Gas Isotopic Compositions Research Articles

Noble gas and carbon isotopes in Mariana Trough basalt glasses

Noble gas elemental and isotopic compositions have been measured as well as the abundance of C and its isotopic ratios in 11 glasses from submarine pillow basalts collected from the Mariana Trough. The 3He/ 4He ratios of 8.22 and 8.51 R atm of samples dredged from the central Mariana Trough (∼18°N) agree well with that of the Mid-Ocean Ridge Basalt (MORB) glasses (8.4±0.3 R atm), whereas a mean ratio of 8.06±0.35 R atm in samples from the northern Mariana Trough (∼20°N) is slightly lower than those of MORB. One sample shows apparent excess of 20Ne and 21Ne relative to atmospheric Ne, suggesting incorporation of solar-type Ne in the magma source. There is a positive correlation between 3He/ 4He and 40Ar/ 36Ar ratios, which may be explained by mixing between MORB-type and atmospheric noble gases. Excess 129Xe is observed in the sample which also shows 20Ne and 21Ne excesses. Observed δ 13C values of ∼20°N samples vary from −3.76‰ to −2.80‰, and appear higher than those of MORB, and the corresponding CO 2/ 3He ratios are higher than those of MARA samples at ∼18°N, suggesting C contribution from the subducted slab.

Factors controlling the origin of gas in Australian Bowen Basin coals

Open system pyrolysis (heating rate 10°C/min) of coal maturity (vitrinite reflectance, VR) sequence (0.5%, 0.8% and 1.4% VR) demonstrates that there are two stages of thermogenic methane generation from Bowen Basin coals. The first and major stage shows a steady increase in methane generation maximising at 570°C, corresponding to a VR of 2 – 2.5%. This is followed by a less intense methane generation which has not as yet maximised by 800°C (equivalent to VR of 5%). Heavier (C 2+) hydrocarbons are generated up to 570°C after which only the C 1 (CH 4, CO and CO 2) gases are produced. The main phase of heavy hydrocarbon generation occurs between 420 and 510°C. Over this temperature range, methane generation accounts for only a minor component, whereas the wet gases (C 2–C 5) are either in equal abundance or are more abundant by a factor of two than the liquid hydrocarbons. The yields of non-hydrocarbon gases CO 2 and CO are greater then methane during the early stages of gas generation from an immature coal, subordinate to methane during the main phase of methane generation after which they are again dominant. Compositional data for desorbed and produced coal seam gases from the Bowen show that CO 2 and wet gases are a minor component. This discrepancy between the proportion of wet gas components produced during open system pyrolysis and that observed in naturally matured coals may be the result of preferential migration of wet gas components, by dilution of methane generated during secondary cracking of bitumen, or kinetic effects associated with different activations for production of individual hydrocarbon gases. Extrapolation of results of artificial pyrolysis of the main organic components in coal to geological significant heating rates suggests that isotopically light methane to δ 13C of −50‰ can be generated. Carbon isotope depletions in 13C are further enhanced, however, as a result of trapping of gases over selected rank levels (instantaneous generation) which is a probable explanation for the range of δ 13C values we have recorded in methane desorbed from Bowen Basin coals (−51 ± 9‰). Pervasive carbonate-rich veins in Bowen Basin coals are the product of magmatism-related hydrothermal activity. Furthermore, the pyrolysis results suggest an additional organic carbon source from CO 2 released at any stage during the maturation history could “mix” in varying proportions with CO 2 from the other sources. This interpretation is supported by C and O isotopic ratios of carbonates that indicate mixing between magmatic and meteoric fluids. Also, the steep slope of the C and O isotope correlation trend suggests that the carbonates were deposited over a very narrow temperature interval basin-wide, or at relatively high temperatures (i.e., greater than 150°C) where mineral-fluid oxygen isotope fractionations are small. These temperatures are high enough for catagenic production of methane and higher hydrocarbons from the coal and coal-derived bitumen. The results suggests that a combination of thermogenic generation of methane and thermodynamic processes associated withCH 4/CO 2 equilibria are the two most important factors that control the primary isotope and molecular composition of coal seam gases in the Bowen Basin. Biological process are regionally subordinate but may be locally significant.

Noble Gases and Carbon Isotopic Composition of Fluid Inclusions from the Larderello Geothermal Field (Italy), Preliminary Data

The intracontinental geothermal field of Larderello (Tuscany, Italy) is a vapour-dominated system, which has been exploited for electricity production throughout this Century. At Lardereil0, studies on hydrothermal mineral assemblages and related fluid inclusions evidenced the presence of several fluid types of different sources and a complex evolution of the geothermal system, from early stage circulation, related to granite emplacement, to present-day condition (Valori et al., 1992; Cathelineau et al., 1994). In this study we measured N2/Ar ratio and the isotopic compositions of noble gases and carbon of CO2 extracted by undervacuum crushing of fluid inclusions in order to: 1) define the origin of the palaeogeothermal fluids, 2) compare noble gas and CO2 isotopic compositions of the fluid trapped in the inclusions with present-day discharged fluid and evaluate if the source of these fluids has been changed with time. The stratigraphy of the deep geothermal wells (up to 4.5 km below the ground level, b.g.l.) schematically consists of (from top to bottom): 1) a cover of siliciclastic sediments, carbonatic rocks, evaporite and ophiolite rocks (Late Triassic to Pliocene), 2) a complex of tectonic slices which include the lower formation of the sedimentary rocks and the uppermost part of the metamorphic basement, 3) a metamorphic basement (Palaeozoic), consisting of phyllitic-quartzitic rocks, micaschists and gneisses. The deepest wells at Larderello have also encountered contact metamorphic rocks and granite dykes and intrusion with ages between 1.3 and 3.8 Ma. Geophysical data suggest that a large intrusion is present below the Larderello area; the geothermal activity can be related to the presence of this intrusion. The productive horizons were found in Triassic carbonate and anhydrite formations (shallow reservoir) of the upper units and in some permeable levels of the Palaeozoic metamorphic basement (deep reservoir).

Mantle-derived Xenoliths with Hot-spot Type Helium in Cenozoic Alkali Basalt, Northwestern Kyushu, Japan

The 3He/4He ratio of mantle-derived material is not uniform and the distinct 3He/4He signatures are found in association with different tectonic environments. Helium isotopic signature of hot-spot regions such as Hawaii, Iceland, and Rbunion range from 9 to 30 Ra, where Ra is 3He/4He of the atmosphere = 1.4 x 10 -6, which indicates that the hot-spot volcanisms tap a deeper region of the mantle enriched in 3He than MORB reservoir. Cenozoic alkaline basalts from back-arc region of the Japanese island-arc system are less depleted in Ta, Nb and Ti and less enriched in K, Sr, Ba and Rb compared with samples from northeastem Japan. These geochemical features lead to Nakamura et al. (1985) to suggest that the alkali volcanisms of this area were not directly related with subduction processes but were triggered by plumes originated from greater depth. Furthermore, it was suggested that the alkali basalt magmatism may represent the final stage of a mantle diapirism which was most active in the Miocene and caused the opening of the Sea of Japan (Iwamori, 1991). We measured noble gas isotopic compositions of xenoliths in Cenozoic alkali basalt, Takashima, northwestern Kyushu to investigate possible contribution of deep mantle plume.

MORB-type neon in an enriched mantle beneath Etna, Sicily

Noble gas elemental and isotopic compositions were determined for five CO 2–CH 4 samples collected around Etna, Sicily, to investigate the geochemical features of the mantle beneath the volcano. The samples contain mantle-derived noble gases. The measured helium isotopic ratios ( 3He/ 4He) vary between 5.9 and 6.4 times atmospheric ratio ( R a=1.4×10 −6), which are comparable to the ratios of olivines (6.1–8.2 R a) in the lavas of the same volcano [1]. Neon in the samples is enriched in both 20Ne and 21Ne ( 20Ne/ 22Ne 9.95–10.7, 21Ne/ 22Ne 0.030–0.037), indicating derivation from the mantle. The δ( 20Ne/ 22Ne)/ δ( 21Ne/ 22Ne) values are identical with that of mid-ocean ridge basalts (MORB), indicating a similarity in the time-integrated (U/Ne) ratio between the Etnean magma source and the depleted upper mantle (MORB source). Argon in the samples has 40Ar/ 36Ar ratios up to of 1800, which are higher than in atmosphere. These ratios are positively correlated with the 20Ne/ 22Ne ratios, indicating a mantle origin of the radiogenic argon. Compared with olivines from the Etnean lavas [1], the argon in our natural gas samples is less contaminated by atmospheric argon. Two samples from a CO 2 well show small but resolvable excess of 129Xe and 134,136Xe. The volcanic rocks of Etna, ranging from tholeiites to alkaline, are enriched in incompatible elements. Nd, Sr and Pb isotopes of the volcanic rocks indicate that the magma source is isotopically heterogeneous and contain a component with high Sr, Pb and low Nd isotopic ratios derived from a mantle region which has been enriched in incompatible elements for a few million years [2,3,4] relative to the depleted upper mantle. The helium isotopic ratios of the gas samples are lower than those of MORB and are consistent with geochemical signatures of the solid elements in the Etnean volcanic rocks. However, the observed MORB-type neon is on a MORB correlation array without accumulation of nucleogenic 21Ne. This apparent decoupling may be explained by a recent mixing of the depleted upper mantle (MORB source) with fluid enriched in both incompatible elements and radiogenic 4He. When the fluid was formed with a small degree of partial melting, fractionation of radiogenic 4He from nucleogenic 21Ne could have occurred because of smaller partition coefficient of He than Ne.

Reconstructing recent atmospheric trace gas concentrations from polar firn and bubbly ice data by inverse methods

We present a method to extract the atmospheric signal of trace gas mixing ratios from firn and bubbly ice measurements. This method, validated using data from Antarctic sites (Vostok and DE08), includes a numerical model that simulates air transport in the firn, and inverse theory. We focus here on atmospheric CH4 reconstruction, but the method can be used to reconstruct recent changes in any trace gas elemental or isotopic composition measured in firn and ice, that has no chemical interaction with solid H2O. This study also provides useful information on the effective diffusity‐porosity relationship and quantifies the smoothing effect due to gas diffusion in firn and at pore closure, showing the typical time periods of events that are filtered by the firn and therefore not observed in the gas record of ice cores.

Chemical features and isotopic composition of gaseous manifestations on Vulcano Island, Aeolian Islands, Italy: An interpretative model of fluid circulation

Between 1987 and 1995, gases were sampled from various manifestations on Vulcano Island including various fumaroles at La Fossa crater and Baia di Levante and diffuse soil gas emissions in several areas which are characterized by high C02 fluxes. Since the end of 1987, volcanic activity has increased considerably, showing a marked rise in temperature (300°C in 1987 to 670°C in December, 1992) and in output of steam from crater fumaroles. In addition, variations have been observed in the chemical and isotopic compositions of gases and thermal waters, which can be attributed to an increased inflow of deeper fluids. The isotopic composition of fumarolec steam seems to be the result of mixing processes between a positive endmember of deep origin (δD = +10‰ ∼ +15‰ δ 18O= +6‰ ∼ +8‰) and two endmembers of meteoric origin with different oxygen-shifts. Variations observed in the C0 2 isotopic composition suggest that all sampled manifestations are fed by a single deep source which may be magmatic. Such a source would be characterized by the 6 13C CO 2 values of high-temperature fumaroles (δ 13C CO 2 ≅0‰) these values are noticeably more positive than those generally attributed to magmatic gases (δ 13C CO 2 = −5‰ ∼ −8‰) implying crustal contamination processes of the magma. The observed decrease in δ 18O CO 2 values of the crater fumaroles (from +20‰ in 1978 to between +12‰ and +15‰ in 1993–1994) was probably caused by two different processes. The first is a reduction of the enrichment factor between C0 2 and H 2O which is related to the increased temperature of the gases, and the second is a mixing with more negative waters of meteoric origin. Finally, C0 2 addition or removal processes caused by interactions between deep gases and shallow hydrothermal waters are likely to be responsible for the different chemical and isotopic compositions of gaseous emissions in the Vulcano Porto area and the crater fumaroles.

Using isotopic disequilibrium of CO 2 to model gas adsorption in mass spectrometric measurements

Mass spectrometric measurements were carried out on a CO 2 sample prepared by mixing known amounts of ‘natural’ CO 2 with CO 2 enriched in 13C and, to a lesser extent, in 17O and 18O. The sample was in isotopic disequilibrium, i.e. with the isotopic species distribution not obeying the statistical probability. The results indicate that isotope scrambling reactions occur during the measurement which tend to shift the gas isotopic composition towards the equilibrium. The isotope exchange reactions take place via gas adsorption in the mass spectrometer inlet system. A Langmuir-type adsorption model, developed independently to describe SiF 4 adsorption in the mass spectrometer inlet system, is applied to the CO 2 scrambling reactions. The model is capable of fitting the experimental data adequately, and sets of values for the isotope exchange kinetics can be derived from it. Small deviations from the model prediction may be due to isotope fractionations which have not been taken into account and require further investigation.

Integration of Natural Gases in Hydrocarbon Systems of the Northern Gulf of Mexico Basin: ABSTRACT

Associated and non-associated natural gases of the northern Gulf of Mexico Basin have been characterized as part of Exxon`s multidisciplinary study of hydrocarbon source, maturation and migration. Analyses of over 600 natural gases were integrated within the hydrocarbon (HC) system context provided by evaluation of more than 2000 reservoired oils, 1200 HC-bearing sea bottom drop cores, and extensive source rock data, all synthesized within a geologic framework developed from 2-D and 3-D seismic. The molecular and isotopic compositions of gases can vary due to type of organic matter and level of maturity of the source, mixing with biogenic methane, and biodegradation. Organic matter type is a primary control on the compositions of natural gases, as documented by the close correspondence of gas compositions to HC systems defined from oil and source rock characterization. For example, the predominance of relatively dry gas on the Texas shelf primarily reflects generation from a terrestrial, gas-prone source. The recognition of significant source controls on gas composition has important implications to evaluation of gas maturity and inferred timing of generation. Complications from widespread occurrence of biogenic gas are incorporated in gas interpretations. Integration of these gas and oil data strongly supports the concept that mostmore » associated thermogenic gases have been cogenerated with oils within the normal oil window. On the slope, large seeps contain early mature oils and abundant wet thermogenic gases, documenting comigration and suggesting that the two have a common origin. In contrast, high maturity gases are distinctly absent.« less

Noble gases and nitrogen released from lunar soils by acid etching

A stepwise acid-etching technique similar to the closed system stepwise etching (CSSE) method developed at ETH Zurich was used to examine the solar wind reservoirs of lunar soil grains. Samples were treated with weak acids (H 2O, H 2SO 3) to facilitate the release of the most shallowly implanted gases. Noble gas abundances and isotopic compositions, including Kr and Xe in some cases, and a few nitrogen data were obtained for mineral or grain-size separates of three lunar soils (plagioclase from 60051, pyroxene from 75081, and <25 ym bulk 79035). The 60051 plagioclase grains, considered to be a possibly unique resource for determining the modern-day solar wind composition, show unusually low contents of solar wind He, Ne, and particularly Ar, but do not otherwise possess any characteristics clearly attributable to a modem-day solar wind exposure. Initial water and acid treatments of the grains, however, release an apparently pure SEP component. The 75081 pyroxene and the size separate of bulk 79035 both yield Kr and Xe compositions in initial etch steps that are characteristic of undiffused solar wind, significantly increasing the database for measurements of solar wind Kr and Xe where possible laboratory thermal diffusion and fractionation effects are not a concern. Pyroxene in particular appears to be a suitable alternative to ilmenite for the purpose of making measurements of this kind. Nitrogen release by acid etching is not at present quantitative, and while it appears possible to obtain reasonable isotopic ratios for solar wind N, we are unable to use the technique to determine solar nitrogen to noble gas ratios. Light noble gases in all three soil separates, other than the aforementioned behavior of 60051, appear to behave in accord with expectations based on acid-etching analyses performed by the Züirich group.

Mass transfer of helium, neon, argon, and xenon through a steady-state upper mantle

We have examined the steady-state upper mantle model for helium, neon, argon, and xenon following the mass transfer approach presented by Kellogg and Wasserburg (1990) for helium and Porcelli and Wasserburg (1995a) for xenon. The model explains the available observational data of mantle helium, neon, argon, and xenon isotope compositions and provides specific predictions regarding the rare gas isotopic compositions of the lower mantle, subduction of rare gases, and mantle rare gas concentrations. Rare gases in the upper mantle are derived from mixing of rare gases from the lower mantle, subducted rare gases, and radiogenic nuclides produced in situ. Isotopic shifts in the closed system lower mantle are due to decay of uranium and thorium decay series nuclides, 40K, 129I, and 244Pu over 4.5 Ga, while isotopic shifts in the steady-state upper mantle are due to decay of uranium and thorium series nuclides, and 40K over a timescale of ∼1.4 Ga. The model predicts that the shift in 21Ne/ 22Ne in the upper mantle relative to that in the lower mantle is the same as that for 4He/ 3He between the mantle reservoirs. This is compatible with the available data for MORB and ocean islands. Subduction of atmospheric helium and neon is not significant. All of the 40Ar in the lower mantle has been produced by 40K decay in the lower mantle. In the upper mantle, 40K decay further increases the radiogenic 40Ar from the lower mantle by a factor of ∼3. The calculated minimum lower mantle 40 Ar 36 Ar ratio is substantially greater than the atmospheric ratio. The inferred rare gas relative abundances of the lower mantle are different from those of the atmosphere and are consistent with possible early solar system reservoirs. Both the calculated 3 He 22 Ne and 20 Ne 36 Ar ratios of the lower mantle are within the range for meteorites with ‘solar’ neon isotope compositions. The 130 Xe 36 Ar ratio of the lower mantle is greater than that of the atmosphere, and may be possibly as high as the ratio found for meteoritic “planetary” rare gases. The model treats the atmosphere as a separate reservoir with rare gas isotope compositions that are distinct from those in the mantle. If the Earth originally had uniform concentrations of rare gases as represented by those in the lower mantle, then degassing of the upper mantle would have provided only a small proportion of the nonradiogenic rare gases presently in the atmosphere. The remainder may have been derived from late-accreted material with a much higher concentration of rare gases than the lower mantle. However, the amount of radiogenic 129Xe and 136Xe in the atmosphere as well as the lower mantle implies a substantial loss of rare gases. It is most likely that rare gases have been lost during late accretion and/or during; the hypothesized moon-forming impact. The nonradiogenic rare gases in the atmosphere were then supplied by subsequently accreted material with nonradiogenic xenon, possibly in comets. Fractionation of atmospheric xenon isotopes relative to other early solar system components must have occurred either on the late-accreting materials or during subsequent loss from the Earth.

Aqueous alteration and brecciation in Bells, an unusual, saponite-bearing, CM chondrite

The petrological and mineralogical characteristics of the unusual CM2 chondrite, Bells, have been investigated in detail by scanning electron microscopy (SEM), electron microprobe analysis (EPMA), and transmission electron microscopy (TEM). Bells is a highly brecciated chondrite which contains few intact chondrules, a very low abundance of refractory inclusions, and is notable in having an unusually high abundance of magnetite, which is disseminated throughout the fine-grained matrix. Fragmental olivines and pyroxenes are common and, based on compositional data, appear to have been derived from chondrules as a result of extensive brecciation. The fine-grained mineralogy of matrix in Bells differs considerably from other CM chondrites and has closer affinities to matrix in CI chondrites. The dominant phases are fine-grained saponite interlayered with serpentine, and phases such as tochilinite and cronstedtite, which are typical of CM chondrite matrices, are entirely absent. Pentlandite, pyrrhotite, magnetite, anhydrite, calcite, and rare Ti-oxides also occur as accessory phases.Based on its oxygen and noble gas isotopic compositions (Zadnik, 1985; Rowe et al., 1994), Bells can be considered to be a CM2 chondrite, although its bulk composition shows some departures from the typical range exhibited by this group. However, these variations in bulk chemistry are entirely consistent with the observed mineralogy of Bells. The unusual fine-grained mineralogy of Bells matrix can be reasonably attributed to the combined effects of aqueous alteration and advanced brecciation in a parent body environment. Extensive brecciation has assisted aqueous alteration by reducing chondrules and mineral grains into progressively smaller grains with high surface areas, which are more susceptible to dissolution reactions involving aqueous fluids. This has resulted in the preferential dissolution of Fe-rich chondrule olivines, which are now completely absent in Bells although present in other CM chondrites. The formation of saponite in Bells probably resulted from the dissolution of relatively silica-rich phases, such as pyroxene and olivine, that were derived from chondrules. The result of such dissolution reactions would be to increase the activity of silica in the fluid phase, at least on a localized scale, stabilizing saponite in preference to serpentine. An increase in aSiO2 would also have destabilized preexisting cronstedtite which may have reacted to form magnetite and Mg-Fe serpentine under conditions of constant ƒO2 .

Noble gas state in the mantle

Noble gas elemental and isotopic data on mantle‐derived materials such as mid‐ocean ridge basalt (MORB), hotspot volcanics, diamonds, and mantle xenoliths published up to August 1993 are reviewed critically. Characteristic features of the mantle‐derived materials, which are important in evaluating the significance of the data, are discussed. We choose MORB glasses, hotspot volcanics, mantle xenoliths, and diamonds as the sources to infer the noble gas state in the mantle: MORB and hotspot volcanics to represent the modern depleted and the less degassed mantle, and diamonds to represent the ancient mantle. Because of various fractionation processes, the elemental abundance data obtained from the mantle‐derived materials is unlikely to constrain the noble gas composition of the mantle, except for the systematic enrichment of the heavier noble gases relative to air. On the other hand, apart from the secondary addition of radiogenic components such as 4He, 21Ne, 40Ar, 129Xe, and 131–136Xe, the noble gas isotopic compositions deduced from the mantle‐derived materials can provide useful information as to the values in the mantle; a best estimate of the mantle noble gas state is presented.

Fractionated martian atmosphere in the nakhlites?

Considerable evidence points to a martian origin of the SNC meteorites. Noble gas isotopic compositions have been measured in most SNC meteorites. The 129Xe/132Xe vs, 84Kr/132Xe ratios in Chassigny, most shergottites, and lithology C of EETA 79001 define a linear array. This array is thought to be a mixing line between martian mantle and martian atmosphere. One of the SNC meteorites, Nakhla, contains a leachable component that has an elevated 129Xe/132Xe ratio relative to its 84Kr/132Xe ratio when compared to this approximately linear array. The leachable component probably consists in part of iddingsite, an alteration product produced by interaction of olivine with aqueous fluid at temperatures lower than 150 degrees C. The elevated Xe isotopic ratio may represent a distinct reservoir in the martian crust or mantle. More plausibly, it is elementally fractionated martian atmosphere. Formation of sediments fractionates the noble gases in the correct direction. The range of sediment/atmosphere fractionation factors is consistent with the elevated 129Xe/132Xe component in Nakhla being contained in iddingsite, a low temperature weathering product. The crystallization age of Nakhla is 1.3 Ga. Its low-shock state suggests that it was ejected from near the surface of Mars. As liquid water is required for the formation of iddingsite, these observations provide further evidence for the near surface existence of aqueous fluids on Mars more recently than 1.3 Ga.

Noble gas isotopic composition of deep underground water in Osaka plain, central Japan: Evidence for mantle He and model for new volcanism

Abstract Elemental and isotopic compositions of noble gases extracted from the bore hole water in Osaka plain, central Japan were examined. The water samples were collected from four shallow bore holes (180‐450 m) and seven deep bore holes (600‐1370 m) which have been used for an urban resort hot spring zone. The water temperatures of the deep bore holes were 22‐50°C and that of the shallow bore holes, 13‐23°C. The elemental abundance patterns show the progressive enrichment of the heavier noble gases compared with the atmospheric noble gas composition except for He, which is heavily enriched in deep bore hole water samples. 3He/4He ratios from the bore holes reaching the Ryoke granitic basement were higher than the atmospheric value (1.4 × 10−6), indicating a release of mantle He through the basement. The highest value of 8.2 × 10−6 is in the range of arc volcanism. On the other hand, the bore holes in sedimentary rocks overlying the basement release He enriched in radiogenic 4He, resulted in a low 3He/4He ratio of 0.5 × 10−6. 4He/20Ne and 40Ar/36Ar ratios indicate that the air contamination is generally larger in shallow bore holes than in deep ones from each site. The helium enriched in mantle He is compatible with the previous work which suggested up‐rising magma in ‘Kinki Spot’, the area of Osaka and western Wakayama, in spite of no volcanic activity in the area. A model to explain an initiation of magma generation beneath this area is presented.

Hydrothermal fields of the Piip submarine volcano, Komandorsky Back-Arc Basin: Chemistry and origin of vent mineralization and bubbling gas

Active thermal vents of the Piip submarine volcano were studied in 1990 from aboard sub­mersibles MIR 1 and 2. Samples of free gas and hydrothermal deposits were collected in the areas of thermal fluid discharge. Mineralogical, isotopic and microprobe studies of samples have shown, as the hydrothermal system cools, the high-temperature anhydrite association displayed at the surface is substituted by calcite-barite and later by calcite-barite-sulfide assemblages. The chemical and isotopic composition of gas and carbonates indicates the significant role of hydrocarbons from the sedimentary layers which, during the low-temperature stage, stimulate the processes of bacterial sulphate reduction. The evolution of a simular hydrothermal system is traced in the Great Caucasus barite deposits.

Gas venting and hydrate deposits in the Okhotsk Sea

Active gas vents and methane hydrate deposits were found in two areas of the Okhotsk Sea in October, 1991 onboard the R. V. Antropov, a Russian research vessel operated by Dalmorgeolo­gia from the Far-East Region of Russia. Gas vents were identified in water depths of 700 to 800 m using a 20 khz fish finding echo sounder. Sediment and hydrate samples were recovered using a 3 m long gravity corer. Some cores contained 30 to 40% by volume methane hydrate in lenses, layers and veins. A small fraction of the venting gas is captured in a methane hydrate phase, while a majority of the vented gas is released and dispersed in the overlying seawater. Small gas bubbles were observed at the sea surface, indicating that some of the venting gas reaches the atmosphere directly. Preliminary hydrocarbon gas and methane isotopic composition suggest that the venting gas is biogenic in origin. Based on the flux of organic carbon reaching the seafloor and pore water ammonia results, recent biogenic processes are not sufficient to support the production of the venting methane. It is concluded that biogenic methane from subsurface sediment layers is migrating upwards along preferential pathways associated with faulting.

Geochemistry of gas emanations: A case study of the Réunion Hot Spot, Indian Ocean

The results of analysis of natural emanations in Réunion Island show a clear magmatic origin for CO 2 and He, while N 2 and Ar are predominantly derived from the atmosphere. The distribution of magmatic gases in the Piton des Neiges massif fits the local volcanotectonic context well and suggests that the areas concerned are still subject to volcanic activity at depth. A simple method is proposed for correcting gas concentration and isotope composition for water degassing. In doing so, the isotope and elemental (C, He) composition of gases is homogeneous for the two volcanoes. The isotope ratio of He (12.5 ± 0.5R/Ra) in the present discharges is in agreement with the results of previous studies on rocks of various ages from the two volcanoes. The isotope ratio of C(δ 13C= −5 ‰ to −4 ‰ vs PDB) and the C/ 3 He ratio (∼4 × 10 9) are similar to those found in other Hot Spot volcanic systems such as Kilauea (Hawaii) and Hengill (Iceland). These similarities suggest comparable volatile history for the respective mantle sources, the main differences being in the relative proportions of radiogenic 4He. In detail, Hot Spots appear enriched in C having a light composition with respect to MORB, possibly due to the addition of a C-rich (e.g. subducted) component, in addition to a relatively undegassed, 3He-rich, component.

Isotopic compositional characteristics of terrigenous natural gases in China

The C and H isotopic compositions of the methane in more than 160 gas samples from 10 basins in China are presented in this paper. The natural gases are classified as four types: biogenic gas, bio- thermocatalytic transitional gas, gas associated with condensate oil, and coal-type gas. The isotopic compositions of these gases are closely related to the depositional basins, the types of organic matter, the stages of thermal evolution and the genetic characteristics of different gas reserviors. Studies of the C and H isotopic compositions of terrigenous natural gases will provide valuable information on the prospecting and development of natural gases of different genetic types.

Noble gases in the mantle wedge and lower crust: an inference from the isotopic analyses of xenoliths from Oki-Dogo and Ichinomegata, Japan.

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.