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Chemical and isotopic studies including analyses of noble gases were comprehensively conducted on the groundwater of the entire Kii Peninsula, which is located in the fore-arc region of southwest Japan. Groundwater of Na–Cl–HCO3, Na–HCO3–Cl, and Na–Cl types was shown to be distributed across the whole area. Groundwater in the inland central part of the peninsula shows relatively low salinity, whereas groundwater from the area along the ENE-trending Median Tectonic Line (MTL), on the north side of the peninsula, shows high salinity (up to 18,800 mg/L of Cl−) and the presence of unusual heavy oxygen isotopes. This trend is similar to that documented in saline waters from the Arima region (the so-called “Arima-type thermal water”). High 3He/4He ratios relative to the atmospheric value (up to 6.7 Ra) were recorded throughout the Kii Peninsula, covering a wider area than documented previously. The saline groundwater is also strongly depleted in 20Ne and heavy noble gases. From the wide distribution of high 3He/4He values and the associated 20Ne and Cl− concentrations, we infer that aqueous fluids derived from dehydration of the subducting slab are present at depth beneath almost the entire Kii Peninsula. These aqueous fluids may ascend along the major north-dipping boundary faults. The isotopic composition of groundwater from the southern part of the peninsula suggests that the contribution from these dehydration-derived fluids is relatively small in this region. However, volatile components (e.g., noble gases and CO2) in the groundwater of this area may originate from the dehydration-derived fluids. Upwelling of Arima-type thermal water of the Na–Cl–HCO3 type is expected to undergo a phase separation of volatile species due to decompression as the fluid ascends. The variety of water types documented may be due to this water–gas separation and the subsequent incorporation of gaseous species into shallow meteoric groundwater. The observed high 3He/4He ratios in the absence of a mantle wedge below the southern part of the Kii Peninsula may reflect the oblique ascent of these fluids along north-dipping boundary faults.

Large earthquakes sometimes trigger local seismicity that is distal to their rupture zones. Various mechanisms for this triggered seismicity have been proposed, based on either the static stress change or ground shaking from seismic waves, but local geological structure is rarely studied to discern why this seismicity is remotely induced. We present the results of a joint three-dimensional resistivity and isotopic analysis of the groundwater system surrounding Mount Fuji, Japan, where increased seismicity was observed following the A.D. 2011 Tohoku-Oki megathrust earthquake. An electrically conductive zone and high concentrations of magmatic gases (He and CO2) correspond to the zone of triggered seismicity. In contrast, a contribution of magmatic water is not suggested from 2H (deuterium, D) and 18O isotope ratios. These results suggest that the earthquakes were triggered within a fractured zone through which magmatic gases preferentially migrated. We hypothesize that the upwelling of gas-rich hydrous fluids and/or gas bubbles occurred along this fracture pathway, causing an increase in the pore pressure and triggering the resultant earthquake sequence.

A geochemical investigation was undertaken at Mt. Etna Volcano to better define groundwater characteristics of its aquifers. Results indicate that the Na?Mg ± Ca-HCO3− ± (SO42− or Cl−) type accounts for more than 80% of the groundwater composition in the volcano. The remaining 20% is characterized by elevated Ca2+. Waters along coastal areas are enriched in SO42− or Cl−, mainly due to mixing with seawater and anthropogenic effects. The majority of the samples showed values between −4% to −9% for δ18O and −19? to −53% for δ2H, suggesting that precipitation is the predominant source of recharge to the aquifers, especially in the west of the study area. The analysis of δ13C and pCO2 shows values 1 to 3 times higher than those expected for waters in equilibrium with the atmosphere, suggesting a partial gas contribution from deep sources. The diffusion of gasses is likely to be controlled by tectonic structures in the volcano. The ascent of deep brines is also reflected in the CO2 enrichment (up to 2.2 bars) and enriched δ2H/δ18O compositions observed in the salt mounts of Paternò. Fil: Bellia, Carmelo . National Institute of Advanced Industrial Science and Technology. Geological Survey of Japan; Japón Fil: Gallardo, Adrian Hugo. Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Departamento de Geologia; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Yasuhara, Masaya . National Institute of Advanced Industrial Science and Technology. Geological Survey of Japan; Japón Fil: Kazahaya, Kohei . National Institute of Advanced Industrial Science and Technology. Geological Survey of Japan; Japón

[1] All available SO2 flux data for 32 years (1975–2006) of Japanese volcanoes, accounting for about 10% of the world's arc volcanoes, were compiled to evaluate the temporal variation of the flux of each volcano and to estimate the time-averaged SO2 flux. The compiled data revealed that 6 volcanoes (Tokachi, Asama, Aso, Sakurajima, Satsuma-Iwojima, and Suwanosejima volcanoes) out of 17 significantly degassing volcanoes usually contributed more than 94% of the total flux. The time-averaged annual flux was 2.2 Tg a−1, which includes intense degassing of Miyakejima volcano after 2000, which raised the figure from 1.4 Tg a−1, indicating that a single huge emitter is capable of significantly skewing regional time-averaged degassing totals and indicating that the time-averaged flux assessments for infrequent huge emitters are important for accurate estimation. The regional SO2 flux distribution in cumulative frequency-flux plot does not obey a power law distribution. It shows a roll-off curve bending at about 500 t d−1, implying that it is misleading to assume the power law distribution for estimation of the global flux. Because the contribution of the major degassing volcanoes including the six volcanoes and additional sporadically degassing volcanoes during eruptive and posteruptive periods to the total flux is more than 95%, measurement of all large flux volcanoes can approximate the global flux.

Knowledge of the groundwater age is indispensable for understanding groundwater flow in crystalline rocks. The present study is the first to discuss the radiocarbon ages of groundwater in Abukuma granite, Fukushima Prefecture, northeast Japan. The vertical profiles of 14C dates and δ13C are obtained from 3 boreholes (depths of 140, 230, and 306 m). Chemical and carbon isotopic compositions suggest that dead-carbon contamination of groundwater occurred during groundwater storage in the fractures. 14C concentration was corrected by using isotopic mass balance in which dead-carbon contamination of the groundwater was considered. The 14C dates ranged from modern to ≃16 ka. The relationship between tritium and 14C data in 1 borehole suggests the simultaneous inflow of shallow groundwater to deeper levels occur for the depths between 60 and 100 m. The vertical profiles of 14C dates indicate a relatively constant age of 10–16 ka for groundwater deeper than 100 m, which may have been influenced by rapid sea-level changes after the glacial period.

Phreatic eruptions occurred at the Meakandake volcano in 1988, 1996, 1998, 2006, and 2008. We conducted geochemical surveillance that included measurements of temperature, SO2 emission rates, and volcanic gas composition from 2003 to 2008 at the Nakamachineshiri (NM), Northwest (NW), and Akanuma (AK) fumarolic areas, and the 96–1 vent, where historical eruptions had occurred. The elemental compositions of the gases discharged from the different areas are similar compared with the large variations observed in volcanic gases discharged from subduction zones. All the gases showed high apparent equilibrium temperatures, suggesting that all these gases originated from a common magmatic gas. The gases discharged from each area also exhibited different characteristics, which are probably the results of differences in the conditions of meteoric water mixing, quenching of chemical reactions, and vapor-liquid separation. The highest apparent equilibrium temperatures (about 500°C) were observed in the case of NW fumarolic gases, despite the low outlet temperature of about 100°C at these fumaroles. Since the NW fumaroles were formed as a result of the 2006 phreatic eruption, the high-temperature gas supply to the NW fumarole suggests that the phreatic eruption was caused by the ascent of high-temperature magmatic gases. The temperatures, compositions, and emission rates of the NM and 96–1 gases did not show any appreciable change after the 2006 eruption, indicating that each fumarolic system had a separate magmatic-hydrothermal system. The temperatures, compositions, and emission rates of the NM fumarolic gases were apparently constant, and these fumaroles are inferred to be formed by the evaporation of a hydrothermal system with a constant temperature of about 300°C. The 96–1 gas compositions showed large changes during continuous temperature decrease from 390° to 190°C occurred from 2003 to 2008, but the sulfur gas emission rates were almost constant at about four tons/day. At the 96–1 vent, the SO2/H2S ratio decreased, while the H2/H2O ratio remained almost constant; this was probably caused by the rock-buffer controlled chemical reaction during the temperature decrease.

To understand deep groundwater flow systems and their interaction with CO2 emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ13CDIC) and DIC concentrations of the sampled waters show that magma-derived CO2 mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ13CDIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ18O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ13CDIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO2 is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO2 mixes into deeper groundwater flowing nearer the magma conduit or chamber.

Two distinctive magmatic fluids were recognized in the Tatun volcanic group (TVG), Taiwan. One is a relatively reduced fluid represented by the fumarolic gases at Hsiao-you-ken (HYK) geothermal field. Another is an oxidized fluid containing high concentrations of HCl represented by the fumarolic gases at Da-you-ken (DYK). An intermediate gas was recognized at Gung-tze-ping (GTP) and She-hung-ping (SHP). The fumarolic gases at HYK and GTP possess the features of so-called primary steam generated on mixing of magmatic gas and meteoric groundwater. The fumarolic gases at DYK are a simple mixture between magmatic gas and water vapor of meteoric origin. The CO 2 /H 2 O molar ratio of the magmatic component in the fumarolic gases at DYK was estimated to be 0.018, meanwhile it was estimated to be 0.027 for the fumarolic gases at HYK and GTP, suggesting the magma beneath DYK is depleted in volatiles relative to the magma beneath HYK and GTP. The estimated CO 2 /H 2 O ratio for the magmatic component is comparable to that of some active volcanoes in Japan, suggesting the enrichment of volatiles in the magmas beneath TVG.

Chemical and isotopic compositions were measured for volcanic gases discharged from the recently erupted Heisei-shinzan dome of Unzen volcano. The volcanic gases are largely mixed with the air, whose content ranges from 60 to 90%. Chemical and isotopic compositions of the gases are corrected for the air mixing accounting not only for the major air components but also for contribution of the atmospheric moisture and CO 2 . The variation of the corrected composition revealed that these gases are derived from various sources including; 1) the air circulating through the lava domes, 2) the local meteoric water at various degrees of evaporation and vapor loss, 3) the magmatic CO 2 likely originated from a deep source, and 4) the gases discharged from the cooling lava dome. The gases discharged from the cooling lava dome are characterized by the oxygen and hydrogen isotopic composition typical of the subduction-type volcanoes but with high Cl content and high Cl/S and F/S ratios. In contrast, discharges of CO 2 with the magmatic δ 13 C values are not correlated with discharge of heat or acid gas species, suggesting the ascent of CO 2 -rich fluids from a deep source, possibly through the volcanic conduit.

Rare earth elements (REEs) of the spring waters upwelling in the non-volcanic fore-arc region of the Kinki district in southwest Japan were investigated to assess their upwelling processes and deep-seated origins. A principal component analysis of the REE data identified three principal components (PCs) that cover 89% of the entire sample variance: (1) PC-01, which corresponds to a dilution process by which fluids are introduced at low concentrations, previously represented by major solute binary trends, including δ18O–δD systematics; (2) PC-02, which is a precipitation process of REEs from the brine; and (3) PC-03, which is an incorporation of REEs from country rock by carbonic acidity, although the types of country rocks may also have a significant impact on the spring water compositions. Based on these three PCs, together with the major solute concentrations and hydrogen, oxygen, and helium isotopic compositions determined in previous studies, five distinct types of spring waters in the Arima and Kii areas were identified: (i) “Tansansen”, (ii) “Kinsen”, (iii) “Ordinary Arima”, (iv) “Ginsen”, and (v) “Eastern Kii”. These five types probably represent (ii) a deep brine, (iii) an evolved deep brine that precipitated REE-bearing minerals, (iv) a mixture of (iii) and meteoric water, (v) a meteoric water carbonated by deep gas derived from (ii), and (i) a spring water similar to (v) with a more significant influence of the country rock constituting the aquifer. A comparison of the spring waters in the Arima and Kii areas revealed systematic geographic distributions. The “Ordinary Arima”-type occurs along the Median Tectonic Line, and the “Eastern Kii”-type occurs in the eastern part of the Kii area. The latter seems to upwell in the restricted region where deep low-frequency tremors are observed. We suggest that the geographical distributions are linked to the tectonic setting and/or temporal evolution of fluid upwelling.

Environmental tracers become a common tool for the groundwater study and a number of methods have been presented in order to understand groundwater flow processes, water budget, origins, chemical reaction processes and retention time. Tracers often used are selected and reviewed for their various methods and advantages as follows; 1) stable 18O, D in water, 2) stable 13C and radioactive 14C in DIC, 3) noble gases such as He, Ne, Ar, Kr, Xe and their isotopes, 4) radioactive 36Cl in dissolved chloride and some heavier isotopes, and 5) inert gaseous species such as CFCs.If they are less reactive species, they likely preserve information at the time of recharge or their origin. Use of D, 18O and the d-value of water is the powerful tool to determine the recharge area because recharged meteoric water have their inherent isotopic ratios correlated with the recharge elevation, distance from the coast, or the local topography. Carbon-bearing species are more reactive though, use of stable isotopes of DIC leads to identify its origin and helps to analyze the chemical reaction between minerals and water or gas addition processes during the groundwater flow in aquifers. Radioactive 14C has been used to estimate groundwater age however special attention should be paid for, i.e., the origin of DIC, before applying the method. Noble gas tracers are the useful species to presume recharge temperature from their concentrations in water using their temperature dependence of solubilities. Radiogenic 4He concentration can be used for the very long-term groundwater dating since the 4He is produced in the crust and is accumulated in the deep aquifers, if the local accumulation rate of 4He is known. Radioactive 36Cl has been used to determine the age of very old saline waters up to million years. This isotope will also be convenient for the dating of very younger waters, by the use of bomb-produced 36Cl resulted from surface nuclear experiments near the seawater in the 1950s. Chlorofluorocarbons (CFCs) are the gas species produced by the recent human activity and dissolve in water during the recharge, therefore, the affected younger groundwater will have equivalent CFCs concentrations with the atmospheric CFCs concentrations at the time of the recharge. As these species are easy to detect with very high sensitivity, this tracer has now been applied not only for the age determination but for the mixing or contamination of shallow young water to a deep old groundwater. As an individual method listed above is valid only for the very simple flow system, appropriate assumptions or coupling of using different tracers is necessary to understand natural complex groundwater flow system where mixing of groundwaters of different origin or age occurs. Combination of tracers helps us simulating the complex system in detail and is being a growing trend in groundwater study.

We report here the first measurements of SO 2 emission rates for three volcanoes in Hokkaido. Observations were made by scattered light ultraviolet (UV) spectroscopy using miniature spectrometers carried in vehicles or on foot, or attached to scanners, with data processing following variants on differential optical absorption spectroscopy. The following mean fluxes were calculated for measurements in 2003 (and 2004): 0.032 (0.14), 0.24 (0.28) and 2.4 kg s − 1 for Meakan-dake, Tarumae, and Tokachi-dake, respectively. These are thought to be the only sites of high temperature volcanic degassing on Hokkaido at present, and thus the modest combined flux of 2.7 kg s − 1 represents, to first order, the total volcanic emission rate of SO 2 for the island. This represents only a small fraction of the SO 2 output of about 80 kg s − 1 for Miyakejima volcano in 2003. The apparently low sulfur emission from Hokkaido volcanoes may reflect the scrubbing efficiency of their hydrothermal systems though H 2 S emissions remain unquantified. The ability to measure comparatively weak fumarole emissions (e.g., at Meakan-dake and Tarumae) emphasizes the value of walking traverse UV spectroscopy.

A method using carbon isotopes (14C and 13C) for quantifying the proportion of CO2 contribution from deep sources, e.g. magma, subducted materials or mantle, in soil air is established, and discussed in comparison with the conventional methods and errors. A relationship between carbon isotopic ratios ( δ 14 C and δ 13 C ) and CO2 concentration has been used to estimate proportions of CO2 reservoirs in conventional studies. Our study indicates that a relationship between δ 14 C and δ 13 C is available to quantify the CO2 derived from deep subsurface, deep CO2 contribution, within low errors (1σ=0.01–0.02). The estimated values of deep CO2 contribution by this method were agree with those estimated by the conventional method, while the estimated results using the relationship between δ 13 C and CO2 concentration show slightly higher values of deep CO2 contribution than those by the method using the δ 14 C in several samples. The quantifying errors estimated by δ 14 C and CO2 concentration are the smallest to be ca. ±0.005, but in case of the CO2 concentration higher than 1% in soil air. With regard to the method using δ 13 C and concentration, the error of estimated results shows ca. ±0.05 for the sample of which CO2 concentration is over 2%.

Volcanic gas of Iwodake has been discharged continuously from high-temperature (900°C maximum) fumaroles and a degassing vent which formed on the central floor of the summit crater after 1994. Although most of the high-temperature fumaroles located on the crater wall before 1991, many fumaroles appeared on the crater floor associated with the vent opening, suggesting a shift of thermal activity from the peripheral to the central crater. A large amount of volcanic gas has been discharged from the fumaroles and vent. Since heat has been transferred from ascending gas to the surrounding soil, a region showing a surface temperature anomaly has developed around the fumaroles and vent. To quantify Iwodake thermal activity, heat and volcanic gas mass fluxes were estimated during 1996–1999 using infrared thermal images, and plume velocity and temperature data which were observed by a pitot tube and thermocouple. The estimated gas mass flux was compared with the COSPEC data to investigate the accuracy of our estimation. The gas mass flux had been decreased from 230 kg/s in October 1996 to 30 kg/s in November 1999. Although the vent diameter had increased from 20 to 70 m during the same period, this mass flux variation had indicated the decline of degassing rate. The degassing depth was estimated from the volcanic gas mass flux and temperature. The depth showed a tendency of magma head migration to shallower depth during 1996–1999, which was consistent with the drastic change of the surface manifestation.

Rare earth elements (REEs) of the eight Arima spring waters in southwest Japan, including Arima-type brine that represents a specific type of deep-seated brine of up to 6 wt.% NaCl in the non-volcanic fore-arc region, have been investigated in order to discuss their upwelling processes and origins. We found four distinct patterns of REE composition of the spring waters within the Arima area of ~1 km2 , based on which two sources for REEs and two aquifers are inferred in the modification of the original deep-seated brine composition. On the basis of the REEs and isotopic compositions of the original deep brine, one of the two sources is thought to be slab-derived fluid dehydrated from the subducted Philippine Sea slab beneath the Arima area, represented by the ‘Kinsen’ hot spring water [1]. The convex-down REE pattern of most Arima spring waters, except for ‘Kinsen’ and ‘Tansansen’, suggests the presence of an oxidizing aquifer deeper than 160 m that causes co-precipitation of REEs with oxyhydroxides. CO2 and He degassed from this aquifer flux the overlying shallow aquifer less than ~50 m in depth, producing highly carbonated water such as ‘Tansansen’ water that was originally derived from meteoric water. The carbonated water may dissolve a significant amount of REEs to the ‘Tansansen’ spring water from the host rocks, which are possibly silicic igneous rocks with Eu-negative anomalies. The four types of REE patterns with a wide concentration range, therefore, provide invaluable information concerning fluid–crust interaction during ascent of the deep brine.

Chemical and stable carbon isotopic analyses of dissolved inorganic carbon (DIC) were carried out for groundwater samples collected from cold springs and shallow wells in the Unzen volcanic region in 1999 and 2000. All of the data sets plotted on the carbon isotope ratio (δ13C) vs 1/DIC diagram can be explained by mixing of volcanic CO2 with DIC equilibrated with soil CO2. Groundwater DIC showing a high mixing ratio of volcanic CO2 appears to have a tendency to distribute along two major faults near the activity center of the 1990–1995 eruption. This suggests that these faults are escape routes of volcanic CO2 diffused into the volcanic edifice. The total flux of the volcanic DIC discharged from the cold springs is shown to be one to two orders of magnitude lower than the roughly estimated flux of volcanic CO2 discharged from the summit during the eruptive period.

Satsuma-Iwojima volcano continuously releases magmatic volatiles from the summit of Iwodake, a rhyolitic lava dome. The temperature of fumaroles is high, between 800° and 900°C, and the water-rich composition of volcanic gases has not changed essentially over the past 10 years. Sulfur dioxide flux measured by COSPEC is almost constant with an average of 550 t/d since 1975. The present volcanic gas is likely degassed from a rhyolitic magma whose composition is similar to that erupted in 1934, 2 km east of Satsuma-Iwojima. Comparison of silicate melt inclusions and volcanic gas compositions indicates that the magma degassing pressure is very low, implying magma-gas separation at a very shallow level. The mass rate of magma degassing is estimated at 10 m3/s using the volatile content of the magma and the fluxes of magmatic volatiles. The rhyolitic parental magma is volatile-undersaturated in the deep magma chamber, as suggested by melt inclusion studies. Magma convection in a conduit, driven by the density difference between higher density degassed and lower density non-degassed magmas, explains the high emission rate of magmatic volatiles released at shallow depth from such a magma chamber, that is gas-undersaturated at depth. Model calculations require the conduit diameter to be greater than 50 m as a necessary condition for convection of the rhyolitic magma. Long-term convective degassing has resulted in the rhyolitic magma in the deep chamber to become depleted in volatile components. The melt-inclusion studies indicate that the rhyolitic magma responsible for discharging the present volcanic gas has been degassed during the long degassing history of the volcano and is now supplied with CO2-rich volatile components from an underlying basaltic magma. The total volcanic gas flux over 800 years requires degassing of 80–120 km3 of basaltic magma.

Up to 15 fumarolic gas samples were collected successively within a few hours at Satsuma-Iwojima and Kuju volcanoes, southwest Japan, with the aim to evaluate the naturally occurring short-term variations in the chemical and isotopic compositions. Variations in the concentrations of the major gas components (CO 2 , total S, SO 2 and H 2 S) and in the hydrogen, oxygen (H 2 O) and carbon (CO 2 ) isotopic compositions were relatively small and were within the analytical errors. Exceptions were HCl at both volcanoes, CO 2 at Satsuma-Iwojima, and D/H and 18 O/ 16 O ratios of water at Kuju volcano. Large variations were found for the minor gas components, i.e., N 2, Ar, CH4 and CO. The observed variations in HCl and CO concentrations probably relate to sampling and analytical procedures. Variations in HCl, N 2, Ar and CH4 concentrations, and D/H and 18 O/ 16 O ratios of water may result from processes such as gain or loss of HClrich liquid, sporadic additions of air and hydrothermally-derived CH 4 and change of mixing ratio between magmatic and meteoric waters. Precision associated with the gas sampling and analytical techniques used in this study were estimated by making the hypothesis that the chemical and isotopic compositions of the fumarolic gases did not change during the sampling period. The estimated precision was 1‐3% for CO2, H2S and SO2, 6‐11% for HCl, 1‐6% for H 2 , 4‐6% for N 2 , 5‐9% for Ar, 2% for CH 4 and 7‐27% for CO. The precision of the isotopic analyses was estimated to be 0.05‐0.2‰ for 13 C/ 12 C ratio of CO 2 . Also the precision was 0.9 and 0.05‐0.11‰ for D/H and 18 O/ 16 O ratios of the condensates, respectively. The results in this study suggest that careful sampling and analysis of volcanic gases provide reliable chemical and isotopic compositions that can be used to monitor volcanic activities.

Non-volcanic hot springs are generally believed to originate through circulation of meteoric or buried sea water heated at depth. In this study, we report the geochemical characteristics of the Arima and Takarazuka hot spring waters, known as Arima-type deep brine, in a forearc region of southwestern Japan. We examine 14 water samples to determine the levels of 12 solute elements or components and the isotopic ratios of H, He, C, O, and Sr, and we perform correlation analysis of the data to deduce the source materials and origin of the deep brine. Moreover, we perform numerical modeling of oxygen and hydrogen isotopic fractionation along subducting slabs to examine the composition of slab-derived fluid as a possible candidate of the deep brine. The results suggest that the high salinity and solute concentrations with characteristic oxygen, hydrogen, carbon, and strontium isotope compositions, as well as high 3He/4He ratios, can be explained by a dehydrated component of the subducted Philippine Sea slab. Hence, this study may provide an invaluable understanding of geofluid processes over a significant depth range.

Chemical analyses of 30 melt inclusions from Satsuma-Iwojima volcano, Japan, were carried out to investigate volatile evolution in a magma chamber beneath the volcano from about 6300 yr BP to the present. Large variations in volatile concentrations of melts were observed. (1) Water concentration of rhyolitic melts decreases with time; 3–4.6 wt.% at the time of latest caldera-forming eruption of Takeshima pyroclastic flow deposit (ca. 6300 yr BP), 3 wt.% for small pyroclastic flow (ca. 1300 yr BP) of Iwodake, post-caldera rhyolitic dome, and 0.7–1.4 wt.% for submarine lava eruption (Showa-Iwojima) in 1934. (2) Rhyolitic melts of the Takeshima and Iwodake eruptions contained CO2 of less than 40 ppm, while the Showa-Iwojima melt has higher CO2 concentration of up to 140 ppm. (3) Water and CO2 concentrations of basaltic to andesitic melt of Inamuradake, a post-caldera basaltic scoria cone, are 1.2–2.8 wt.% and ≤290 ppm, respectively. Volatile evolution in the magma chamber is interpreted as follows: (1) the rhyolitic magma at the time of the latest caldera-forming eruption (ca. 6300 yr BP) was gas-saturated due to pressure variation in the magma chamber because the large variation in water concentration of the melt was attributed to exsolution of volatile in the magma prior to the eruption. Iwodake eruption (ca. 1300 yr BP) was caused by a remnant of the caldera-forming rhyolitic magma, suggested from the similarity of major element composition between these magmas. (2) Volatile composition of the Showa-Iwojima rhyolitic melt agrees with that of magmatic gases presently discharging from a summit of Iwodake, indicating the low pressure degassing condition. (3) The degassing of the magma chamber by magma convection in a conduit of Iwodake during non-eruptive but active degassing period for longer than 800 years decreased water concentration of the rhyolitic magma. (4) Geological and petrological observations indicate that a stratified magma chamber, which consists of a lower basaltic layer and an upper rhyolitic layer, might have existed during the post-caldera stage. Addition of CO2 from the underlying basaltic magma to the upper gas-undersaturated (degassed) rhyolitic magma increased CO2 concentration of the rhyolitic magma.

To understand degassing processes during the 2000 Miyakejima volcanic activity, we applied the following two methods : 1) repeated analyses of adhered water-soluble gas component such as SO4and Cl ions on ashes produced at eruptions from July 8 and mid-September, and 2) SO2 flux measurements by COSPEC since August 26.The repetitive analyses of soluble component show remarkable changrd in Cl/S. Until August 18, the adhered SO4 concentrations are quite high and Cl concentrations are always low with Cl/S of 0.01-0.05, indicating that a certain mass of groundwater existed in aquifers beneath the summit crater and Cl component selectively dissolved in the groundwater. Since August 29, chlorine concentrations became greater and Cl/S ratios were determined as 0.1-0.14 on August 29 and 0.5-1.5 in September, which coincided with the strong volcanic gas emissions which started in mid-August. Groundwater boiling and establishment of gas conduit are likely to occur to prevent HCl from being absorbed in the groundwater.Sulfur dioxide emission rate has been monitored since August 26. The SO2flux increased in midSeptember from thousands to tens of thousands tons a day. The average SO2 flux after midSeptember to the present is 48 ktons/day. The highest flux was observed on December 7 to be 230 ktons/day. The mass rate of the magma degassing is estimated as 20 Mtons/day and the total volume of the degassed magma is calculated to be 1 km3 so far. The continuous magma degassing without eruptions occurs at a shallow environment by the convective transport of magma from a chamber to a magma head through conduits. A huge degassing rate is likely to be due to a large surface at the magma head, which would be made by a piston-cylinder type of collapse of the volcanic body.

To understand the eruptive mechanism of the 2000 Miyakejima volcanic activity, we conducted intensive geological, petrographic, and mineralogical studies on the pyroclastics of the August 18 eruption. Volcanic ashes, which were rich in accretionary lapilli, covered most of the islands. Cauliflower-shaped bombs and lapilli were ejected along with accidental lava blocks. Black-colored angular scoriaceous particles with abundant vesicles 10 -100 μm in diameter are found among ashes, comprising about 40 wt. % of total constituents. These bombs, lapilli, and black ashes have identical bulk chemical compositions and constituent mineral compositions, suggesting a common origin. Existence of oxidized ashes and accretionary lapilli attached to a large flattened bomb and chemicallyreacted anhydrite particles trapped in the voids of bombs suggest that bombs were still hot and ductile when they were emplaced on the ground. We, therefore, conclude that the August 18 eruption was a phreatomagmatic eruption and cauliflower-shaped bombs and black ashes were essential magmatic materials. Significant SO2 emissions from the volcano after August 18 also suggest convective upwelling of magmas to a shallower level beneath the volcanic edifice. We propose a magma-ascending model in which vesiculating magmas continuously ascend through the wall of subsided piston-like blocks.

Among the series of eruptions at Miyakejima volcano in 2000, the largest summit explosion occurred on 18 August 2000. During this explosion, vesiculated bombs and lapilli having cauliflower-like shapes were ejected as essential products. Petrological observation and chemical analyses of the essential ejecta and melt inclusions were carried out in order to investigate magma ascent and eruption processes. SEM images indicate that the essential bombs and lapilli have similar textures, which have many tiny bubbles, crystal-rich and glass-poor groundmass and microphenocrysts of plagioclase, augite and olivine. Black ash particles, which compose 40% of the air-fall ash from the explosion, also have similar textures to the essential bombs. Whole-rock analyses show that the chemical composition of all essential ejecta is basaltic (SiO2=51–52 wt%). Chemical analyses of melt inclusions in plagioclase and olivine phenocrysts indicate that melt in the magma had 0.9–1.9 wt% H2O

A 40-year-long record of the sulfur dioxide (SO2) emission rate of Asama volcano, Japan, is presented including high-temporal-resolution data since the 2004 eruption. The 2004 and 2008–2009 eruptive activities were associated with high SO2 emission, and SO2 emission rates markedly fluctuated. In contrast, stable and weak SO2 emissions have been observed for the rest of the investigated interval. The fluctuation of the SO2 emission rates is correlated with the number of shallow low-frequency B-type earthquakes, implying that increased flows of gas and/or magma induced the B-type earthquakes along the shallow conduit. The total volumes of outgassed magma during the 2004 and 2008–2009 eruptive activities are estimated to be 1.9 × 108 and 1.5 × 108 m3, respectively. These volumes are about 100–200 times larger than those of the erupted magma, indicating that the large volumes of the magma were outgassed without being erupted (i.e., excess degassing/outgassing). Degassing and outgassing driven by magma convection rather than by permeable gas flow in the conduit is concluded as the probable degassing/outgassing process of Asama volcano based on model examinations, and is thought to occur regardless of the outgassing intensity. Production rates of outgassed magma related to the 2004 and 2008–2009 eruptive periods are estimated to have been 7.4 × 103 and 6.7 × 103 kg/s, respectively. These values are one order of magnitude higher than the average production rate of 0.92 × 103 kg/s for the inactive periods. Increased supply of fresh magma is thought to activate magma convection in the conduit and to thereby increase magma degassing/outgassing.

In hydrothermal quartz from the Kaneuchi tungsten deposit of Honsyu island, Japan, the smaller primary fluid inclusions have the power δ18O values. This correlation is well accounted for by a quantitative model based on (isotropic or anisotropic) 18O diffusion through quartz. The model yields a room temperature 18O diffusivity that is close to the extrapolated diffusivity of water molecules measured by Cordier (1988) but much higher than the extrapolated diffusivity of structural oxygen measured by Farver and Yund (1991). The observed correlation between the size of primary fluid inclusions in quartz and their δ18O(water) is thus, well explained by relatively fast diffusion of oxygen as water molecules.

[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 3He flux is estimated by an applied method using helium isotopes. The three-dimensional distribution of 3He flux can be obtained as well as that of the concentrations. High 3He 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 3He flux with high concentration in deep groundwater suggests the slow and long-term accumulation of magmatic volatiles. High 3He 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 3He flux, and the total supply rates are estimated to be 2.3 × 10−3 (3He), 5.1 × 108 (magmatic C), 1.3 × 108 (Cl), and 1.3 × 108 (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.

Excess degassing of magmatic H2O and SO2 was observed at Izu-Oshima volcano during its latest degassing activity from January 1988 to March 1990. The minimum production rate for degassed magma was calculated to be about 1×104 kg/s using emission rates of magmatic H2O and SO2, and H2O and S contents of the magma. The minimum total volume of magma degassed during the 27-month period is estimated to be 2.6×108 m3. This volume is 20 times larger than that of the magma ejected during the 1986 summit eruption. Convective transport of magma through a conduit is proposed as the mechanism that causes degassing from a magma reservoir at several kilometers depth. The magma transport rate is quantitatively evaluated based on two fluid-dynamic models: Poiseuille flow in a concentric double-walled pipe, and ascent of non-degassed magma spheres through a conduit filled with degassed magma. This process is further tested for an andesitic volcano and is concluded to be a common process for volcanoes that discharge excess volatiles.

The chemical and isotopic (D, 18O, 34S) compositions were determined for 19 fumarolic gases, 11 hot spring waters and 3 meteoric waters collected from Satsuma-Iwojima volcanic island, Japan. The fumarolic gases discharge over a wide range of temperature (from 877 to 99°C), but are likely to be derived from a common parent gas with a deduced composition of (μmol/mol): H2O:975000, CO2:3800, total S:9800 (with an average oxidation state of 3.4), HCl:5800, HF:300, H2:5000, CO:15, He:0.05, Ar:0.25, and N2:60. The variation in gas composition from different fumaroles may largely be explained in terms of re-equilibration at lower temperature, addition of earlier deposited elemental S and mixing with meteoric waters at shallow depths. The chemical and isotopic composition of the highly acid hot spring discharges, pH 1.2 to 1.8, indicate that they are formed through absorption of Cl-depleted volcanic gas into local groundwaters in close to equal amounts.