The degassing state of the Earth's interior is one of the main issues in relation to the evolution of the Earth and has been discussed by many investigators over several decades. I outline some trials to reveal the state of degassing based on noble gas isotopes. Then, based on noble gas isotope signatures so far obtained, I discuss models of the chemical structure of the Earth's interior which are inevitably related to the degassing state of the Earth. I propose a preferred model which suggests the possibility for the occurrence of relatively undegassed and possibly less fractionated reservoir(s) in the Earth's deep interior, which might be located in a relatively non-convective mantle.

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This review gives an overview of the estimates of volatile emissions from arc and mid-ocean ridge volcanoes to the atmosphere and hydrosphere with particular focus on H₂O, CO₂, N₂, Cl and F. The gas compositions of high temperature (>500°C) fumaroles are compiled and used to derive magmatic H₂O/SO₂, CO₂/SO₂, HCl/SO₂ and HF/SO₂ ratios on an arc-by-arc basis to obtain new estimates of major volatile fluxes from arcs globally. The estimate of F flux from arcs is two orders of magnitude smaller than the amount of F released from mid ocean ridges whereas the arc Cl flux exceeds the ridge flux. An important observation is that globally the water budget of subduction zones seems to be balanced and the amount of water degassed through arc volcanism is within the estimates of the amount of water released from the slab below the volcanic front. Recent work that focused on the Central American arc shows that detailed knowledge of the subduction input compositions, coupled with gas emission studies is critical to further constrain the fate of volatiles during the subduction processes.

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We have carried out noble gas analysis for gases released by a high-resolution stepwise crushing on basaltic glass and olivine separated from a new set of pillow basalts collected form the Loihi seamount, Hawaii. One basaltic glass sample yielded quite consistent ³He/⁴He ratios of about 35 Ra (Ra denotes the atmospheric ratio) for helium released over six successive crushing steps. This ratio is the highest among those so far reported for Loihi samples, suggesting that this sample represents the most primitive Loihi magma in terms of helium isotope composition. In contrast, the argon isotope ratios (⁴⁰Ar/³⁶Ar) observed in this sample appeared to be low (<400) compared with the ratio suggested for the source of Loihi magma (i.e., 8000 by Trieloff et al., 2000), implying a significant contribution from the atmospheric argon. A clear binary mixing trend was defined between ³He/³⁶Ar and ⁴⁰Ar/³⁶Ar ratios measured during successive crushing steps, which is extending from an atmospheric component to a mantle-derived component. This mixing trend permits us to estimate ⁴⁰Ar/³⁶Ar ratios of the mantle by assuming the mantle ³He/³⁶Ar ratio of 0.7 (Moreira et al., 1998). For the Loihi basaltic glass sample with 35 Ra, the ⁴⁰Ar/³⁶Ar ratio without atmospheric contribution is estimated to be 3000, whereas the ratio estimated for the olivine separates with ³He/⁴He ratio of 27 Ra appeared to be 11000. We found that ³He/⁴He and ⁴⁰Ar/³⁶Ar (corrected for air addition) ratios determined for the present samples, as well as for other plumes (Reunion and Iceland) and MORB-popping rock, plot on a rough negative trend, suggesting that helium and argon isotope ratios in plume-derived samples might be controlled by a variable contribution from the MORB-type component to the plume endmember. Extrapolation of this trend to ³He/⁴He of 50 Ra as found in the Baffin Island picrite (Stuart et al., 2003) yielded the plume source ⁴⁰Ar/³⁶Ar ratio of about 1000, suggesting that the source region of the high ³He/⁴He mantle plume might have ⁴⁰Ar/³⁶Ar ratio being significantly less radiogenic than has usually been anticipated.

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The ³He/⁴He ratios (Ra) of natural gas, volcanic fluid, and groundwater are compiled in the Japanese Islands and their geographical distributions are discussed in the tectonic frame work of subduction zone together with precisely-determined seismic velocity structures. In Northeastern (NE) Japan where typical island arc signatures are developed, there is a clear contrast of ³He/⁴He ratios perpendicular to the trench axis, low-Ra in the frontal arc and high-Ra in the volcanic arc. This may reflect the presence or absence of magma with high-Ra in the shallow crust. As a carrier of primordial helium, source melt may be generated in low-V zone of the wedge mantle by dehydration of Pacific slab at about 150 km deep and may flow upward sub-parallel to the slab, which is well constrained by S-wave velocity perturbation. In the Chugoku and Shikoku districts of Southwestern (SW) Japan, there is a geographical contrast of Ra similar to NE Japan except for the region at about 100 km from the volcanic front where medium-Ra was found. High-Ra observed in volcanic arc of the Chugoku district may be attributable to the mantle helium derived from the magma source generated below the Philippine Sea slab. Medium-Ra in the Shikoku district is explained by dehydration of the young slab with a moderate aging effect. These features are again consistent with the results of seismic tomography. In the Kinki district of SW Japan, anomalously high-Ra was observed in the frontal arc region that was called by “Kinki Spot”. Since the high-Ra is located at much wider region from the volcanic front when compared with NE Japan, the melt generated below the Philippine Sea slab may penetrate into the fissure of the slab tear and may arrive at the shallow crust by upwelling flow.

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The relationship between geological structure and helium isotopes is discussed for deep groundwaters from the Osaka sedimentary basin, southwest Japan, to understand dissolved He in groundwater for hydrological application. Although this area shows no Quaternary volcanic activity, nearly upper mantle-like ³He/⁴He ratio (1.1 × 10^-5) has been observed in the area along the fault system where the basement rocks are outcropped. In contrast, deep groundwaters from the lowest part of the aquifers beneath the Osaka Basin showed a wide variation in ³He/⁴He ratio (0.27-8 × 10^-6), which seems to reflect the geological structure. The moderately high ³He/⁴He ratios, not as high as those in the mantle value, are due to a contribution of radiogenic ⁴He within the aquifer. The flux of mantle He through the Uemachi thrust fault into the aquifers in the east block (hanging wall of the fault) of the Osaka Basin will be smaller than those in the west block (foot wall), because mantle He through this fault encountered the aquifers in the west block and dissolved in this aquifer. The model for the Osaka Basin presented in this study implies that there are at least two different sources of He flux into an aquifer; mantle He flux through the fault system and crustal He from the underlying formation. The former should be spatially heterogeneous, while crustal ⁴He flux is rather spatially constant throughout the lowermost part of the Osaka Basin.

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The northwestern region of the Iberian Peninsula was traditionally considered a seismically stable area. However, the 1995 and 1997 Galicia earthquakes of magnitude 4.6 and 5.3 produced significant damage in the region (intensity EMS: V), and these events changed the seismic hazard map used for the building code in Spain. Searching for earthquake precursors of these seismic events was performed, and precursory geochemical signatures of the 1995 and 1997 Galicia earthquakes were detected. These precursory signatures were observed in CO₂-rich groundwaters from a 120 m deep well at Cabreiroá (Ourense, Galicia), ∼90 km distance from the epicentres. Secular variations of the dissolved Cl^- in these CO₂-rich groundwaters showed clearly an increase trend three months before the occurrence of two earthquakes of magnitude 4.6 (November 29 and December 24, 1995) in Lugo (Galicia). After the precursory variations of Cl^- prior to the earthquakes in 1995, a continuous radon monitoring system was installed at Cabreiorá's well. A significant increase of radon activity in the CO₂-rich ground waters was detected four weeks prior to the magnitude 5.3 Galicia earthquake of May 21, 1997. Precursory changes in the chemical and isotopic composition of terrestrial fluids have been mainly attributed to the development of microcracks caused by the regional tectonic strain. The occurrence of these precursory geochemical signatures of CO₂-rich groundwaters of magmatic origin in the NW Iberian peninsula probably suggests that high fluid pressure at depth play a major role in triggering earthquakes in the region.

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Simultaneous monitoring of the gas concentration and water level in groundwater was achieved at the Omaezaki open well. Coaxially arranged pipes inserted into the well enabled us to pump groundwater directly from an aquifer and to return it the aquifer without disturbing the groundwater level. Concentration of methane, as one subsurface-origin gas, was monitored using a quadrupole mass spectrometer equipped with a gas separation tube. The methane concentration intermittently showed spike-like increases during the observation period. The temporal change of the methane concentration was independent of the seismic activity around the station, the groundwater temperature, the ambient temperature, and the pumping rate, with the exception of the groundwater level. The methane concentration was associated with the difference of the groundwater level, of which the amplitude of a barometric response is greater than that of a tidal one. Spike-like increases of the methane concentration were explained using a logistic model as a function of the groundwater level change.

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Evolution of CO₂ in Lakes Monoun and Nyos (Cameroon) before and during controlled degassing is described using results of regular monitoring obtained during the last 21 years. The CO_2(aq) profiles soon after the limnic eruptions were estimated for Lakes Monoun and Nyos using the CTD data obtained in October and November 1986, respectively. Based on the CO_2(aq) profiles through time, the CO₂ content and its change over time were calculated for both lakes. The CO₂ accumulation rate calculated from the pre-degassing data, was constant after the limnic eruption at Lake Nyos (1986-2001), whereas the rate appeared initially high (1986-1996) but later slowed down (1996-2003) at Lake Monoun. The CO₂ concentration at 58 m depth in Lake Monoun in January 2003 was very close to saturation due to the CO₂ accumulation. This situation is suggestive of a mechanism for the limnic eruption , because it may take place spontaneously without receiving an external trigger. The CO₂ content of the lakes decreased significantly after controlled degassing started in March 2001 at Lake Nyos and in February 2003 at Lake Monoun. The current content is lower than the content estimated soon after the limnic eruption at both lakes. At Monoun the degassing rate increased greatly after February 2006 due to an increase of the number of degassing pipes and deepening of the pipe intake depth. The current CO₂ content is ∼40% of the maximum content attained just before the degassing started. At current degassing rates the lower chemocline will subside to the degassing pipe intake depth of 93 m in about one year. After this depth is reached, the gas removal rate will progressively decline because water of lower CO_2(aq) concentration will be tapped by the pipes. To keep the CO₂ content of Lake Monoun as small as possible, it is recommended to set up a new, simple device that sends deep water to the surface since natural recharge of CO₂ will continue. Controlled degassing at Lake Nyos since 2001 has also reduced the CO₂ content. It is currently slightly below the level estimated after the limnic eruption in 1986. However, the current CO₂ content still amounts to 80% of the maximum level of 14.8 giga moles observed in January 2001. The depth of the lower chemocline may reach the pipe intake depth of 203 m within a few years. After this situation is reached the degassing rate with the current system will progressively decline, and it would take decades to remove the majority of dissolved gases even if the degassing system keeps working continuously. Additional degassing pipes must be installed to speed up gas removal from Lake Nyos in order to make the area safer for local populations.

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The chemical trap method has been used for monitoring of volcanic activity. In this method, a strong alkaline water solution was exposed in ambient air. Acidic gases, such as SO₂, H₂S, HCl and HF are absorbed in KOH solution without any instrumental support. For sulfur isotopic monitoring with this method, however, a sulfur isotopic fractionation between fumarolic gas and sulfur trapped in KOH solution occurs (Sakai and Ueda, 1984). In this study, the magnitude of isotopic fractionation was experimentally determined for SO₂ and H₂S gases. The δ³⁴S of SO₂ and H₂S gases absorbed by KOH solution is, respectively, 2.4 and 6.5‰ lower than the gases in air, which agreed with the theoretical prediction based on molecular diffusion of SO₂ and H₂S in air. The KOH solution trap method was applied to the volcanic gas at Miyakejima. The δ³⁴S of the KOH solution gradually increased from 2001 to the summer of 2002, then abruptly decreased. After the summer of 2002, the δ³⁴S of KOH solution again gradually increased until 2005. The increase of δ³⁴S in both periods can be explained by open system degassing of magma. A value of -0.3 to -1.0 for 1000lnα_gas-magma produces the increase in the δ³⁴S, where α_gas-magma is the fractionation factor between gas and magma. The fractionation factor α_gas-magma is a function of oxygen fugacity (ΔNNO). The value of +0.6 to +1.2 for ΔNNO required to obtain -0.3 to -1.0 for 1000lnα_gas-magma is significantly higher than the oxygen fugacity of magma estimated petrologically by Yasuda et al. (2001), suggesting a disequilibrium between gas and magma in terms of oxygen fugacity. The intrinsic δ³⁴S_CDT was estimated to be +4.4 to +5.7‰ for the magma of Miyakejima, which is consistent with the value of Quaternary volcanoes in the Japanese island arc estimated by Ueda and Sakai (1984).

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Remote measurements of chemical compositions of the Aso volcanic plume using an FT-IR spectral radiometer were carried out at the 1st crater of Mt. Nakadake, Aso volcano, Japan, for six times from 1996 to 2003. We have succeeded in detecting 6 volcanic gas species (SO₂, HCl, HF, CO, CO₂, COS) using the InSb detector. The equilibrium temperatures estimated from observed CO/CO₂ ratios have remained high at more than 700°C during the observation period. However, the temporal variation of the ratios between volcanic gas components showed significant scrubbing of HCl compared to other species by hydrothermal interaction in 1998. From the visual and FT-IR observations, we presume that SO₂ flux from the volcano was lower in 1998 than in other years. This decrease in flux was not due to hydrothermal scrubbing of SO₂ but was due to decrease in a total supply rate of gas from depth. This assumption is supported by relatively stable CO₂/SO₂ ratio which is probably reflecting stable gas chemistry at the gas source.

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