Soil gas prospection of He,222Rn and CO2: Vulcano Porto area,Aeolian Islands,Italy

In March 1994, soil gases were sampled in the area of Vulcano Porto, on the island of Vulcano, using a grid of about 200 points/km². Analysed gases were CO₂, He and²²²Rn and, over a smaller area, H₂S. Some of the samples were also analysed for the isotope composition of CO₂C. Three anomalous CO₂ degassing areas were identified: Grotta dei Palizzi, the area near the Telephone Exchange, and the area near the beach fumaroles. The behaviour of He and²²²Rn is different in these 3 areas. The concentration of He is much lower than that of atmospheric He (down to −3950 ppb) in the isthmus, and only in the area near Grotta dei Palizzi does it have values significantly higher than atmospheric ones (up to 1900 ppb). The activity of²²²Rn, always significantly positively related to CO₂ concentrations, peaked in the 2 fumarole areas (isthmus, Telephone Exchange).     

Subsidence due to crack closure and depressurization of hydrothermal systems: a case study from Mt Epomeo (Ischia Island,Italy)

Levelling surveys carried out between 1990 and 2003 on the Mt Epomeo resurgent block (Ischia Island) record negative dislocations on its northern and southern flanks with a maximum subsidence rate of 1.27 cm yr⁻¹. This deformation is not associated with the cooling, crystallization or lateral drainage of magma and cannot be explained by a pressure point or prolate ellipsoid source. Results from dislocation models and the available structural and geochemical information indicate that the subsidence is due to crack closure processes along two main ENE–WSW and E–W preexisting faults, which represent the preferred pathway of CO₂ degassing from the hydrothermal system located beneath Mt Epomeo. The monitoring of the dislocations and CO₂ flux along these faults could give useful information on the dynamics of the hydrothermal system.     

青藏高原南部及邻区深部碳释放规模与成因

不同构造背景下的深部碳释放通量与机制研究对于深刻理解长时间尺度的气候变化具有重要意义,以往的相关研究多集中在洋中脊、大洋俯冲带和大陆裂谷等地质单元,缺少对大陆碰撞带深部碳释放规模与机理的关注,从而制约了对大陆碰撞带深部碳循环过程及其气候环境效应的进一步认识。青藏高原起源于印度和欧亚大陆的碰撞,是研究大陆碰撞带深部碳循环的理想地区。为此,在近年来青藏高原温室气体释放野外观测与研究的基础上,本文估算了高原南部及邻区火山-地热区的CO₂释放规模并探讨了其释放模式。气体He-C同位素地球化学与温泉水热活动特征等显示,青藏高原南部及邻区的深部碳释放主要受深部岩浆房、断裂和浅部水热系统等因素的控制。依据深部流体源区和上升运移控制因素的差异,可以将青藏高原南部及邻区的深部碳释放划分为三大类：(1)以壳内水热系统脱碳为主的藏南地区;(2)深大断裂控制的以水热系统脱碳为主的川西地区;(3)深部岩浆房和浅部水热系统共同控制的滇西南地区。青藏高原南部土壤微渗漏CO₂释放通量介于18.7～52.3Mt/yr之间,温泉溶解无机碳释放通量约为0.13Mt/yr;高原邻区...     

Helium,Argon and Carbon Isotopic Compositions of Spring Gases in the Hainan Island,China

Chemical and isotopic compositions have been measured for N2-He-rich bubbling gases discharging from hot springs in the Hainan Island, Southern China. Observed 3He/4He ratios (0.1–1.3 RA) indicate the occurrence of a mantle component throughout the island, which has been highly diluted by a crustal radiogenic 4He component. The occurrence of mantle-derived helium is high in the northern island (12%–16% of total He) and gradually decreases towards southern coast (1%–3% of total He). Such a distribution pattern is most likely controlled by the Pleocene-Quaternary volcanic activities in the northern island and groundwater circulation along the deep major faults. The 40Ar/36Ar and N2/Ar ratios suggest that N2 and Ar of the hot spring gases are mostly meteoric. Although δ13C values of CO2 (–20‰ to –27‰) with low concentrations are consistent with the biogenic origin, the combination of 3He/4He and d13CCO2 suggests a two end-member mixing of mantle and crustal components with CO2/3He ratios of 2×109 and 8×1011, respectively. However, the low CO2/3He ratios (1–22×106) can not be ascribed in terms of the simple mixing but has to be explained by the addition of radiogenic 4He and loss of CO2 by calcite precipitation in the hydrothermal system, which is most likely controlled by the degree of gas-water-rock interaction.     

CO2 gas pools in Jiyang sag,China

The CO₂ gas pools of Jiyang sag are located along the Gaoqing–Pingnan fault within a region of alkaline basalts. The concentration of CO₂ in the gas pools is in the range of 68.85–96.99%. All of the geochemical tracers for the CO₂ gas pools support the suggestion that CO₂ was mainly derived from mantle degassing. The δ¹³C values of CO₂ in the gas pools are in the range of −5.67–−3.41‰, which are higher than those of organogenic CO₂, and near to those of abiogenic CO₂. Their ³He/⁴He ratios are 2.80–4.47×10⁶, i.e. the R/Ra ratios are 2.00–3.19, showing that the Jiyang sag had undergone strong mantle degassing. CO₂/³He ratios are 0.59–0.89×10⁹, which are identical to those for N-MORB, indicating that CO₂ in these CO₂ gas pools was mainly derived from the mantle. Accompanying the intrusion of mantle-derived magma, the mantle-derived CO₂ migrated upwards along deep faults and was trapped in advantageous structures forming gas pools.     

An estimate of gas emissions and magmatic gas content from Kilauea volcano

Emission rates of CO₂ have been measured at Kilauea volcano, Hawaii, in the east-rift eruptive plume and CO₂ and SO₂ have been measured in the plume from the noneruptive fumaroles in the summit caldera. These data yield an estimate of the loading of Kilauean eruptive gases to the atmosphere and suggest that such estimates may be inferred directly from measured lava volumes. These data, combined with other chemical and geologic data, suggest that magma arrives at the shallow summit reservoir containing (wt.%) 0.32% H₂O, 0.32% CO₂ and 0.09% S. Magma is rapidly degassed of most of its CO₂ in the shallow reservoir before transport to the eruption site. Because this summit degassing yields a magma saturated and in equilibrium with volatile species and because transport of the magma to the eruption site occurs in a zone no shallower than the summit reservoir, we suggest that eruptive gases from Kilauea characteristically should be one of two types: a ‘primary’ gas from fresh magma derived directly from the mantle and a carbon-depleted gas from magma stored in the summit reservoir.     

Volcanic degassing at Somma–Vesuvio (Italy) inferred by chemical and isotopic signatures of groundwater

A geochemical model is proposed for water evolution at Somma–Vesuvio, based on the chemical and isotopic composition of groundwaters, submarine gas emission and chemical composition of the dissolved gases. The active degassing processes, present in the highest part of the volcano edifice, strongly influence the groundwater evolution. The geological–volcanological setting of the volcano forces the waters infiltrating at Somma–Vesuvio caldera, enriched in volcanic gases, to flow towards the southern sector to an area of high pCO₂ groundwaters. Reaction path modelling applied to this conceptual model, involving gas–water–rock interaction, highlights an intense degassing process in the aquifer controlling the chemical and isotopic composition of dissolved gases, total dissolved inorganic C (TDIC) and submarine gas emission. Mapping of TDIC shows a unique area of high values situated SSE of Vesuvio volcano with an average TDIC value of 0.039 mol/L, i.e., one order of magnitude higher than groundwaters from other sectors of the volcano. On the basis of TDIC values, the amount of CO₂ transported by Vesuvio groundwaters was estimated at about 150 t/d. This estimate does not take into account the fraction of gas loss by degassing, however, it represents a relevant part of the CO₂ emitted in this quiescent period by the Vesuvio volcanic system, being of the same order of magnitude as the CO₂ diffusely degassed from the crater area.     

Accumulation of mantle gases in a permanently stratified volcanic lake (Lac Pavin,France)

Lac Pavin is a volcanic crater lake in the Massif Central (France), characterized by a permanent vertical density stratification resulting from a strong and persistent chemocline between about 60 and 70 m depth. The deep water below the chemocline forms the monimolimnion, in which most dissolved ions as well as helium, carbon dioxide, and methane are strongly enriched. The ³He/⁴He isotope ratio of the excess helium is (9.09 ± 0.01) · 10⁻⁶, or (6.57 ± 0.01) R_a. These findings clearly indicate a flux of mantle-derived magmatic gases into the monimolimnion.In order to derive the fluxes of magmatic volatiles into Lac Pavin, it is essential to understand the hydrologic characteristics of the lake. Previously published two-box models have assumed groundwater input at the lake bottom, a short residence time in the monimolimnion, and biogenic origin of the CO₂. We propose an alternative model with a flux of magmatic gases, but not of water, into the monimolimnion, and a weak diffusive coupling between the monimolimnion and the overlying mixolimnion which leads to a long deep-water residence time (≈ 70 yr). We reassess the carbon budget of the lake and conclude that the major part of the accumulated CO₂ in the monimolimnion is of magmatic origin. From the model-derived water exchange rates, we calculated a mantle ⁴He flux of (6 ± 2) · 10¹¹ atoms m⁻² s⁻¹. This value lies near the lower end of the range found in comparable volcanic lakes. The flux of magmatic CO₂ is estimated as (1.2 ± 0.4) · 10⁻⁷ mol m⁻² s⁻¹, which is also comparatively low. The monimolimnion appears to be in steady state with respect to these fluxes, therefore no further, potentially hazardous, accumulation of CO₂ takes place.     

Genesis of fumarolic emissions as inferred by isotope mass balances: CO2 and water at Vulcano Island, Italy

We have developed a quantitative model of CO₂ and H₂O isotopic mixing between magmatic and hydrothermal gases for the fumarolic emissions of the La Fossa crater (Vulcano Island, Italy). On the basis of isotope balance equations, the model takes into account the isotope equilibrium between H₂O and CO₂ and extends the recent model of chemical and energy two-end-member mixing by Nuccio et al. (1999). As a result, the H₂O and CO₂ content and the δD, δ¹⁸O, and δ¹³C isotope compositions for both magmatic and hydrothermal end-members have been assessed. Low contributions of meteoric steam, added at a shallow depth, have been also recognized and quantified in the fumaroles throughout the period from 1988 to 1998. Nonequilibrium oxygen isotope exchange also seems to be occurring between ascending gases and wall rocks along some fumarolic conduits.The δ¹³C_CO2 of the magmatic gases varies around −3 to 1‰ vs. Peedee belemnite (PDB), following a perfect synchronism with the variations of the CO₂ concentration in the magmatic gases. This suggests a process of isotope fractionation because of vapor exsolution caused by magma depressurization. The hydrogen isotopes in the magmatic gases (−1 to −‰ vs. standard mean ocean water [SMOW]), as well as the above δ¹³C_CO2 value, are coherent with a convergent tectonic setting of magma generation, where the local mantle is widely contaminated by fluids released from the subducted slab. Magma contamination in the crust probably amplifies this effect.The computed isotope composition of carbon and hydrogen in the hydrothermal vapors has been used to calculate the δD and δ¹³C of the entire hydrothermal system, including mixed H₂O-CO₂ vapor, liquid water, and dissolved carbon. We have computed values of about 10‰ vs. SMOW for water and −2 to −6.5‰ vs. PDB for CO₂. On these grounds, we think that Mediterranean marine water (δD_H2O ≈ 10‰) feeds the hydrothermal system. It infiltrates at depth throughout the local rocks, reaching oxygen isotope equilibrium at high temperatures. Interaction processes between magmatic gases and the evolving seawater also seem to occur, causing the dissolution of isotopically fractionated aqueous CO₂ and providing the source for hydrothermal carbon. These results have important implications concerning fluid circulation beneath Vulcano and address the more convenient routine of geochemical surveillance.     

Magma-derived gas influx and water-rock interactions in the volcanic aquifer of Mt. Vesuvius, Italy

We report in this paper a systematic investigation of the chemical and isotopic composition of groundwaters flowing in the volcanic aquifer of Mt. Vesuvius during its current phase of dormancy, including the first data on dissolved helium isotope composition and tritium content. The relevant results on dissolved He and C presented in this paper reveal that an extensive interaction between rising magmatic volatiles and groundwaters currently takes place at Vesuvius.Vesuvius groundwaters are dilute (mean TDS ∼ 2800 mg/L) hypothermal fluids ( mean T = 17.7°C) with a prevalent alkaline-bicarbonate composition. Calcium-bicarbonate groundwaters normally occur on the surrounding Campanian Plain, likely recharged from the Apennines. δD and δ¹⁸O data evidence an essentially meteoric origin of Vesuvius groundwaters, the contribution from either Tyrrhenian seawater or ¹⁸O-enriched thermal water appearing to be small or negligible. However, the dissolution of CO₂-rich gases at depth promotes acid alteration and isochemical leaching of the permeable volcanic rocks, which explains the generally low pH and high total carbon content of waters. Attainment of chemical equilibrium between the rock and the weathering solutions is prevented by commonly low temperature (10 to 28°C) and acid-reducing conditions.The chemical and isotope (C and He) composition of dissolved gases highlights the magmatic origin of the gas phase feeding the aquifer. We show that although the pristine magmatic composition may vary upon gas ascent because of either dilution by a soil-atmospheric component or fractionation processes during interaction with the aquifer, both ¹³C/¹²C and ³He/⁴He measurements indicate the contribution of a magmatic component with a δ¹³C ∼ 0‰ and R/R_a of ∼2.7, which is consistent with data from Vesuvius fumaroles and phenocryst melt inclusions in olivine phenocrysts.A main control of tectonics on gas ascent is revealed by data presented in this paper. For example, two areas of high CO₂ release and enhanced rock leaching are recognized on the western (Torre del Greco) and southwestern (Torre Annunziata-Pompeii) flanks of Vesuvius, where important NE-SW and NW-SE tectonic structures are recognized. In contrast, waters flowing through the northern sector of the volcano are generally colder, less saline, and CO₂ depleted, despite in some cases containing significant concentrations of magma-derived helium. The remarkable differences among the various sectors of the volcano are reconciled in a geochemical interpretative model, which is consistent with recent structural and geophysical evidences on the structure of Somma-Vesuvius volcanic complex.     

Soil CO2 flux measurements in volcanic and geothermal areas

The accumulation chamber methodology allows one to obtain reliable values of the soil CO₂ flux, ϕ_{soil CO2}, in the range 0.2 to over 10 000 g m⁻² d⁻¹, as proven by both laboratory tests and field surveys in geothermal and volcanic areas. A strong negative correlation is observed between Δϕ_{soil CO2}/Δt and ΔP_atm/Δt. Maps of classes of log ϕ_{soil CO2} for the northern sector of Vulcano Island, Solfatara of Pozzuoli, Nea Kameni Islet and Yanbajain geothermal field evidence that active faults and fractures act as uprising channels of deep, CO₂-rich geothermal or magmatic gases. The total diffuse CO₂ output was evaluated for each surveyed area.     

汶川M_s 8.0地震破裂带CO_2、CH_4、Rn和Hg脱气强度

地震活动断裂带能够向大气释放大量的温室气体、放射性气体和有毒气体(CO_2、CH_4、Rn和Hg),并对大气环境的影响产生复杂的影响。利用静态暗箱法,对汶川M_s8.0地震破裂带CO_2、Rn和Hg脱气强度进行实地测量,并计算了CO_2和Hg脱气对大气的年贡献量。结果表明:(1)破裂带土壤气中CO_2、CH_4、Rn和Hg异常浓度最大值分别可以达到7.98%、2.38%、524.30k Bq/m~3和161.00ng/m~3;破裂带CO_2、Rn和Hg脱气平均通量是34.95g·m~(-2)d~(-1)、36.11m Bq·m~(-2)s~(-1)和26.56ng·m~(-2)h~(-1),最大值分别达到259.23g·m~(-2)d~(-1)、580.35m Bq·m~(-2)s~(-1)和387.67ng·m~(-2)h~(-1);(2)汶川Ms8.0地震破裂带向大气脱气的CO_2年贡献量是0.95Mt,Hg的年贡献量是15.94kg。汶川Ms8.0地震破裂带破裂CO_2、CH_4、Rn和Hg等的脱气强度,不仅与破裂带渗透率有关,还与断裂带浅部存在的气藏、煤层以及磷矿层等气体源有重要的联系。     

断层与幔源二氧化碳气藏的形成和分布——以渤海湾盆地济阳坳陷为例

幔源CO_2气的形成和分布与不同级别断层早白垩世以来的活动密切相关。郯庐断裂带是研究区最主要的成气断层,拆离断层和变换断层这些地壳断层是次要的成气断层,二者于早白垩世143Ma、124Ma、新生代~43Ma、~24Ma和~8Ma的走滑或伸展活动,以及与之准同时的新生代碱性玄武岩浆活动,控制了幔源CO_2气的分散和聚集。它们与基底断裂、盖层断裂共同组成运移通道,其中拆离滑脱处的低速带和盖层断裂中的顺层断层是重要的水平运移通道。早白垩世古太平洋板块俯冲脱水脱气,产生的幔源CO_2气沿着郯庐断裂带向上分散聚集;新生代以来受控于太平洋板块俯冲方向和速度的改变以及印欧板块碰撞的远程效应,形成幔源CO_2气。与此同时郯庐断裂带切割深度亦逐渐加大,~43Ma碱性岩浆活动亦开始形成幔源CO_2气并主要位于断裂带,24Ma和8Ma(5Ma)为新近纪碱性岩浆活动脱气两个主要形成时期。郯庐断裂带的活动使地幔脱气形成的CO_2沿断层走向向上运移,并在作为重要横向运移通道的拆离断层拆离滑脱处,与因岩浆脱气形成的CO_2汇合,再通过陡倾斜、缓倾斜基底断层、盖层断层的接力传递在浅部聚集成藏。预测郯庐断裂带附近是无机成因油气重要的聚集分布区带。     

Determining the origin of carbon dioxide and methane in the gaseous emissions of the San Vittorino plain (Central Italy) by means of stable isotopes and noble gas analysis

The chemistry and isotope ratios of He, C (δ¹³C) and H (δD) of free gases collected in the San Vittorino plain, an intramontane depression of tectonic origin, were determined to shed light on mantle degassing in central Italy. The C isotopic composition of CO₂ (δ¹³C–CO₂ −2.0‰ to −3.8‰) and He isotope ratios (R/R_A 0.12–0.27) were used to calculate the fraction of CO₂ originating from mantle degassing vs. sedimentary sources. The results show that CO₂ predominantly (average of 75%) derives from the thermo-metamorphic reaction of limestone. Between 6% and 22% of the CO₂ in the samples derives from organic-rich sedimentary sources. The mantle source accounts for 0–6% of the total CO₂; however, in two samples, located in proximity to the most important faults of the plain, the mantle accounts for 24% and 42%. The presence of faults and fractures allows upward gas migration from a deep source to the Earth’s surface, not only in the peri-Tyrrhenian sector, as generally reported by studies on natural gas emissions in central Italy, but also in the pre-Apennine and Apennine belts. Isotope ratios of CH₄ (δ¹³C–CH₄ −6.1‰ to −22.7‰; δD–CH₄ −9‰ to −129‰) show that CH₄ does not appear to be related to mantle or magma degassing, but it is the product of thermal degradation of organic matter (i.e. thermogenic origin) and/or the reduction of CO₂ (i.e. geothermal origin). Most of the samples appear to be affected by secondary microbial oxidation processes.     

Analysis of long- and short-term temporal variations of the diffuse CO2 emission from Timanfaya volcano,Lanzarote, Canary Islands

Reported herein are the results of eight soil CO₂ efflux surveys performed from 2006 to 2011 at Timanfaya Volcanic Field (TVF), Lanzarote Island with the aim of evaluating the long- and short-term temporal variations of the diffuse CO₂ emission. Soil CO₂ efflux values ranged from non-detectable up to 34.2 g m⁻² d⁻¹, with the highest values measured in September 2008. Conditional sequential Gaussian simulations (sGs) were applied to construct soil CO₂ efflux distribution maps and to estimate the total CO₂ output from the studied area at the TVF. Soil CO₂ efflux maps showed a high spatial and temporal variability. Total CO₂ emission rates ranged between 41 and 518 t d⁻¹, February 2011 (winter) being the season when maximum diffuse CO₂ emission rates were observed. To investigate the influence of external variables on the soil CO₂ efflux, a geochemical station (LZT01) was installed at TVF to measure continuously the soil CO₂ efflux between July 2010 and March 2012 Since external factors such as barometric pressure, rainfall, soil water content, soil and air temperatures, and wind speed influence strongly the observed soil CO₂ effluxes, multiple regression analysis was applied to the time series recorded by the automatic geochemical station LZT01 to remove the contribution of these external factors. The influence of meteorological variables on soil CO₂ efflux oscillations accounts for 13% of total variance, with barometric pressure, rainfall and/or soil water content having the most influence in the control of the soil CO₂ efflux. These observations along with the results from the eight soil gas surveys performed at TVF indicate that the short and long-term trends in the diffuse CO₂ degassing are mainly controlled by environmental factors.     

Noble gas isotopic compositions and water/gas chemistry of soda springs from the islands of Bioko,São Tomé and Annobon,along with Cameroon Volcanic Line,West Africa

Chemical and isotopic compositions are reported for water, and CO₂ and noble gases in groundwater and soda springs from Bioko, Principé, São Tomé and Annobon, all islands located in the off-shore part of the Cameroon Volcanic Line in West Africa. The soda spring waters are of Ca–Mg–HCO₃ type, with δD and δ¹⁸O values that range from −20 to −8‰ and −5.4 to −2.7‰ respectively, indicative of a meteoric origin. CO₂ is the main gas species in the springs. δ¹³C–CO₂ values vary from −2.8 to −5.0‰, overlapping the observed mantle C range (−3 to −8‰). CO₂/³He ratios (3–9×10⁹) suggest that most C (∼90%) in the samples is derived from the mantle. Neon has atmospheric isotopic compositions, while Ar is slightly enriched in radiogenic ⁴⁰Ar. ³He/⁴He ratios (3.0 to 10.1×10⁻⁶ or 2.1 to 7.2R_a, where R_a is the atmospheric ratio of 1.4×10⁻⁶) are much higher than those for typical crustal fluids (∼10⁻⁸) but lower than those expected for fluids derived from ‘high-³He/⁴He’ hotspots like Loihi and Iceland. This precludes significant contributions of such fluids in the source regions of the gases, and by inference, in the magmatism of these oceanic islands. Alternatively, approximately 90% of the He in São Tomé gases is inferred to be derived from a source similar to the MORB source. The ³He/⁴He ratio for the Bioko gas (6.6×10⁻⁶) may be derived from a source with a higher time integrated (U+Th)/³He ratio than the MORB source.     

Helium and carbon isotope systematics of natural gases from Taranaki Basin,New Zealand

The chemical and isotopic compositions of gases from hydrocarbon systems of the Taranaki Basin of New Zealand (both offshore and onshore) show wide variation. The most striking difference between the western and south-eastern groups of gases is the helium content and its isotopic ratio. In the west, the Maui gas is over an order of magnitude higher in helium concentration (up to 190 μmol mol⁻¹) and its ³He/⁴He ratio of 3.8 R_A (where R_A=the air ³He/⁴He ratio of 1.4×10⁻⁶) is approximately half that of upper mantle helium issuing from volcanic vents of the Taupo Volcanic Zone. In the SE, the Kupe South and most Kapuni natural gases have only a minor mantle helium input of 0.03–0.32 R_A and low total helium concentrations of 10–19 μmol mol⁻¹. The ³He/C ratio (where C represents the total carbon in the gas phase) of the samples measured including those from a recent study of on-shore Taranaki natural gases are generally high at locations where the surface heat flow is high. The ³He/CO₂ ratio of the Maui gases of 5 to 18×10⁻⁹ is higher than the MORB value of 0.2 to 0.5×10⁻⁹, a feature found in other continental basins such as the Pannonian and Vienna basins and in many high helium wells in the USA. Extrapolation to zero CO₂/³He and CO₂/C indicates δ¹³C(CO₂) values between −7 and −5‰ close to that of MORB CO₂. The remaining CO₂ would appear to be mostly organically-influenced with δ¹³C(CO₂) c.−15‰. There is some evidence of marine carbonate CO₂ in the gases from the New Plymouth field. The radiogenic ⁴He content (He_rad) varies across the Taranaki Basin with the highest He_rad/C ratios occurring in the Maui field. δ¹³C(CH₄) becomes more enriched in ¹³C with increasing He_rad and hydrocarbon maturity. Because ³He/⁴He is related to the ratio of mantle to radiogenic crustal helium and ³He/C is virtually constant in the Maui field, there is a correlation between R_C/R_A (where R_C=air-corrected ³He/⁴He) and δ¹³C(CH₄) in the Maui and New Plymouth fields, with the more negative δ¹³C(CH₄) values corresponding to high ³He/⁴He ratios. A correlation between ³He/⁴He and δ¹³C(CO₂) was also observed in the Maui field. In the fields adjacent to Mt Taranaki (2518 m andesitic volcano), correlations of some parameters, particularly CO₂/CH₄, C₂H₆/CH₄ and δ¹³C(CH₄), are present with increasing depth of the gas reservoir and with distance from the volcanic cone.     

A geochemical traverse across the Eastern Carpathians (Romania): constraints on the origin and evolution of the mineral water and gas discharges

The inner sector of the Eastern Carpathians displays a large number of Na–HCO₃, CO₂-rich, meteoric-originated cold springs (soda springs) and bore wells, as well as dry mofettes. They border the southern part of the Pliocene–Quaternary Calimani–Gurghiu–Harghita (CGH) calc-alkaline volcanic chain. Both volcanic rocks and CO₂-rich emissions are situated between the eastern part of the Transylvanian Basin and the main east Carpathian Range, where active compression tectonics caused diapiric intrusions of Miocene halite deposits and associated saline, CO₂-rich waters along active faults. The regional patterns of the distribution of CO₂ in spring waters (as calculated pCO₂) and the distribution pattern of the ³He/⁴He ratio in the free gas phases (up to 4.5 R_m/R_a) show their maximum values in coincidence with both the maximum heat-flow measurements and the more recent volcanic edifices. Moving towards the eastern external foredeep areas, where oil fields and associated brines are present, natural gas emissions become CH₄-dominated. Such a change in the composition of gas emissions at surface is also recorded by the ³He/⁴He ratios that, in this area, assume ‘typical’ crustal values (R_m/R_a=0.02).In spite of the fact that thermal springs are rare in the Harghita volcanic area and that equilibrium temperature estimates based on geothermometric techniques on gas and liquid phases at surface do not suggest the presence of shallow active hydrothermal systems, a large circulation of fluids (gases) is likely triggered by the presence of mantle magmas stored inside the crust. If total ³He comes from the mantle or from the degassing of magmas stored in the crust, CO₂ might be associated to both volcanic degassing and thermometamorphism of recently subducted limestones.     

Mantle-derived CO_2 in Hot Springs of the Rehai Geothermal Field,Tengchong,China

Gas concentrations and isotopic compositions of He and CO2 were determined on free gas samples from ten hot springs of the Rehai geothermal field, Tengchong, China. The results showed that hot-spring CO2 gas, together with He,was derived mainly from the mantle, indicating the accumulation of mantle-derived volatiles beneath the survey area. The δ^13C values of CO2, higher than those of the typical mantle-derived carbon and the isotopic composition of hot-spring-free CO2 in unequilibrium with dissolved CO2, are recognized only in the Rehai geothermal field, suggesting that there seems to be a still-degassing magmatic intrusion at depths, which provides mantle-derived volatiles to the hydrothermal system above. The accumulation of those volatiles has probably played an important role in triggering earthquakes in this region.In addition, the isotopic characteristics of He and C also indicate that the magmatic intrusion seems to have been derived from the MORB source, and could be contaminated by crustal materials during its upwelling through the continental crust.     

Composition and origin of the volatile components released from the Pesyanoe aubrite by stepwise crushing and heating

Aubrites are achondritic meteorites (enstatite pyroxenites) that were formed in highly reduced magmatic environments on a differentiated parent body sharing a common oxygen isotope reservoir with enstatite chondrites (EC), Earth and Moon, and could be considered as a geochemical model of the early proto-Earth. Some pyroxenes of the Pesyanoe aubrite have high abundance of gaseous inclusions, captured during the crystallization of the rocks. Investigation of the inclusions by IR spectroscopy reveals presence of OH⁻ groups and C–H bonds. The former are assigned to protonated point defects in enstatite lattice and the latter to compounds occupying void walls. Molecular water and CO₂ were not observed. Volatile components released from the samples of the Pesyanoe enstatite by stepwise crushing and heating are composed of CO₂, H₂O and a non-condensable phase. Hydrogen isotopic composition of volatiles extracted in form of molecular water in Px-separates varies in the range δD = −61 – −84‰ with mean value of δD = −73 ± 16‰ VSMOW and is within the ranges of ECs and Earth’s mantle. The total abundance of H₂ in the pyroxene of Pesyanoe were estimated as at least 0.047 ppm that is too low in comparison with that of enstatite chondrites (≥30 ppm H₂) and could indicate nearly complete degassing of the Pesyanoe primitive precursor material during the Pesyanoe parent body accretion or a mantle degassing in igneous differentiation process. In a last case a primitive precursor could have D/H ratio different from that of enstatite chondrites.