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1.
The combined use of geophysical and soil gas composition exploration methods allows to rapidly obtain at relative low cost information that might be related to seismic activity conditions. In this study, we carried out geochemical soil gas sampling (222Rn, 220Rn and CO2), electrical resistivity tomography and seismic refraction profiles in two selected zones near the town of Amer in the Spanish Pyrenees, where the presence of recent fractures is evident in the field. Data analysis clearly reveals anomalous values for each gas at specific positions along the electrical imaging transects. Geomorphologic and hydrogeologic data and the integration of geophysical data and soil gas measurements indicate that: (1) endogene gases radon (222Rn) and carbon dioxide (CO2) are released from the meta-sedimentary basement rocks across the main fractured zones with higher permeability values, while lower Cenozoic detrital sedimentary formations act as an impervious boundary; (2) sites with highest radon concentrations (52?kBq?m?3) coincide with the zones in the Amer fault showing more recent geomorphic evidence of activity, and more specifically with those areas covered by thinner surficial formations; (3) the lowest 222Rn values (0.2?C0.4?kBq?m?3) were recorded just on the master active fault plane. This pattern could be explained by a dilution effect resulting from high rates of soil CO2 efflux (267?g?m?2?day?1); (4) soil thoron (220Rn) activity is maximum (143?kBq?m?3) in areas with high surficial fracturing; (5) groundwater pumping may cause important distortions in the natural flow dynamics and in the measured concentrations of gases. The agreement between the different data (geochemical, geophysical, and hydrogeological) and field observations (geology and geomorphology) leads us to propose a preliminary tectonic-gravitational model for the study area.  相似文献   

2.
Groundwater surveys were performed by detailed(around 300 sites) grid-analysis of water temperature, pH, redox potential, electrical conductivity, 222Rn, alkalinity and by calculating the pCO2, throughout the Ciampino and Marino towns in the Alban Hills quiescent volcano (Central Italy). Following several episodes of dangerous CO2 exhalation from soils during the last 20 years and earlier ashistorically recorded, the work aimed at assessing the Natural Gas Hazard (NGH) including the indoor-Rn hazard. The NGH was defined as the probability of an area to become a site of poisonous peri-volcanic gas exhalations from soils to the lower atmosphere (comprising buildings). CO2 was found to be a ``carrier' for the other poisonous minor and in trace components (HsS, CH4, 222Rn, etc.). This assessment was performed by extrapolating in the aquifer CO2 and 222Rn conditions, and discriminating sectors where future CO2 flux in soils as well as indoor-Rn measurements have to be noted. A preliminary indoor-Rn survey was performed at about 200 sites. The highest values were found in the highest pCO2 and high 222Rn values in groundwater. This indicates convection and enhanced permeability in certain sectors of the main aquifer, i.e., along the bordering faults and inside the gas-trap of the Ciampino Horst., where ``continuous gas-phase micro-macro seepage mechanism' is invoked to explain the high peri-volcanic gases flux.  相似文献   

3.
Controls governing the production of 4He and 222Rn in the solid phase as well as parameters and processes contributing to their transport into the pore fluid are discussed. 222Rn activity is used to quantify the uranium sources for 4He and the result is a simplified pore-fluid age equation which is virtually independent of the porosity, the water/rock ratio, the rock density and the uranium concentration, but does require a Th/U estimate. The crucial parameter is the ratio of the release factors for the two species, /GLRn//GLHe, which is discussed in terms of three possible release mechanisms: (1) recoil; (2) recoil followed by diffusion; and (3) weathering release of accumulated 4He. It was found that /GLRn//GLHe can vary over several orders of magnitude, but can be expressed in terms of the effective grain size r, and the diffusive half-length for 222Rn decay, re.4He measurements are used to “date” gases from known gas fields and the agreement with the assumed source-rock age is good. Application of 4He/ 222Rn measurements to continental freshwater springs indicates that the weathering release of accumulated 4He dominates the input and results in a large overestimate of groundwater age. Measurement in the Lardarello geothermal field indicates that the 4He/222Rn method can indicate relative transport direction. Other possible applications in various geochemical fields are suggested.  相似文献   

4.
Controls governing the production of 4He and 222Rn in the solid phase as well as parameters and processes contributing to their transport into the pore fluid are discussed. 222Rn activity is used to quantify the uranium sources for 4He and the result is a simplified pore-fluid age equation which is virtually independent of the porosity, the water/rock ratio, the rock density and the uranium concentration, but does require a Th/U estimate. The crucial parameter is the ratio of the release factors for the two species, /GLRn//GLHe, which is discussed in terms of three possible release mechanisms: (1) recoil; (2) recoil followed by diffusion; and (3) weathering release of accumulated 4He. It was found that /GLRn//GLHe can vary over several orders of magnitude, but can be expressed in terms of the effective grain size r, and the diffusive half-length for 222Rn decay, re.4He measurements are used to “date” gases from known gas fields and the agreement with the assumed source-rock age is good. Application of 4He/ 222Rn measurements to continental freshwater springs indicates that the weathering release of accumulated 4He dominates the input and results in a large overestimate of groundwater age. Measurement in the Lardarello geothermal field indicates that the 4He/222Rn method can indicate relative transport direction. Other possible applications in various geochemical fields are suggested.  相似文献   

5.
Carbon and noble gas isotope analyses are reported for bubbling gas samples from the Tengchong volcanic geothermal area near the Indo-Eurasian suture zone. All samples contain a resolvable component of mantle-derived 3He. Occurrence of mantle-derived 3He coincides with surface volcanism. However, 3He occurs over a larger geographic areathan do surface volcanics. δ13C values for CO2 and CH4 vary from -33.4‰ to 1.6 ‰ and from -52.8‰ to -2.8‰, respectively. He and C isotope systematics indicate that CO2 and CH4 in the CO2-rich gases originated predominantly from magmatic component mixed with crustal CO2 produced from carbonate. However, breakdown of organic matter and near-surface processes accounts for the CH4 and CO2 in N2-rich gases. 3He/4He ratio distribution pattern suggests that mantle-derived He and heat sources of high-temperature system in central Tengchong originate from a hidden magma reservoir at subsurface. CO2-rich gases with the highest 3He/4He ratio (5.2 Ra) may be representative of the  相似文献   

6.
《Applied Geochemistry》2005,20(2):317-340
Sardinia is typically seismically quiescent, displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity. This evidence may be in agreement with the presence of a ductile layer in the northern sector of the island, as suggested by the He isotopic signature in fluids rising to the surface through quiescent fault systems. The fault systems have been found to be “segmented” and therefore isolated in fluid circulation. The study of fluid behaviour along fault systems becomes strategically important when applied to solve some geological risk assessments such as Rn-indoor, or to define geological structures like potential CO2 storage sites. Both of these have been recently requested by the exploitation in Italy of the Euratom Directive and the evolution of the KyotoProtocol policy.Four water-dominated hydrothermal areas of Sardinia, located along regional fault systems, were considered: Campidano Graben, Tirso Valley, Logudoro and Casteldoria. A fluid geochemical survey was carried out taking into account physical–chemical and environmental parameters, major elements within gaseous and liquid phases, a few minor and trace elements, selected isotope ratios (2H, 18O, 13C, 3He/4He), 222Rn concentration, and some dissolved gases.Two different fluids have been recognised as regards both water chemistry and dissolved gases: (i) CO2-rich gases, poor in He and Rn, with a relatively high 3He/4He ratio (up to R/Ra = 2.32), associated with Na–HCO3–(Cl) thermal and cold groundwater; (ii) gases rich in He and N2, poor in CO2 and Rn, with a low 3He/4He ratio, associated with alkaline thermal and cold waters. The distribution of these two groups of fluids characterises the Sardinian tectonic systems. In fact, gas fluxes are not homogeneous, being mainly related to the different fault segments and to the areas where Quaternary basalts crop out. The underground geochemical evolution of the Sardinian fluids, as a function of the geological and tectonic systems, provides some suggestions for solving one of the most important problems: CO2 geological sequestration. In order to reduce the CO2 excess produced by human activity, the best geological disposal sites are reservoirs with low hydraulic conductivity, sealed to fluid movement, or aquifers characterised by maximum pH buffering capacity of their mineralogical matrix. The knowledge of the role of faults, as permeability barriers or as deep fluid uprising pathways, is prerequisite.  相似文献   

7.
A geochemical survey, in shallow aquifers and soils, has been carried out to evaluate the feasibility of natural gas (CH4) storage in a deep saline aquifer at Rivara (MO), Northern Italy. This paper discusses the areal distribution of CO2 and CH4 fluxes and CO2, CH4, Rn, He, H2 concentrations both in soils and shallow aquifers above the proposed storage reservoir. The distribution of pathfinder elements such as 222Rn, He and H2 has been studied in order to identify potential faults and/or fractures related to preferential migration pathways and the possible interactions between the reservoir and surface. A geochemical and isotopic characterization of the ground waters circulating in the first 200 m has allowed to investigation of (i) the origin of the circulating fluids, (ii) the gas–water–rock interaction processes, (iii) the amount of dissolved gases and/or their saturation status. In the first 200 m, the presence of CH4-rich reducing waters are probably related to organic matter (peat) bearing strata which generate shallow-derived CH4, as elsewhere in the Po Plain. On the basis of isotopic analysis, no hints of thermogenic CH4 gas leakage from a deeper reservoir have been shown. The δ13C(CO2) both in ground waters and free gases suggests a prevalent shallow origin of CO2 (i.e. organic and/or soil-derived). The acquisition of pre-injection data is strategic for the natural gas storage development project and as a baseline for future monitoring during the gas injection/withdrawing period. Such a geochemical approach is considered as a methodological reference model for future CO2/CH4 storage projects.  相似文献   

8.
The study reports the age evolution of groundwater as it flows from the recharge area through a regional alluvial aquifer system in North Gujarat-Cambay region in western India. Radiocarbon (14C), 4He and 4He / 222Rn dating methods have been employed. Sediments from a drill core in the Cambay Basin were also analysed for uranium (U) and thorium (Th) concentrations and the measured values have been used to estimate the 4He and 222Rn production rate for groundwater age calculations. Additionally, factors controlling the distribution of 222Rn, 4He and temperature anomalies in groundwater, vis-à-vis their relation to the tectonic framework and lithology of the study area, have also been examined.The multi-isotope study indicated a reasonable correspondence in groundwater age estimates by the three methods employed. The groundwater 14C ages increased, progressively, in the groundwater flow direction: from the foothills of Aravalli Mountains in the east, and reached a value of ∼35 ka towards the region of lowest elevation, linking Little Rann of Kachchh (LRK)-Nalsarovar (NS)-Gulf of Khambhat (GK) in the western part of the study area. In this region, groundwater ages obtained for free flowing thermal wells and springs employing 4He and 4He / 222Rn systematics are in the order of million years. Such anomalous ages are possibly due to enhanced mobilisation and migration of ‘excess helium’ from hydrothermal circulation vents along deep-seated faults. Excluding such anomalous cases and considering all uncertainties, presently estimated 4He and 4He / 222Rn groundwater ages are in reasonable agreement with 14C age estimates in the Cambay Basin for helium release factor (ΛHe) value of 0.4 ± 0.3. The 4He method also indicated west-southwards progression of groundwater ages up to ∼100 ka beyond the Cambay Basin.Large ‘excess helium’ concentrations are also seen to be generally associated with anomalous groundwater temperatures (> 35 °C) and found to overlie some of the basement faults in the study area, particularly along the east and the west flanks of the Cambay Basin. Groundwater 222Rn activities in most of the study area are 800 ± 400 dpm/l. But, a thermal spring at Tuwa on the east flank of the Cambay Basin, having granitic basement at shallow depth, recorded the highest 222Rn activity (∼63,000 dpm/l).  相似文献   

9.
In the Alban Hills area, strong areally diffuse and localised spot degassing processes occur (Tivoli, Cava dei Selci, Solforata, Tor Caldara). The gas comprises a large proportion of CO2, with minor CH4, H2S and Rn. These advective features are generated by fluid leakage from buried reservoirs hosted in the structural highs of the Mesozoic carbonate basement. Gas migration towards the surface is controlled by fault and fracture systems bordering the structural highs of the carbonate formations (e.g. Ciampino high). His release is triggered when the total pressure of the fluid phase exceeds the hydrostatic pressure, thus forming a free gas phase. Furthermore, both the sudden and catastrophic, and slow and continuous gas release at surface, of naturally occurring toxic species (CO2, H2S and Rn) poses a serious health risk to people living in this geologically active area.This paper presents data obtained from soil gas and gas flux surveys, as well as gas isotopes analyses, which suggest the presence of a deep origin gas flux enriched in carbon dioxide and minor species (CH4 and H2S), as well as a channelled migration of geogas mixtures having a Rn component which is not produced in situ.In regards to the health risk to local inhabitants, it was found that some anomalous areas had been zoned as parkland while others had been heavily developed for residential purposes. For example, many new houses were found to have been built on ground which has soil gas CO2 concentrations of over 70% and a CO2 flux of about 0.7 kg m−2 day−1, as well as radon values of more than 250 kBq/m3. In addition, an indoor radon survey has been conducted in selected houses in the town of Cava dei Selci to search for a possible correlation between the local geology and the radon concentration in indoor air. Preliminary results indicate high indoor values at ground floor levels (up to 1000 Bq/m3) and very high values in the cellars (up to 250.000 Bq/m3). It is recommended that land-use planners incorporate soil gas and/or gas flux measurements in the environmental assessment of areas of possible risk (i.e. volcanic or structurally active areas).  相似文献   

10.
To test the usefulness of groundwater for U prospecting, 130 domestic wells were sampled from a 22,000 km2 rectangular area extending south and west from Ottawa, Ontario. The waters were analyzed for twenty variables including ten gases and five trace elements.The dissolved gases give information on subsurface chemical and geological environments. Carbon dioxide highs occur at the contact of granites and limestones; He highs are along major faults; CH4 and H2S reveal strongly reducing environments, and O2 in wells indicates shallow wells or active water turnover. Rn reveals U mineralization and granitic rocks.The South March U-Cu surface occurrence gives coincident U, Cu, Zn, Pb and Rn highs. A weak U-Rn-Cu high in the Richmond-North Gower area suggests more radioactive subcropping March formation there.A strong regional He anomaly coincident with the outcropping Rockcliffe Formation and controlled by the Hazeldean Fault is believed to be due to deep-seated He, but its coincidence with a weak regional Cu anomaly, a Rn anomaly in the Bells Corners area, its linear northeast trend, the presence of Helikian U-rich source rocks to the southwest, and northeast-dipping Paleozoic sandstones provide both indications of, and the right environment for epigenetic type U-Cu occurrences.  相似文献   

11.
《Applied Geochemistry》1998,13(5):543-552
The accumulation chamber methodology allows one to obtain reliable values of the soil CO2 flux, ϕsoil 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 Δϕsoil CO2/Δt and ΔPatm/Δt. Maps of classes of log ϕsoil 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.  相似文献   

12.
Soil–gas measurements of different gas species were performed in two distinct areas of the Corinth Gulf Rift (Greece): the Aigion-Neos Erineos-Lambiri (ANEL) fault zone and the Rion-Patras fault zone. Both zones lie in one of the most seismically active areas of the Euro-Mediterranean region, where a fast-opening continental rift is located. In particular, the geochemical investigations were focused on fault segments and fracture systems previously inferred by geomorphological, lithological and structural studies.In this work the applicability of soil–gas geochemistry surveys for the exploration of buried/hidden faults was tested by using various statistical methods. Moreover, a comprehensive geostatistical treatment of the collected data provided new insights into the control exerted by active structures on deep-seated gas migration towards the surface. In both investigated areas, the highest 222Rn and CO2 concentration peaks correspond with zones where the interaction among fracture and fault segments was inferred by structural and morphological methods. This indicates a clear correlation between the shape and orientation of the anomalies and the different attitude and kinematic behavior of the faults recognized in the two areas. Furthermore, obtained results show that gases migrate preferentially through zones of brittle deformation by advective processes, as suggested by the relatively high rate of migration needed to obtain anomalies of short-lived 222Rn in the soil pores.  相似文献   

13.
The transport of dissolved gases in the anoxic sediments of Cape Lookout Bight, North Carolina, is controlled by diffusion and bubble ebullition and exhibits a distinct seasonal cycle. Detailed seasonal profiles of CH4, N2 and 222Rn and direct gas flux measurements indicate that ebullition dominates the flux of all dissolved gases across the sediment-water interface during summer months, and is of major importance on an annual basis. Transport within the upper 6–8 cm of sediment appears to be controlled by molecular diffusion of gases. Transport at greater depths is controlled by diffusion in winter and ebullition in summer. Rn-222 profiles were used to estimate the rate of stripping of dissolved gases within the CH4 production zone (10–30 cm); rates averaged 3–5 percent per day and agreed with estimates derived from N2 profiles. As a result of summer ebullition, the sediments of the bight are never at saturation with respect to the major gases present in seawater. Evidence from other sites suggests that ebullition may be an important transport process in many coastal sediments, and may provide mechanism for the transport of volatile reduced compounds between anoxic sediments and the atmosphere. 222Rn is a useful tracer for quantifying this transport.  相似文献   

14.
A calculation method for determining the amount of Rn isotopes and daughter products at the start of measurement (CRAS) is proposed as a more accurate means of estimating the initial Rn concentration in soil gas. The CRAS utilizes the decay law between 222Rn and 220Rn isotopes and the daughter products 218Po and 216Po, and is applicable to α-scintillation counter measurements. As Rn is both inert and chemically stable, it is useful for fault investigation based on the soil gas geochemistry. However, the total number of α particles emitted by the decay of Rn has generally been considered to be proportional to the initial Rn concentration, without considering the gas condition with respect to radioactive equilibrium. The CRAS method is shown to be effective to derive Rn concentration for soil gases under both nonequilibrium conditions, in which the total number of decays increases with time, and equilibrium conditions, which are typical of normal soil under low gas flux. The CRAS method in conjunction with finite difference method simulation is applied to the analysis of two active fault areas in Japan, and it is demonstrated that this combination could detect the sharp rises in 222Rn concentrations associated with faults. The method also allows the determination of fault geometry near the surface based on the asymmetry variation of the Rn concentration distribution when coupled with a numerical simulation of 222Rn transport. The results for the new method as applied to the two case studies are consistent with the data collected from the geological survey. It implies that the CRAS method is suitable for investigating the fault system and interstitial gas mobility through fractures. The present analyses have also demonstrated that high Rn concentrations require the recent and repeated accumulation of 222Rn parents (230Th and 226Ra) in fault gouges through deep gas release during fault movement.  相似文献   

15.
Chemical and isotopic compositions have been measured for CO2-rich bubbling gases discharging from cold springs in Wudalianchi intra-plate volcanic area, NE China. Observed 3He/4He ratios (2–3 RA) and δ13C values of CO2 (−5‰ to −3‰) indicate the occurrence of a mantle component released and transferred to the surface by the Cenozoic extension-related magmatic activities. The CO2/3He ratios are in wide range of (0.4–97 × 109). Based on the apparent mixing trend in a 3He/4He and δ13C of CO2 diagram from all published data, the extracted magmatic end-member in the Wudalianchi Volcano has 3He/4He, δ13C and CO2/3He value of ∼3.2 RA, ∼−4.6‰ and ∼6 × 1010, respectively. These values suggest that the volatiles originate from the sub-continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events, as revealed by geothermal He and CO2 from the adjacent Changbaishan volcanic area.  相似文献   

16.
The spatio-temporal variations of soil gas in the seismic fault zone produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake were investigated based on the field measurements of soil gas concentrations after the main shock. Concentrations of He, H2, CO2, CH4, O2, N2, Rn, and Hg in soil gas were measured in the field at eight short profiles across the seismic rupture zone in June and December 2008 and July 2009. Soil-gas concentrations of more than 800 sampling sites were obtained. The data showed that the magnitudes of the He and H2 anomalies of three surveys declined significantly with decreasing strength of the aftershocks with time. The maximum concentrations of He and H2 (40 and 279.4 ppm, respectively) were found in three replicates at the south part of the rupture zone close to the epicenter. The spatio-temporal variations of CO2, Rn, and Hg concentrations differed obviously between the north and south parts of the fault zone. The maximum He and H2 concentrations in Jun 2008 occurred near the parts of the rupture zone where vertical displacements were larger. The anomalies of He, H2, CO2, Rn, and Hg concentrations could be related to the variation in the regional stress field and the aftershock activity.  相似文献   

17.
《Applied Geochemistry》2001,16(4):419-436
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 RA (where RA=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 RA 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 (Herad) varies across the Taranaki Basin with the highest Herad/C ratios occurring in the Maui field. δ13C(CH4) becomes more enriched in 13C with increasing Herad 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 RC/RA (where RC=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.  相似文献   

18.
Many studies have shown the soil gas method to be one of the most reliable investigation tools in the research of earthquake precursory signals and fault delineation. The present research is aimed finding the relationship between soil gas distribution and tectonic systems in the vicinity of the Hsinhua Fault zone in the Tainan area of Southern Taiwan. More than 110 samples were collected along 13 traverses to find the spatial distribution of Rn, He, CO2 and N2. The spatial congruence of all the gases shows that N2 is the most probable carrier gas of He, whereas CO2 seems to be a good carrier gas of Rn in this area. From the spatial distribution of Rn, He, CO2 and N2 the trace of Hsinhua Fault and neotectonic features can be identified. The spatial distribution of studied gases shows a clear anomalous trend ENE–SWS along the Hsinhua Fault.  相似文献   

19.
《Applied Geochemistry》2002,17(10):1329-1342
Chemical (major and trace elements) and isotopic compositions (δD and δ18O in waters and δ13C in CO2 and 3He/4He in gases) of natural thermal (11) and cold (39) fluids (spring waters and gases) discharging from a tectonic window of Mesozoic limestones in central Italy, have proved to be the result of mixing processes inside the limestone formations. The limestones provide a preferential route for subsurface fluid migration and they gather both descending cold, Ca-HCO3, B-depleted groundwaters and rising convective Ca-SO4(HCO3), CO2-saturated, B-rich thermal waters. Atmospherically-derived descending gas components (N2, Ne, He), dissolved in rainfall that infiltrates the limestone system mix with N2, Ne, He-depleted hot rising waters. Boron in the liquid phase and N2 and Ne in the gas phase are the most useful elements to trace the mixing process. The deeper gas samples recognised in the area are associated with the hotter waters emerging in the area. In spite of being depleted in Ne and He and light hydrocarbons they have the higher measured 3He/4He ratios, suggesting a contribution of mantle 3He to the gas phase. This contrasts with deep circulation in the crust which would lead to increased concentration of 4He in the deeper gases. Paradoxically, there is more relative concentration of 4He in the more air-contaminated gas samples than in the deeper gas samples. A similar paradox exists when the δ13C of CO2 in the deeper gas samples is considered. The shallower air-contaminated gas samples, although they should be affected by the addition of soil-13C depleted organic C, have δ13C in CO2 more positive than the deeper gas samples recognized. Since any deep hydrothermal source of CO2 should generate CO2 with more positive values of δ13C than those measured at surface, a multiple (single) calcite precipitation process from hydrothermal solutions, with C isotopic fractionation along the rising path inside the Mesozoic limestone formations, is proposed.  相似文献   

20.
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 δ13CCO2 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 δ13CCO2 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 (δDH2O ≈ 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.  相似文献   

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