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。     

Soil–gas monitoring: A tool for fault delineation studies along Hsinhua Fault (Tainan), Southern Taiwan

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.     

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).     

Short- and long-term gas hazard: the release of toxic gases in the Alban Hills volcanic area (central Italy)

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).     

玉树Ms7.1级地震甘达村段构造地球化学特征

为探究地震对断层带气体逸出的影响和震后气体地球化学特征随时间的变化,以及地震动力学特征,利用土壤气地球化学方法,于2010年4月14日青海玉树Ms7.1级地震断层甘达村段测量地表破裂特征和震后断层气氡和汞浓度,发现断层带土壤的气氡和汞浓度的平均值分别为9246 Bq/m3和4.2 ng/m3。破裂带中Rn浓度变化幅度大,而Hg的浓度变化较小。断层主滑面附近Rn浓度相对降低,为地表破裂后断层气逃逸所致。泉水气氡观测发现,震前氡浓度增高,表明地震断层活动性增强。因此巴颜喀拉地块向东不均匀挤压,使深部气体沿地震断裂逃逸至地表,这也是造成断层带气氡、汞浓度的增高的原因。     

Geochemistry of shallow aquifers and soil gas surveys in a feasibility study at the Rivara natural gas storage site (Po Plain,Northern Italy)

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.     

Helium, carbon dioxide and oxygen soil gases: Small-scale variations over fractured ground

To investigate the control on small-scale variation of He in soil gas exercised by minor fracturing, shallow surveys have been carried out over a cave system formed along an approximately orthogonal set of fractures in Devonian limestone in southwest England. The possibilities that He variation could be related to deep-seated, major fractures or hidden mineralisation, and that other soil gases may also be affected by minor fracturing were assessed by contemporary surveys for CO₂ and O₂. Comparisons of soil gas values with variations in electrical apparent resistivity were also carried out. Location of fractures with direct connection to the cave system was determined by spiking the cave atmosphere with He and then resurveying after equilibration.The results for CO₂ and O₂ show anomalies of low magnitude (with respect to atmospheric concentrations), and although they display an antithetic relationship, this is generally poor. There is also no strong correlation with the results of the He surveys either before or after spiking, or with the apparent resistivity values. Moreover, anomalies in CO₂ + O₂ do not support the pattern of variation shown by the individual gases. These negative results suggest the absence of deep-seated fractures or hidden mineralisation, and show that CO₂ and O₂ values are independent of minor fracturing. The origin of the variation is attributed to bacteriological productivity.He soil gas concentrations were obtained as disequilibrium values relative to Field Atmospheric Air (ΔHe/ppb-FAA). Positive ΔHe values were generally found to correlate with areas of thin, dry soil cover, enhancement occurring along fracture lines. Negative ΔHe values were also found to correlate with fracture lines, but in these areas the fractures are overlain by a thick soil cover with a high moisture content. It is considered that distinctions can be drawn between water-conducting and dry fractures, and that negative ΔHe values are likely only to be encountered with shallow soil gas samples. Results of spiking with He show a clear distribution of enhanced values along the set of orthogonal fractures, irrespective of the sign of the natural He anomaly. It is considered, therefore, that major deep-seated fractures may result in positive He anomalies superimposed upon negative ΔHe values. This implies that He anomalies must always be related to a local datum value. It is concluded that resolution of interacting variables in the interpretation of soil gas data is facilitated by integrated soil gas surveys.     

Environmental influences upon mercury, radon and helium concentrations in soil gases at a site near Denver, Colorado

Variations in soil gas Hg, Rn and He concentrations and meteorological variables were monitored daily at one site over a period of 22 months. Air and soil temperature, humidity, barometric pressure, soil moisture, wind direction and velocity, soil freeze-thaw, water table elevation, crystal strain and gas emissions were determined simultaneously in order to assess the influence of the environmental variables on gas emission.Mercury concentrations were found to be higher in the summer while Rn and He concentrations were higher in the winter. It is hypothesized that adsorption-desorption controls the migration of Hg whereas Rn and He concentrations are controlled by diffusion and mass transport. Gas emissions respond to seasonal and shorter-term changes in environmental conditions. Stepwise multiple regression using gas emissions as the dependent variables suggests that environmental parameters account for 62% of the total Hg variance, 83% of the total Rn variance, and 33% of the total He variance. Temperature, barometric pressure and soil moisture exert the most influence on gas emissions with temperature effects dominating gas emissions throughout the year. Soil gas emissions display a predictable behavior during winter and summer when more stable meteorological conditions exist. During the transitional seasons of spring and fall, soil gas emissions become erratic and exhibit increased variability. Environmental variables are interrelated and appear to control the manner of gas migration.Increased adsorption by solids during the season of falling temperature decreases soil gas Hg<0.0001 ng L⁻¹, which is desorbed as the soil warms in the spring. Transport of Rn and He is primarily by convection. During the winter months, air temperatures are less than soil temperatures promoting upward movement of Rn and He by convection. During the summer, soil temperatures are less than air temperatures and an inversion layer below the level of sampling reduces upward flux and observed concentration.     

Statistical approach for the geochemical signature of two active normal faults in the western Corinth Gulf Rift (Greece)

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.     

Nitrogen as the carrier gas for helium emission along an active fault in NW Taiwan

Variations of He gas concentration are widely applied in studies devoted to the location of faults and to monitor seismic activities. Up to now, its migration mechanism in soil is not fully understood. A systematic soil gas survey across an active fault in NW Taiwan provides the opportunity to closely examine the mechanism of He migration in the fault zone. Significant spatial and temporal correlations observed between soil N₂ and He gas support the hypothesis that N₂ is the probable carrier gas for He emission in the studied area. Based on N₂/Ar ratios and N₂ isotopic results, the excess soil N₂ in this study is considered to be largely derived from ancient atmospheric air which was dissolved in groundwater. Furthermore, observations rule out the possibility of CO₂ being the dominant carrier gas for He in the studied area based on the C and He isotopic compositions and the relationship between concentrations of these gases. At least two soil gas sources, A and B, can be identified in the studied area. Source A is an abiogenic gas source characterized by excess N₂ and He, and very low O₂ and CO₂ content. Source B, on the other hand, is a mixture of biogenic gas and atmospheric air. The development of the fault system is an important factor affecting the degree of mixture between sources A and B. Therefore, variations of soil gas composition, in particular those derived from source A, could be a useful proxy for tracing faults in the area.     

车载小型气体质谱仪现场地气分析应用

OmniStar小型气体质谱仪用于车载实验室,可根据野外情况采取灵活的采样方式,实现对H2、He、CH4、N2、O2、Ar、CO2以及C3H8、C4H10的现场分析。对浓度为100×10-6的H2、He、CH4、N2、O2、Ar、CO2混合标气进行100次连续测定,得到的各组分的精密度(RSD)均优于1%,H2、He、Ar、CO2准确度(RE)分别为3.74、0.94、1.83、10.5;对室内空气进行连续3 500次测定,得到的H2、He、CO2的精密度分别为2.25%、5.55%、4.06%,N2、O2、Ar的精密度优于1%;对CH4、C2H6、C3H8、C4H10混合标准气体进行连续13次测量,得到的C3H8、C4H10的精密度(RSD)优于1%,测量的平均值与标准值完全吻合,能够满足野外现场分析需求。在内蒙古西乌旗及新疆金窝子地区应用该仪器,采用采样袋取气、螺旋钻及井孔原位在线测定等方式得到样品数据,并发现两区域内部分采样点的CO2、H2及He异常。     

Fracture detection and groundwater flow characterization using He and Rn in soil gases,Manitoba, Canada

Surveys of the distribution and migration of He and Rn were carried out in the well-characterized granitic terrane of the AECL Underground Research Laboratory (URL), Manitoba as part of a joint AECL Research, United Kingdom Department of the Environment, and United States Department of Energy research initiative. The investigations were designed to determine whether concentrations of He and Rn in soil gases could be used to identify locations of groundwater recharge and discharge from bedrock fractures. The results obtained indicate that subsurface transport of He and possibly Rn in this setting appears to be controlled largely by the groundwater flow system in the bedrock. Release of dissolved gases near the ground surface causes soil gas anomalies, which reflect discharge from the deeper flow system. In the recharge area of the deep groundwater flow system at the URL site, He abundances are close to the atmospheric level, but the discharge area of the deep flow system is characterized by significant He anomalies (up to 0.5 ppm above atmosphere levels). For Rn, the recharge area has broadly distributed high concentrations, probably caused by local Rn production in U-rich overburden, while the discharge area has only localized concentrations of Rn, which are not at the same location as the He anomalies. The general nature of the groundwater flow regime in both areas is reflected in the presence and distribution of the soil gas anomalies. In addition, major fractures in bedrock, which act as preferential groundwater flow paths, have been located from soil gas anomalies, even when obscured by overburden of variable thickness and character. The distribution of He in soil gas appears to be most representative of groundwater recharge and discharge conditions in the granitic rock, while Rn may be useful for locating specific channels where more rapid groundwater discharge is occurring from deep fracture zones.     

Integrated geophysics and soil gas profiles as a tool to characterize active faults: the Amer fault example (Pyrenees, NE Spain)

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.     

Detailed compositional analysis of gas seepage at the National Carbon Storage Test Site,Teapot Dome,Wyoming, USA

A baseline determination of CO₂ and CH₄ fluxes and soil gas concentrations of CO₂ and CH₄ was made over the Teapot Dome oil field in the Naval Petroleum Reserve No. 3 (NPR-3) in Wyoming, USA. This was done in anticipation of experimentation with CO₂ sequestration in the Pennsylvanian Tensleep Sandstone underlying the field at a depth of 1680 m.The baseline data were collected during the winter, 2004 in order to minimize near-surface biological activity in the soil profile. The baseline data were used to select anomalous locations that may be the result of seeping thermogenic gas, along with background locations. Five 10-m holes were drilled, 3 of which had anomalous gas microseepage, and 2 were characterized as “background.” These were equipped for nested gas sampling at depths of 10-, 5-, 3-, 2-, and 1-m depths. Methane concentrations as high as 170,000 ppmv (17%) were found, along with high concentrations of C₂H₆, C₃H₈, n-C₄H₁₀, and i-C₄H₁₀. Much smaller concentrations of C₂H₄ and C₃H₆ were observed indicating the beginning of hydrocarbon oxidation in the anomalous holes. The anomalous 10-m holes also had high concentrations of isotopically enriched CO₂, indicating the oxidation of hydrocarbons. Concentrations of the gases decreased upward, as expected, indicating oxidation and transport into the atmosphere. The ancient source of the gases was confirmed by ¹⁴C determinations on CO₂, with radiocarbon ages approaching 38 ka within 5 m of the surface.Modeling was used to analyze the distribution of hydrocarbons in the anomalous and background 10-m holes. Diffusion alone was not sufficient to account for the hydrocarbon concentration distributions, however the data could be fit with the addition of a consumptive reaction. First-order rate constants for methanotrophic oxidation were obtained by inverse modeling. High rates of oxidation were found, particularly near the surface in the anomalous 10-m holes, demonstrating the effectiveness of the process in the attenuation of CH₄ microseepage. The results also demonstrate the importance of CH₄ measurements in the planning of a monitoring and verification program for geological CO₂ sequestration in sites with significant remaining hydrocarbons (i.e. spent oil reservoirs).     

Natural Gas Hazard (CO2, 222 Rn) within a Quiescent Volcanic Region and Its Relations with Tectonics: The Case of the Ciampino-Marino Area, Alban Hills Volcano, Italy

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 (H<sub>s</sub>S, CH<sub>4</sub>, <sup>222</sup>Rn, etc.). This assessment was performed by extrapolating in the aquifer CO<sub>2</sub> and <sup>222</sup>Rn conditions, and discriminating sectors where future CO<sub>2</sub> 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 pCO<sub>2</sub> and high <sup>222</sup>Rn 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.     

Hazardous gas emissions from the flanks of the quiescent Colli Albani volcano (Rome,Italy)

Gas hazard was evaluated in the three most important cold gas emission zones on the flanks of the quiescent Colli Albani volcano. These zones are located above structural highs of the buried carbonate basement which represents the main regional aquifer and the main reservoir for gas rising from depth. All extensional faults affecting the limestone reservoir represent leaking pathways along which gas rises to the surface and locally accumulates in shallow permeable horizons forming pressurized pockets that may produce gas blowout when reached by wells. The gas, mainly composed of CO₂ (>90 vol.%), contains appreciable quantities of H₂S (0.35–6 vol.%), and both represent a potentially high local hazard. Both gases are denser than air and accumulate near ground where they may reach hazardous concentrations, and lethal accidents frequently occur to animals watering at local ponds. In order to evaluate the rate of degassing and the related hazard, CO₂ and H₂S diffuse soil flux surveys have been repeatedly carried out using an accumulation chamber. The viscous gas flux of some important discrete emissions has been evaluated and the CO₂ and H₂S air concentration measured by portable devices and by Tunable Diode Laser profiles. The minimum potential lethal concentration of the two gases (250 ppm for H₂S and 8 vol.% for CO₂) is 320 times higher for CO₂, whereas the CO₂/H₂S concentration ratio in the emitted natural gas is significantly lower (15–159). This explains why H₂S reaches hazardous, even lethal, concentrations more frequently than CO₂. A relevant hazard exists for both gases in the depressed zones (channels, excavations) particularly in the non-windy early hours of the day.     

Origin of hydrogen-nitrogen gas seeps,Oman

Six gas samples were collected from five thermal springs in the Semail Nappe ophiolite and the calcareous (calcite and dolomite) Hajar Formation, northern Oman. The³He/⁴He,⁴He/²⁰Ne,⁴⁰Ar/³⁶Ar and³⁸Ar/³⁶Ar ratios, chemical compositions (H₂, N₂, CO₂, CH₄, O₂, Ar and He), and stable isotope compositions (δD_H2, δD_H2O, δ¹³C_CO2, δ¹³C_CH4, and δ¹⁵N_N2) are reported. Samples from the ophiolite region are significantly anoxic with major constituents of H₂, CH₄ and N₂, while those from calcite and dolomite regions are ordinary gas seeps, consisting of N₂, CO₂ and/or O₂. The former H₂-rich gas is characterized by relatively high³He/⁴He ratio (0.4–0.8 R_atm) with low He content (<5 ppm), atmospheric⁴⁰Ar/³⁶Ar ratio, low N₂/Ar ratio (<55) and high δ¹⁵N_N2 value (∼1 ‰). On the other hand, the latter N₂-rich gas shows relatively low³He/⁴He ratio (0.1–0.4 R_atm) with high He concentration (>300 ppm), slight radiogenic⁴⁰Ar/³⁶Ar ratio, high N₂/Ar ratio (77–97) and low δ¹⁵N_N2 value (<0‰). Observed δD_H2 value of −536‰ in H₂-rich gas is distinguished from the literature value of −699‰ in the ophiolite region, giving discrepant isotope formation temperatures.     

Sources, degassing, and contamination of CO2, H2O, He, Ne, and Ar in basaltic glasses from Kolbeinsey Ridge, North Atlantic

New volatile data (CO₂, H₂O, He, Ne, and Ar) are presented for 24 submarine basaltic glasses from the Kolbeinsey Ridge, Tjörnes Fracture Zone and Mohns Ridge, North Atlantic. Low CO₂ and He contents indicate that magmas were strongly outgassed with the extent of degassing increasing toward the south, as expected from shallower ridge depths. Ne and Ar are significantly more abundant in the southernmost glasses than predicted for degassed melt. The strong atmospheric isotopic signal associated with this excess Ne and Ar suggests syn- or posteruptive contamination by air. Degassing, by itself, cannot generate the large variations in δ¹³C values of dissolved CO₂ or coupled CO₂-Ar variations. This suggests that δ¹³C values were also affected by some other processes, most probably melt-crust interaction. Modelling indicates that degassing had a negligible influence on water owing to its higher solubility in basaltic melt than the other volatiles. Low H₂O contents in the glasses reflect melting of a mantle source that is not water-rich relative to the source of N-MORB.Before eruption, Kolbeinsey Ridge melts contained ∼400 ppm CO₂ with δ¹³C of −6‰, 0.1 to 0.35 wt.% H₂O, ³He/⁴He ∼11 R_A, and CO₂/³He of ∼2 × 10⁹. We model restored volatile characteristics and find homogeneous compositions in the source of Kolbeinsey Ridge magmas. Relative to the MORB-source, He and Ne are mildly fractionated while the ⁴⁰Ar/³⁶Ar may be low. The ³He/⁴He ratios in Tjörnes Fracture Zone glasses are slightly higher (13.6 R_A) than on Kolbeinsey Ridge, suggesting a greater contribution of Icelandic mantle from the south, but the lack of ³He/⁴He variation along the Kolbeinsey Ridge is inconsistent with active dispersal of Icelandic mantle beyond the Tjörnes Fracture Zone.     

Application of soil-gas geochemistry to mineral exploration in Africa

Field methods of soil-gas analysis for CO₂, O₂, Rn and Tn are described and applied to the exploration for copper sulphide and gold/sulphide mineralisation in terrains ranging from tropical rain forest to sand desert in Africa. CO₂ concentrations are high in soil gases overlying sulphide-bearing mineralisation and the patterns can be followed under increasing cover of sand to a depth of several tens of metres. The radioactive gases show promise in the mapping of basic rocks which are potential hosts to gold mineralisation. Strong seasonal effects on the CO₂ distribution are noted in tropical rain forests, the absolute levels being much higher during or following the rainy season.     

Environmental conditions affecting concentrations of He,CO2, O2 and N2 in soil gases

The measurement of concentrations of volatile species in soil gases has potential for use in geochemical exploration for concealed ore deposits and for monitoring of subsurface contaminants. However, the interpretation of anomalies in surficial gases can be difficult because soil-gas concentrations are dependent on both meteorological and environmental conditions.For this study, concentrations of He, CO₂, O₂ and N₂ and meteorological conditions were monitored for 10–14 months at eight nonmineralized sites in both humid and dry environments. Gases were collected at 0.6–0.7-m depth at seven sites. At one site, gases were collected from 0.3-, 0.6-, 1.2-, and 2.0-m depths; diurnal monitoring studies were conducted at this site also. Rain and snowfall, soil and air temperatures, barometric pressure, and relative humidity were monitored at all the sites. The sand, silt and clay content, and the organic carbon content of surficial soil were measured at each site.Meteorological conditions generally affected He and CO₂ concentrations in the same way at all the sites; however, these effects were modified by local environmental conditions. Both seasonal and diurnal concentration changes occurred. The most important seasonal concentration changes were related to rain and snowfall and soil and air temperatures. Seasonal changes tended to be larger then the diurnal changes, but both could be related to the same processes. Local conditions of soil type and organic content affected the amount of pore space and moisture present in the soil and therefore the soil-gas concentrations.