divide given in Figure 2. Solid and dashed lines show the active faults and concealed faults, respectively. 4 of 16

5 observed results. Correction for atmospheric contamination was basically performed according to the procedure of Morikawa et al. [2008a]. Observed helium and neon concentrations in groundwater can be expressed as follows: 3 He obs ¼ 3 He asw þ 3 He ea þ 3 He mag þ 3 He rad þ 3 He tri ð1þ 4 He obs ¼ 4 He asw þ 4 He ea þ 4 He mag þ 4 He rad ð2þ 20 Ne obs ¼ 20 Ne asw þ 20 Ne ea ð3þ where the subscripts obs, asw, ea, mag, rad and tri mean observed, solubility equilibrium, excess air, magmatic, radiogenic (including crustal component) and tritiogenic, respectively. If both magmatic and radiogenic Ne component are assumed to be negligible (although these were not exactly examined due to the lack of Ne isotopic composition), the excess air component of 4 He ( 4 He ea ) can be obtained using 20 Ne concentration as following equation: 4 He ea ¼ 20 Ne obs 20 Ne asw 4 He= 20 Ne ð4þ where 20 Ne obs and 20 Ne asw are the observed 20 Ne concentration and the solubility 20 Ne value, respectively. ( 4 He/ 20 Ne) atm is the 4 He/ 20 Ne ratio in the atmosphere (0.318) [Ozima and Podosek, 2002]. The solubility equilibrium value of 20 Ne at 10 C, mean annual temperature of studied area, is mmol/kg from Weiss [1971]. By subtracting the excess air and atmospheric He component, we can obtain the subsurface 4 He concentration ( 4 He sub = 4 He mag + 4 He rad ). The 3 He sub concentration can also be calculated in the same way. In the case of the sample which carried out the helium correction because the Ne is depleted, it assumes that the excess air is absent, therefore only the value of solubility equilibrium is subtracted. The 3 He/ 4 He ratio of subsurface origins which subtracted the excess air and solubility equilibrium He component is expressed as follows: 3 He= 4 He ¼ 3 He mag þ 3 He rad = 4 He mag þ 4 He rad ð5þ sub and is listed in Table 1. Both magmatic 3 He ( 3 He mag ) and radiogenic 4 He ( 4 He rad ) concentrations were calculated by multiplying relative proportions of helium and obtained the 3 He sub and 4 He sub concentrations. The relative proportions of magmatic helium (M) and radiogenic helium (R) can be calculated by the following isotopic mass balance equations: 3 He= 4 He ¼ 3 He= 4 He M þ 3 He= 4 He R ð6þ sub mag rad M þ R ¼ 1 We applied the values of ( 3 He/ 4 He) mag for 7.9 Ra [Graham, 2002] and ( 3 He/ 4 He) rad for Ra, which was calculated from the data of the chemical composition of the total crust as reported by Rudnick and Gao [2003]. The radiogenic component will separate into two more components, in situ produced and a crustal component [e.g., Stute et al., 1992], but we did not separate these in this study as discussed later. atm ð7þ The calculated 3 He mag and 4 He rad concentrations are shown in Table 1. In these calculations, it was assumed that the amount of tritiogenic 3 He was negligible. If all tritium in groundwater with 5.5 T.U. (natural production level) [Kaufman and Libby, 1954] decays, tritiogenic 3 He of mmol/kg is generated. However, in the case of significantly low 3 He concentration, the influence of tritiogenic 3 He cannot be ignored. Therefore, the low 3 He data lower than mmol/kg were excluded from the later discussions. Moreover, the data with 4 He/ 20 Ne ratios lower than 0.3 were also excluded, because the gases are composed of mostly atmospheric noble gas and the error in the calculation of the correction for the atmospheric addition would become too high. 5. Magmatic 3 He Flux Into the Groundwater [16] For the quantitative discussion on the magmatic volatile supply from Iwate volcano, the estimation of the magmatic volatile flux is considered one of the useful methods. The magmatic 3 He supply rate into the groundwater (F( 3 He mag ), cm 3 STP/y) can be denoted by following equation [Morikawa et al., 2008a]: F 3 He mag ¼ C 3 He mag Fo ¼ C 3 He mag pv=tr ¼ C 3 He mag p S h=tr ð8þ where the parameters in equation (8) are summarized in Table 2. The residence time (T r ) of groundwater is obtained by a simple one-box model calculation using the helium isotopic compositions [Morikawa et al., 2005]: T r ¼ pv=f o ¼ C 4 He o 1 Ro =R mag p rw = P 4 He þ F 4 He =h ð9þ The notations for the parameters in equation (9) are also listed in Table 2. In equation (9), two sources of radiogenic 4 He accumulated into the groundwater are defined, the in situ production one (P( 4 He): in situ production flux of 4 He) and the external one from deeper regions (F( 4 He): crustal 4 He flux). The F( 4 He) used cm 3 STP/cm 2 /y [Sano, 1986], whose value was obtained for the Kanto Plain, eastern Japan, as the representative value in the subduction area, because the crustal 4 He flux around Iwate volcano is unknown. When the value of the thickness of the reservoir (h) ranged from 50 m to 400 m, the F( 4 He)/h become to cm 3 STP/cm 3 /y. These values are far higher than P( 4 He) which are calculated cm 3 STP/cm 3 /y (Quaternary aquifer) and to cm 3 STP/cm 3 /y (pre-tertiary aquifer). Thus a denominator of equation (9) can be replaced by F( 4 He)/h. Therefore, the 3 He mag flux (cm 3 STP/cm 2 /y) can be obtained by combining equation (8) with equation (9): F 3 He mag =S ¼ C 3 He mag =C 4 He o 1= rw 1 R o =R mag F 4 He ð10þ The flux value can be obtained by giving the He isotope ratio (R o = ( 3 He/ 4 He) sub ), 4 He concentration (C( 4 He o ) = C( 4 He mag )+C( 4 He rad )) and magmatic 3 He concentration (C( 3 He mag )) without the value of porosity and thickness 5of16

6 Table 1. Calculated Helium Isotopes, Magmatic Carbon, and Magmatic Volatile Fluxes in the Springs, Volcanic Hot Springs, Shallow and Deep Groundwaters in Iwate Volcano, NE Japan a Sample Name Category Geology 3 He/ 4 He sub (R/Ra) 3 He mag (10 12 ) 4 He rad (10 6 ) Ct mag Ct mag / 3 He 3 He mag Flux (10 12 ) Ct mag Flux Area N 1 IWTW_036 s IWTW_037 s IWTW_038 s IWTW_039 s IWTW_040 s IWTW_041 s nd IWTW_044 s nd 8 IWTW_043 s nd IWTW_042 s IWTW_179 s IWTW_450 s IWTW_046 s IWTW_047 s IWTW_049 s IWTW_058 s IWTW_169 vhs nd IWTW_170 vhs nd IWTW_101 sgw Q IWTW_097 sgw Q IWTW_094 sgw IWTW_105 sgw Q nd IWTW_093 sgw Q IWTW_045 sgw Q IWTW_048 sgw Q IWTW_487 sgw Q IWTW_175 sgw Q IWTW_178 sgw Q IWTW_479 sgw Q IWTW_484 sgw IWTW_486 sgw Q IWTW_514 sgw Q IWTW_525 sgw Q nd 33 IWTW_083 dgw IWTW_100 dgw Ter IWTW_092 dgw Ter IWTW_057 dgw pre-ter IWTW_177 dgw Ter IWTW_024 dgw Ter 2.85 Area NE 39 IWTW_022 s IWTW_080 s IWTW_180 s IWTW_417 s IWTW_415 s IWTW_061 s IWTW_023 sgw Ter IWTW_081 sgw Q IWTW_082 sgw Q IWTW_483 sgw Q IWTW_026 sgw IWTW_056 sgw Q IWTW_517 sgw Q IWTW_078 sgw Q IWTW_079 sgw Q IWTW_520 sgw Q IWTW_473 sgw Q IWTW_475 sgw Ter IWTW_499 sgw Q IWTW_472 sgw Q IWTW_106 sgw Q IWTW_107 sgw Ter IWTW_502 sgw Q IWTW_077 sgw Q IWTW_075 sgw Q IWTW_076 sgw Q IWTW_007 sgw Q of16

7 Table 1. (continued) Sample Name Category Geology 3 He/ 4 He sub (R/Ra) 3 He mag (10 12 ) 4 He rad (10 6 ) Ct mag Ct mag / 3 He 3 He mag Flux (10 12 ) Ct mag Flux 66 IWTW_008 sgw Q IWTW_011 sgw Q IWTW_012 sgw Q IWTW_013 sgw Q IWTW_014 sgw Q IWTW_054 sgw Q IWTW_050 sgw Q IWTW_494 sgw Q IWTW_051 sgw Q IWTW_052 sgw Q IWTW_437 sgw IWTW_009 sgw Q IWTW_010 sgw Q IWTW_096 dgw Ter IWTW_025 dgw pre-ter IWTW_055 dgw pre-ter IWTW_053 dgw pre-ter Area SE 83 IWTW_183 s nd IWTW_108 s IWTW_182 s IWTW_006 sgw Q IWTW_029 sgw Q IWTW_030 sgw Q IWTW_031 sgw Q IWTW_059 sgw Q IWTW_003 sgw Q IWTW_027 sgw Q nd 93 IWTW_104 sgw Q IWTW_015 sgw Q IWTW_151 sgw Q IWTW_128 sgw Q IWTW_001 sgw Q IWTW_002 sgw Ter IWTW_507 sgw Ter IWTW_005 sgw Ter IWTW_121 sgw Q IWTW_116 sgw Q IWTW_063 sgw Q IWTW_150 sgw IWTW_115 sgw IWTW_155 sgw Q IWTW_114 sgw Ter IWTW_149 sgw IWTW_148 sgw Ter IWTW_156 sgw Q IWTW_124 sgw Q IWTW_123 sgw Ter nd 113 IWTW_157 sgw Ter IWTW_127 sgw Ter IWTW_132 sgw IWTW_109 sgw Ter IWTW_158 sgw pre-t IWTW_154 sgw Q IWTW_139 sgw Q IWTW_161 sgw Q IWTW_125 sgw nd 122 IWTW_126 sgw IWTW_165 sgw Q IWTW_137 sgw Q IWTW_152 sgw IWTW_144 sgw Q IWTW_147 sgw IWTW_163 sgw IWTW_159 sgw Q IWTW_060 dgw Ter IWTW_028 dgw Ter IWTW_103 dgw pre-ter IWTW_102 dgw Ter of16

8 Table 1. (continued) Sample Name Category Geology 3 He/ 4 He sub (R/Ra) 3 He mag (10 12 ) 4 He rad (10 6 ) Ct mag Ct mag / 3 He 3 He mag Flux (10 12 ) Ct mag Flux 134 IWTW_072 dgw Ter IWTW_071 dgw Ter nd 0.29 Area S 136 IWTW_185 s nd IWTW_167 s IWTW_018 sgw Q IWTW_168 sgw Q IWTW_099 sgw Q IWTW_509 sgw Q IWTW_016 sgw Q IWTW_017 sgw Q IWTW_021 sgw Q IWTW_019 sgw Q IWTW_085 sgw Q IWTW_034 sgw Q IWTW_035 sgw Q IWTW_033 sgw Q IWTW_089 sgw Q IWTW_091 sgw Q IWTW_066 sgw Q IWTW_348 sgw Ter IWTW_088 sgw Ter IWTW_087 sgw Ter IWTW_032 sgw Ter IWTW_413 sgw Q IWTW_357 sgw Ter IWTW_359 sgw Q IWTW_373 sgw Q nd 161 IWTW_490 sgw Ter IWTW_384 sgw Ter nd 163 IWTW_389 sgw Q IWTW_368 sgw Ter IWTW_412 sgw Ter IWTW_402 sgw Ter IWTW_405 sgw Ter IWTW_064 dgw Ter IWTW_098 dgw Ter IWTW_020 dgw Ter IWTW_084 dgw pre-ter IWTW_070 dgw Ter IWTW_062 dgw Ter IWTW_090 dgw Ter IWTW_065 dgw Ter IWTW_073 dgw Ter IWTW_067 dgw Ter nd 178 IWTW_068 dgw Ter nd IWTW_069 dgw Ter IWTW_074 dgw Ter a Abbreviations: s, spring; vhs, volcanic hot spring; sgw, shallow groundwater; dgw, deep groundwater; Ct mag, total magmatic carbon; nd, no data; Q, Quaternary volcanic rocks; Ter, Tertiary volcanic and sedimentary rocks; pre-ter, pre-tertiary basement rocks. of the aquifer. In this calculation, the value of crustal 4 He flux (F( 4 He)) includes an uncertainty of factor of 2 3. The flux of 3 He mag calculated in equation (10) would therefore have an uncertainty of factor of 2 3. The obtained 3 He mag fluxes are listed in Table 1. In this study, since the aquifer of the spring is same as that of the shallow groundwater, hereafter, we treat springs as shallow groundwater after discussion. 6. Other Magmatic Volatile (C, Cl and S) Flux Into the Groundwater [17] In addition to He, there are the CO 2, Cl and S as other chemical species related to volcanic activity. These are the major species that exsolve from magma as it ascends from the depth, and are usually the most abundant volatile species in magma next to water [e.g., Giggenbach, 1996; Symonds et al., 2001]. Indeed, the concentrations of the CO 2, Cl (HCl) and S(H 2 S and SO 2 ) in the fumarolic gas from the Oojigokudani changed corresponding to the volcanic activity in 1998 [Ohba et al., 2011]. [18] The concentrations of Cl, HCO 3,SO 4 2, the total amount of DIC (afterward referred to as Ct) and the d 13 C values of DIC are given in Table S1 of the auxiliary material. The amount of Ct was determined by the thermodynamic calculation using the HCO 3 concentration, temperature and ph. Figure 4 shows the relationship between d 13 C and Ct. The d 13 C values of high temperature volcanic gas from a 8of16

9 Table 2. The Notations and Values of Parameters of Equations (8), (9) and (10) for Calculation of the Magmatic 3 He Flux and the Residence Time of Groundwater Symbol Description Units Quaternary Tertiary Tertiary, Pre-Tertiary Reference C(X) Concentration of component X cm 3 STP F o Water flow rate from reservoir cm 3 /y T r Residence time y r w Density of water g/cm r R Density of the reservoir rock g/cm PROCK [Murata et al., 1998] p Porosity of the reservoir R mag Helium isotope ratio of original Graham [2002] magmatic volatiles F( 4 He) Crustal 4 He flux from the bottom cm 3 STP/cm 2 /y Sano [1986] of the reservoir P( 4 He) a in situ production flux of 4 He from the reservoir cm 3 STP/cm 3 /y (sediment), (granite) [U] U concentration in the reservoir rock mg/g 1.2 nd 2.8 (sediment), 3.0 (granite) This study [Th] Th concentration in the reservoir rock mg/g 4.5 nd 9.7 (sediment), 19 (granite) This study h Thickness of the reservoir cm Kitamura and Onishi [1972] V Volume of the reservoir cm 3 S Area of the reservoir cm 2 a P( 4 He) = (1-p) r R { [U] [Th]}. subduction zone are in the range of 5.5 to 2.0 [Marty et al., 1989]. On the other hand, the d 13 C values of DIC equilibrated with soil CO 2 which is considered biogenic origin are said to be approximately in the range from 23 to 14 [Clark and Fritz, 1997]. Almost all the data are plotted between the two mixing lines of biogenic and magmatic end-members. The contribution ratio of total magmatic carbon (Ct mag ) can be calculated by the isotopic mass balance using the d 13 C values of the two end-members assumed the magmatic d 13 C= 4 and biogenic d 13 C= 20. The obtained concentrations of Ct mag are listed in Table 1. The flux of Ct mag is estimated using the 3 He mag flux and Ct mag / 3 He ratio and also listed in Table 1. The fluxes of Cl and S are estimated by the same procedure. Since the groundwater likely contains components from non-magmatic Cl and S of anthropogenic sources such as agricultural and urban activities, the estimated Cl and S fluxes could possibly be overestimated. The average fluxes of Cl and S for each area and aquifer are listed in Table 3 with those of 3 He mag and Ct mag. 7. Three-Dimensional Distribution of Magmatic Volatiles Into the Groundwater 7.1. Concentrations of 3 He mag, 4 He rad and Ct mag and the ( 3 He/ 4 He) sub Ratios [19] The ( 3 He/ 4 He) sub ratio, concentrations of 3 He mag, and Ct mag, are used as parameters which represent the contribution of magmatic volatiles, and the concentration of 4 He rad is related to the residence time of groundwater. The threedimensional distributions of the ( 3 He/ 4 He) sub ratio, and the concentrations of 3 He mag, 4 He rad and Ct mag are shown in Figure 5. [20] The many shallow groundwaters having the higher ( 3 He/ 4 He) sub ratio than 2.0 Ra are found in the northern part of Iwate volcano (areas N and NE) and around the Table 3. The Average Magmatic Volatile Fluxes to the Groundwaters Figure 4. Relationship between d 13 C of DIC and the concentrations of total carbon (Ct) of springs, and shallow groundwaters and deep groundwaters around Iwate volcano. The d 13 C values of magmatic and biogenic end-members ranged from 5.5 to 2.0 [Marty et al., 1989], and ranged from 23 to 14 [Clark and Fritz, 1997], respectively. Two curved lines indicate the mixing lines among the magmatic and biogenic end-members. Area Category 3 He mag ( mol/m 2 /y) Ct mag Cl S N sgw dgw NE sgw dgw SE sgw dgw S sgw dgw of16

10 Figure 5. Three-dimensional distribution maps of (a) subsurface origin 3 He/ 4 He ratios, ( 3 He/ 4 He) sub, (b) magmatic 3 He ( 3 He mag ) concentrations, (c) radiogenic 4 He ( 4 He rad ) concentrations, and (d) total magmatic carbon (Ct mag ) concentrations of the springs, shallow groundwaters and deep groundwaters in Iwate volcano. Four vertical profiles which encircled the map indicate the areas of north, south, east and west to the volcano center, respectively. The simplified geologic profile is also drawn in the vertical profiles in the E-W direction. Legend of the geological profile is same as in Figure of 16

11 Figure 6. Three-dimensional distribution maps of (a) magmatic 3 He ( 3 He mag ) flux and (b) total magmatic carbon (Ct mag ) flux of springs, shallow groundwater and deep groundwater in Iwate volcano. Four vertical profiles which encircled the map indicate the areas of north, south, east and west to the volcano center, respectively. The simplified geologic profile is also drawn in the vertical profiles in the E-W direction. Legend of the geological profile is same as in Figure 1. Shizukuishi fault zone placed in area S. The highest ( 3 He/ 4 He) sub ratio of 6.4 Ra is found at No.48 in area NE which is larger than the value of the fumarolic gas at Oojigokudani [Ohba et al., 2011]. These shallow groundwaters in areas N and NE and around the Shizukuishi fault zone have comparatively high Ct mag and 3 He mag concentrations, indicating a high magmatic contribution, but the 4 He rad concentrations are relatively low, which indicated either the high mobility of groundwater or a shorter residence time. In contrast, in the area SE and the area S far from the fault zone, most of the shallow groundwater has the lower ( 3 He/ 4 He) sub ratios, and lower concentrations of Ct mag, 3 He mag, 4 He rad than that for areas N and NE, indicating that the contribution of magmatic volatiles is smaller. [21] For the deep groundwater in the areas N, NE, and SE, although the concentrations of Ct mag and 3 He mag are considerably high, the ( 3 He/ 4 He) sub ratios are lower than 2.0 Ra. In particular, all the ( 3 He/ 4 He) sub ratio in the areas NE and SE, are lower than 0.5 Ra. The very high 4 He rad concentration for the deep groundwater in all the areas suggested that the relatively large amount of magmatic volatiles have been supplied in a longer time period necessary for high concentrations of 4 He rad accumulation. The aquifers of shallow groundwater and deep groundwater in the areas N, NE and SE are thus separate and isolated, and the groundwater flow system differs from each other. In the deep groundwater around the Shizukuishi fault zone in particular, high values of magmatic parameters ( 3 He/ 4 He) sub, 3 He mag,ct mag and high 4 He rad concentrations suggest long residence times. The magmatic volatiles with high ( 3 He/ 4 He) sub ratios are supplied both to the shallow and the deep groundwater systems whose residence times are different. The rising of magmatic volatiles through the fault is reported in some volcanic areas [e.g., Federico et al., 2002; Saar et al., 2005; Morikawa et al., 2008a]. The fault thus acts as a supply path of magmatic volatiles, and affects both the deep and the shallow aquifers. However, the deep groundwaters in area S of 5 km far from the fault have similar features to those of the area SE. [22] From the results of the distribution of the ( 3 He/ 4 He) sub ratio and the concentrations of magmatic volatiles, the high concentration of the magmatic volatiles is caused either by two factors: either the large supply of magmatic volatiles or the long-term accumulation of volatiles such as long residence times of groundwater. Therefore, high magmatic concentration does not readily mean the high flux of magmatic volatiles. To treat the magmatic activity using groundwater parameters, it is necessary to take into consideration not only the concentrations but the fluxes which can neglect the time scales of groundwater flow for the evaluation of magmatic activity Fluxes of the 3 He mag and Ct mag [23] The magmatic volatile fluxes are estimated by the equation (10) and the composition ratio of each component. Here, we use the 3 He mag and Ct mag as representatives of magmatic volatile species. The three-dimensional distributions of 3 He mag and Ct mag fluxes are shown in Figure 6. It is remarkable that fluxes for the shallow groundwater in 11 of 16

12 Figure 7. A schematic model of the supply of magmatic volatiles in Iwate volcano. Most of the magmatic volatiles are caught by the shallow groundwater system, and residual appears as volcanic gases. Values indicate the average supply rate (mol/y) of magmatic volatiles ( 3 He mag,ct mag, Cl, and S) into the shallow (upper part) and deep (lower part) groundwater aquifers for each divided area. Fluxes in the shallow groundwater at the areas N and NE, and the shallow and deep groundwater at the fault zone (area S) are revealed to be very large. areas N and NE are larger than those of deep groundwater even though the deep groundwater contains a lot of magmatic species as seen in Figure 5. The flux values between the shallow groundwater and deep groundwater differ in two orders at a maximum, implying the difference in the supply process of magmatic volatiles for each groundwater system. In addition, the fluxes for the shallow groundwater in the area SE are one order of magnitude smaller than that in the areas N and NE, indicating that magmatic volatiles have been supplied anisotropically in the shallow groundwater aquifers around Iwate volcano. This anisotropy is likely related to the shallow groundwater flow as will be described later. Around the Shizukuishi fault zone in area S, the fluxes show large values in both shallow and deep groundwater, which is due to the fault zone acting as the flow path of magmatic volatiles as described in the previous section. The large flux value suggests the supply of large amounts of magmatic volatiles through the fault. [24] The flux of 3 He into groundwater has been estimated for several aquifers worldwide. Large flux values associated with faults are also reported by Kulongoski et al. [2005] for Morongo Basin (California, USA). In Iwate volcano, although the large 3 He mag fluxes above mol/m 2 /y are found in the shallow groundwater in the areas N and NE, these values are smaller than those of thermal waters around Unzen volcano (southwest Japan), which erupted in and reached up to mol/m 2 /y [Morikawa et al., 2008a]. However, 3 He mag fluxes from Iwate volcano are significantly larger than the natural gas field of northern Taiwan (total 3 He flux of (2 3.8) mol/m 2 /y) [Sano et al., 1986] and average 3 He flux values of subduction area [Torgersen, 1989]. This implies that the large amounts of magmatic volatiles are effectively supplied to the groundwater for Iwate volcano with passive activity. In contrast, the lower flux values than mol/m 2 /y which are found in deep groundwaters in the area NE and SE are similar or only slightly high level compared with those of the aquifers from non-volcanic area; e.g., mol/m 2 /y for Great Artesian Basin [Torgersen and Clarke, 1987], ( ) mol/m 2 /y for Great Hungarian Plain [Stute et al., 1992], ( ) mol/m 2 /y for central part of Paris Basin [Marty et al., 1993; Castro et al., 1998]. This confirms the inefficient accumulation of magmatic volatiles into deep aquifers around Iwate volcano. [25] Spatial distribution feature of Ct mag flux are found to be very similar to that of the 3 He mag flux. This indicates that the He and C components originate from the same magma, and the pathway of the magmatic volatiles must also be similar. However, the range of fluctuation of Ct mag flux is larger than those of 3 He mag flux, and the Ct mag flux to the deep groundwater is very small. This difference likely caused by the carbon depletion by depositing of C-bearing minerals such as carbonates. [26] The high concentrations of the magmatic volatiles are observed in both the shallow groundwater and the deep groundwater. However, the flux value to these groundwaters is revealed to be totally different. The differences between the concentration and the flux are considered to be linked to the difference of the residence time of groundwater as mentioned before. To discuss the evaluation of magmatic 12 of 16

13 Table 4. Magmatic Volatile Supply Rate to the Groundwaters and Magmatic Volatile Emission Rate Category 3 He mag ( 10 6 mol/y) Ct mag (10 6 mol/y) Cl ( 10 6 mol/y) S ( 10 6 mol/y) Magmatic volatile supply rate to groundwater a Area N (150 km 2 ) sgw dgw Area NE (120 km 2 ) sgw dgw Area SE (180 km 2 ) sgw dgw Area S (200 km 2 ) sgw dgw Total Volcanic gas Total emission rate from Iwate volcano a The number put in the parenthesis shows the area of the groundwater aquifer. activity, those of volatile flux is more useful than the concentration of magmatic species. 8. Magmatic Volatile Supply Model to the Groundwater System [27] The results of three-dimensional distributions of magmatic volatile concentrations and fluxes reveal the three particular magmatic volatile supply systems; the shallow groundwater system, the deep groundwater system and the fault zone system, and show the anisotropic supply of magmatic volatiles to the shallow groundwater. The supply of magmatic volatiles in Iwate volcano is summarized schematically in Figure 7, which shows the magmatic volatile supply rate (mol/y) of magmatic volatiles ( 3 He mag, Ct mag, Cl and S) into the groundwater. The magmatic volatile supply rate at each groundwater system is calculated using the magmatic volatile flux with an assumption that the area of groundwater system (m 2 ) is equal to the areas of N, NE, SE, and S flanks, respectively, and listed in Table 4. [28] The areas of high supply rate in the shallow groundwater are placed at the areas N and NE, where the groundwater is flowing down along the collapsed slope inside the volcanic body which forms a boundary surface underlying the permeable Iwate volcanic body, and many springs are discharging in the low elevation areas [Kazahaya, 2000]. Most of the shallow groundwaters of the N and NE flanks are also taken from the low elevation area, where the large volume of debris avalanche deposit covers. The groundwaters discharging at area N and at area NE have recharge areas containing the Oojigokudani caldera area of Nishi- Iwate, and the Yakushidake summit area of Higashi-Iwate, respectively (Figure 2). Because both of the Oojigokudani and Yakushidake are the fumarolic areas, the recharged groundwater would contact with the ascending magmatic gases through the conduit of the summit of Higashi-Iwate and Oojigokudani caldera of Nishi-Iwate and dissolve them. As a result, the groundwaters containing the large amount of magmatic volatiles flow to the N and NE areas, respectively, and the magmatic volatile supply rate of the shallow groundwaters in the areas N and NE are found to be high. [29] The magmatic volatile supply rates of deep groundwater are lower than those of shallow groundwater, but both of the concentrations of magmatic volatiles and 4 He rad are higher (Figure 5). This suggests that the magmatic volatiles are gradually accumulated with relatively longer time. The approximate residence times of deep groundwater can be calculated to be hundreds to tens of thousands of years from the equation (9), and may be quite longer than the shallow groundwater. The less permeable aquifer of deep groundwater in the Tertiary and pre-tertiary geologic unit probably makes a longer residence time for deep groundwater and the low mobility of magmatic volatiles which result from the low supply rate. The lowest supply rates to the deep groundwater are also found in the area NE, whose geologic unit of aquifer is pre-tertiary. However, the supply rate to the deep groundwater around the fault zone (area S) is high, because the large amounts of magmatic volatiles ascend via a fault zone where permeable fractures exist. And the supply rate to the deep groundwater in area N also slightly high, and this may also be supplied from a concealed fault. [30] Generally, volcanic gas emission is the major process of the magmatic volatile release from a volcano. The magmatic volatile emission rate (mol/y) from volcanic gas in Iwate volcano can be calculated using the magmatic volatile compositions and the sulfur dioxide (SO 2 ) flux of fumarolic gas (0.1 tons/day = mol/y) in 2004 (Mori et al., unpublished data, 2007). Therefore, the magmatic volatile emission rates of 3 He mag,ct mag, Cl and S from volcanic gas are calculated from the these volatile compositions of fumarolic gas in 2004 [Ohba et al., 2011], and are shown in Table 4. The emission rate is calculated as total S considering the major gaseous species of H 2 S. [31] The comparison between the supply rate of groundwater with the emission rate of volcanic gas revealed significantly low emission of magmatic volatiles from volcanic gas. From the results of this study, it is suggested that the groundwater which recharged at the summit and the caldera area forms the huge groundwater flow system and the recharged groundwater interact with the ascending volcanic gas. The fumarolic activity of the summit area in Higashi- Iwate is less active, where a weak and diffusive steam discharge occurred with temperatures lower than the boiling point of water. Although the fumarolic activities of Oojigokudani in Nishi-Iwate are active, the high temperature fumarole exceeding 100 C is placed in a narrow area, and most fumaroles have temperatures of around the boiling point of water. Therefore it is suggested that the volcanic gases emitted from the fumaroles in Iwate volcano are the gases after interactions with groundwater. Most of the magmatic volatiles are concluded to be discharged through 13 of 16