On the basis of recent high quality noble gas experimental data, we propose a scenario for the origin of terrestrial noble gases. The isotopic compositions of non-radiogenic, terrestrial noble gases (except for atmospheric neon) were established, prior to the Earth's accretion, in planetesimals by gravitational capture of solar-type noble gases from the solar nebula. Gravitational capture by planetesimals produces a pattern in which the strongest fractionations occur in the heaviest elements. After the formation of the Earth, neon was further fractionated during hydrodynamic escape of hydrogen from the primitive atmosphere to give rise to the present atmospheric neon isotopic composition. Therefore, except for neon, isotopic compositions of nonradiogenic noble gases both in the atmosphere and the solid earth are the same. Noble gas degassing from the solid earth occurred during the magma ocean stage through bubble (gas)/melt partition. Subsequent magma production and eruption throughout geological time has contributed little to atmospheric noble gases except for radiogenic components. From the comparision of ³He/⁴He ratios between the upper and lower mantles, we estimate that the ⁴⁰Ar/³⁶Ar ratio in the lower mantle is more than 6, 000. Hence, we argue that the lower mantle has also undergone some degree of degassing.

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Noble gas isotopic ratios and their abundance patterns reveal the characteristics of their sources and processes that have modified them. Although isotopic signatures reflect the characteristics of source materials, absolute abundances and their relative patterns are controlled by magmatic processes such as the partial melting, fractional crystallization, transportation and degassing. The degree of partial melting affects the concentration of noble gases in a lava. However, this effect is apparent only for heavier noble gases (Ar, Kr, Xe), in the case of Loihi samples, because kinetic processes work more effectively for lighter noble gases (He, Ne). Further, the amounts of atmospheric Ar observed in volcanic rocks are sometimes lower than those expected from the solubility data for Ar in the atmosphere at the surface. This suggests that the samples did not equilibrate with the atmospheric Ar, and the degassing of the magmatic Ar is incomplete. The ³He/⁴He-³He/³⁶Ar diagram indicates that the ³He/³⁶Ar ratios are quite variable even though the ³He/⁴He ratios are relatively uniform for MORBs and for some samples from hot spot areas. The higher ³He/³⁶Ar ratios observed for dunites relative to olivine phenocrysts could be a result of secondary introduction of He into dunites from surrounding magma.

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A simple model for equilibrium noble gases partitioning amongst solid, melt, and gas phases, and extraction of the gas (vesicles) from the system was used to study the relationships between the rate of noble gas fractionation and parameters controlling the degassing process.. The following results are inferred: (i) the lower the ratio of a gas-phase volume (vesicles) divided by the volume of melt, the higher the degree of noble gas fractionation in the melt; (ii) relative excess of both lighter (Ne) and heavier (Xe) noble gases over Ar occurs providing a high solid/melt partition coefficient for Xe and a small melt fraction.

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He, Ar, Sr, Nd and Pb isotopic ratios have been determined for a set of submarine basaltic glasses from the Gulf of Tadjoura and subaerial lavas from Afar, Republic of Djibuti. Rare gases were recovered by vacuum crushing and analysed with a new analytical system. Helium isotopic ratios up to 15 Ra show the occurrence of a “high ³He” hot spot beneath Afar and indicate the occurrence of a plume originating from a gas-rich, presumably deep, region of the mantle. He-Ar isotope systematics indicate a mixing between three geochemical end-members: the plume component, enriched in ³He, a radiogenic, possibly crustal, component, and the atmosphere. Low ⁴⁰Ar/³⁶Ar ratios are interpreted as the result of selective atmospheric contamination of Afar magmas, but the process of contamination is unclear. In a He-Sr-Nd-Pb space, Afar data are plotted in a field similar to basic lavas emitted in Iceland (Djibuti territory) and in the Reykjanes Ridge (Gulf of Tadjoura) and differ by their He isotopic ratios from lavas emitted in EM (e.g., Gough and Tristan da Cunha) and HIMU (e.g., Saint Helena and Tubuaii) hot spots analysed so far.

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The noble gas isotopic compositions of xenoliths sampled from Oki-Dogo island and Ichinomegata crater, Japan, have been measured to understand better the origin of noble gases in the mantle wedge and lower crust beneath the Japanese Island Arc which represents a typical convergent plate boundary. The ³He/⁴He ratios of lherzolites and a websterite derived from the upper mantle are the same as MORB-type He isotopic ratios, i.e., 8 ± 1 times the atmospheric value. The ⁴⁰Ar/³⁶Ar ratios (307–1870) of these samples are generally lower than that assumed for MORB-type Ar (>20000). These results indicate that the He and Ar initially present in the mantle wedge were similar to the MORB-type gases but were later contaminated by atmospheric noble gases in the subducting slab. This resulted in the decreased ⁴⁰Ar/³⁶Ar ratios and a negligible change in ³He/⁴He ratios. A high He concentration (1.2 × 10^–6 cm³ STP/g) and ³He/⁴He ratio (10.5 × 10^–6), similar to arc volcanic gases, was observed in an amphibolite sample which represents lower crust beneath Ichinomegata. This He isotopic ratio implies that magmatic noble gases associated with arc volcanism do not ascend directly from the mantle wedge but from the lower crust where they accumulate.

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More than hundred carbon isotopic data were examined statistically for the CO₂ discharges from the Carpathian Basin, central Europe. The δ¹³C values from the central Carpathian Basin show a normal distribution with the average value of –5 ± 2.5‰, whereas those from north-eastern part of Slovakia show a distribution slightly skewed towards heavier values. The average δ¹³C value indicates the mantle origin of the carbon, despite the lack of recent volcanic activity in the region. This is consistent with the existence of mantle He in the CO₂ discharges.

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Volcanic and hot spring gases and mineral waters containing dissolved CO₂ were collected from the Unzen volcanic area in the Shimabara peninsula, Kyushu, Japan, before and after the Mt. Fugen eruption of dacitic magma on November 17, 1990. The relationships between the ³He/⁴He and ⁴He/²⁰Ne ratios for samples collected from the eastern part of the peninsula including Mt. Fugen indicate that a mixing has occurred between air and present magmatic gas with the ³He/⁴He ratio of 10 × 10^–6, which is one of the highest values reported for volcanic and geothermal gases in Japanese island. On the other hand, the gases from two spas in the western part before and a year after the onset of the eruption show the mixings between air and gases with lower ³He/⁴He ratios of 6 × 10^–6 and 7 × 10^–6, having no effects by presently active magma. The seismic data related to the present eruption suggest that the present magma migrated from the west to the center of the Shimabara peninsula during the period of Nov. 1989 to May, 1991. These incompatible observations suggest that there are several independent channels for magmatic He to reach the earth's surface. The gases sampled from Obama and Unzen spas in the western part of Shimabara peninsula have been contaminated with ⁴He which had accumulated in cracks along the ascending route from a deep-seated magma having non-contaminated ³He/⁴He ratios of 10 × 10^–6.

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The chemical and isotopic compositions were determined for fumarolic gas samples collected between May 1988 and January 1991 from the summit (“A” terrace) and from the south caldera rim of Izu-Oshima volcano, Japan. The contribution of meteoric water and/or seawater to the “A” terrace fumarolic discharges increased as the fumarole temperature decreased. A large contribution of seawater to the fumarolic gas was inferred during the period from December 1988 to May 1989. The fumaroles which first appeared in June 1989 on the south caldera rim discharge the water vapor derived from local meteoric water. The isotopic compositions of water vapor followed a Rayleigh fractionation process; a part of vapor condensed during ascent in a gas conduit. A scenario for the changes in the discharge system of fumarolic gases at Izu-Oshima volcano is proposed as follows: The proportion of groundwater to the “A” terrace fumarolic gas increased with time as the magma head subsided. The fracture network which formed beneath the caldera during 1986 eruption may have allowed seawater incursion from the coast to the central part of the island. Loss of a large amount of groundwater in the form of vapor from the summit crater may have accelerated infiltration of seawater. The discharge system developed over a wider area as the deeper magma head subsided.

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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.

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Terrestrial gas samples from thermal springs located along and behind the volcanic arc of Japan have a variable chemical and isotopic composition, largely reflecting their different positions with respect to a convergent plate boundary. The CO₂ largely originates from subduction-related magmatism, though in N₂-rich gases the CO₂ may also have been derived from an organic sedimentary source. The chemical and isotopic compositions of the CO₂-CH₄ pair indicate that both isotopic and chemical equilibrium are commonly attained (and maintained) in the CO₂-rich gases, whereas CH₄ in the N₂-rich gases appears to be affected by bacterial oxidation. In some samples with low contents of CH₄, heavy δ¹³C_CH4 (–13 to –17‰) may result from quenching from magmatic conditions, though bacterial oxidation cannot be ruled out. In combination with the chemical composition of the thermal gases, noble gas isotopic signatures indicate two sources: a magmatic source containing a mantle component and gases from melting of the subducting slab; these are mixed with atmospheric gases near the surface.

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The fluctuating behaviour of the volcanic system of Vulcano, which caused changes in chemical-physical properties of surface manifestations, moderate ground inflation and seismicity, is interpreted to be the result of different contributions of the deep-seated magma (fire) and the near-surface aqueous environment (water). The phenomena observed during the last fifteen years are explained by variations in these two contributions. A factor analysis method was applied to the variation of temperature and chemical composition of fumarolic gases. The results support the model. The absence of major seismic events rules out eruptive activity caused by ascending magma. Significant thermal output from the deeper portions of the system into shallow aquifers, however, caused vaporization of water and, consequently, pressure increase and inflation of the volcano. Landslides in the weakest sectors of the crater slopes can be caused by inflation. The resulting pressure decrease beneath the volcano may have triggered the emission of gases.

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Thermodynamic evaluation and restoration procedures are applied to a set of 10 volcanic gas analyses obtained by modern collection and analytical methods. The samples were collected from a vigorously fuming fissure during episode 1 of the Puu Oo eruption of Kilauea Volcano in 1983. Collections were made through titanium and mullite tubing at 895–935°C in evacuated bottles containing ⁴N NaOH solution. A variety of analytical techniques were used to determine the gas compositions. In most samples, the combined amounts of N₂+Ar+O₂ are far less abundant than H₂, CO, or H₂S, suggesting little or no contamination or reaction with atmospheric gases. Analyses of samples collected through titanium and mullite tubing are similar, indicating chemical alterations from gas-titanium interactions were insignificant. Thermodynamic evaluation shows that 6 of the 10 analyses are equilibrium compositions, and 4 analyses are disequilibrium compositions. Three of the disequilibrium analyses involve samples affected by minor spilling of NaOH solution from the sample bottles during collection. The deviation of these analyses from equilibrium is dominated by the effects of disequilibrium water-loss. The fourth disequilibrium analysis, is contaminated with meteoric water. In all 4 cases, the restoration procedures retrieve the original equilibrium compositions. Minor loss of sulfur by elemental S precipitation is restored for all samples. All gas samples approach closely to equilibrium compositions at temperatures up to 117°C above collection temperatures. The equilibrium preserved in the gases, however, actually reflects oxygen buffering of the gases by coexisting lavas, not just simple internal homogeneous equilibrium among the gas species. The oxygen buffering process is not evident if oxygen fugacities or R_H redox parameter values are calculated and plotted directly from the analytical data. It is only evident after thermodynamic evaluation and restoration of the analyses. Direct use of gas analytical data in thermodynamic calculations and diagrams without prior evaluation of equilibrium/disequilibrium and restoration is not recommended, despite the undeniable improvements in collection and analytical techniques.

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During the survey on the eruption of the volcano “La Soufriere de la Guadeloupe” (French West Indies), the behaviour of sulfur in volcanic gases was examined. Matsuo (1962) studied chemical equilibrium in volcanic gases which led us to develop the “in situ” gas analysis. A field gas chromatograph allowed direct injection of hot gases, before water and sulfur condensation occurred. A silica tube equipped with thermocouples was used for sublimates sampling and for measuring the condensation temperature. Conventional condensors and caustic soda bottles were used for sampling and later complementary analyses in the laboratory. A free energy minimisation computational method modelled the physical and chemical changes that occurred during cooling of volcanic gases. The high temperature composition of the .gas mixture was recalculated from the concentrations of the gaseous and solid components obtained during sampling. The equilibrium composition was first calculated at the collection temperature for 22 elements. The model then calculated the equilibrium compositions at 50°C intervals using the residual gas composition after condensation at the previous temperature. The depositional sequence observed in the silica tube depend on the temperature and the concentration of elements in the initial mixture. The computational method was applied to gases sampled from Mt. St Helens. The calculated results agree with observed sublimates. A new method for volcano monitoring is proposed which allow to determine the magmatic origin of volcanic gases and their emission temperature from remote plume analysis. The model is also applicable to estimate the temperature and the composition of the gases entering hydrothermal systems or participating in ore deposits in the basement of the volcano. The model predicts the behaviour of the main and minor species emitted in the volcanic gases. This approach is not only restricted to the volcanic gas studies but can be applied to studies of high temperature reactive gas reactor to simulate cooling reactions.

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Sublimates were sampled from high-temperature (up to 800°C) fumaroles at Merapi volcano, Indonesia in January 1984. Sampling is accomplished by inserting silica tubes into high-temperature vents. Volcanic gas flows through the tubes and sublimates precipitate on the inner walls in response to the temperature gradient. With decreasing temperature (800–500°C) in the tubes, there are five sublimate zones: 1) cristobalite, magnetite, and halite; 2) halite, sylvite, K-Ca sulfate (K₂Ca[SO₄]₂), acmite, wollastonite, and pyrite; 3) halite, sylvite, galena, Pb-Bi sulfide, aphthitalite, sphalerite, and Cs-K sulfate; 4) halite, sylvite, galena, PbKCl₃, aphthitalite, and Na-K-Fe sulfate ([Na, K]₂Fe[SO₄]Cl₂); and 5) halite, sylvite, galena, PbKCl₃, and various sulfates of Pb, Cu, and Zn. Texturally, the sublimate phases grade from large, well-formed crystals at their highest-temperature occurrence to more numerous, smaller crystals that are less perfect at lower temperatures. These changes imply that the crystal nucleation and growth rates increase and decrease, respectively, as temperature decreases. Several of the sublimate phases also exhibit highly anisotropic morphologies (whiskers, platelets, spheres), especially below their highest temperature of deposition. Overall, the textural data suggest that the gas is saturated or slightly supersaturated with the phases at their hottest occurrence, but that the gas becomes increasingly supersaturated with the phases at lower temperatures. The anisotropic morphologies probably form because the crystals grow toward more supersaturated conditions in the center of the tube. The gas fails to maintain equilibrium with the precipitating sublimates because the high velocity of the carrier gas prevents complete mass transfer from the gas stream to the tube walls.

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Arsenic contents were determined for volcanic gases, sulfur sublimates, and alteration products from Mts. Tokachi, Meakan, Tarumae, Esan, Unzen and Iwo-dake (Satsuma-Iwojima Island), in Japan. The gas sampled from >300°C vents contain between 700 and 4000 ppb As with a positive temperature dependence, regardless of their major compositions. However, the As contents of <300°C gases are scattered over a wide range, possibly because of contamination from or loss to sublimates and condensates. The As/S ratios in the sulfur sublimates are highly variable, without definite relations to those in the parent-gases. However, when this is coupled with the fact that only a small fraction of total sulfur is sublimated out of the gas, it is concluded that a large part of the emitted As escapes to the atmosphere. Alteration products are enriched in As relative to the fresh lava, due to the deposition of As-bearing sublimates and/or the condensation of volcanic gases on the surrounding altered rocks; also possibly due to gas-rock interactions. This study shows that As is an essential part of magmatic emanations which underwent almost no modification during ascent from magma top to the surface. Arsenic content in volcanic gases can be a good indicator for fumarolic activity.

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We have determined SO₂ concentration in fumarolic gas released from the summit crater of Asama volcano, Japan, using an infrared spectral radiometer installed in a site about 4 km away from the summit. During the observation period from 31 October to 4 November 1991, weak fumarolic activities with a height less than 100 m were observed intermittently. The infrared spectrum of the fumarolic gas just above the summit showed slight absorption due to rotational bands associated with v, vibration (1151 cm^–1) of the SO₂ molecule. Compared with the spectrum of standard SO₂ gas, this absorption corresponds roughly to the column amount of 250 ppm-m SO₂. This is the first result of remote detection of SO₂ in volcanic gas from a ground station by means of infrared spectroscopy. Infrared spectral radiometer is a promising tool for remote detection of various chemical species in volcanic gas.

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