Controls on pore-fluid concentration of 4He and 222Rn and the calculation of 4He/222Rn ages

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, /GL_Rn//GL_He, 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 /GL_Rn//GL_He 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, r_e.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.     

Identification of He sources and estimation of He ages in groundwater of the North China Plain

Dissolved helium concentrations and ³He/⁴He ratios were measured for 18 groundwater samples collected from the Quaternary confined aquifers in the North China Plain (NCP). The dissolved helium concentrations ranged from 1 × 10⁻⁷ to 1 × 10⁻⁶ cm³STP·g⁻¹ in the 14 samples from the central plain, but was approximately two orders of magnitude higher, between 6 × 10⁻⁶ and 9 × 10⁻⁵ cm³STP·g⁻¹, in 4 samples from the coastal plain. Based on these concentrations and the corresponding ³He/⁴He ratios varying from 0.09 to 0.55 R_a (where R_a is the ³He/⁴He ratio of air), the dissolved helium in groundwater in the central plain was identified to be primarily a mixture of atmospheric helium with radiogenic helium and a representative radiogenic helium ratio was estimated to be 0.035 R_a. Despite the high fraction of terrigenic ⁴He in the samples from the coastal plain, their ³He/⁴He ratios were found to be significantly above this radiogenic value, ranging between 0.20 and 0.37 R_a, indicating the presence of a mantle-derived He component in this area. About 2–4% mantle helium was estimated to be present in the groundwater of the coastal plain, which probably is associated with the regional Cangdong fault and tectonic activities. Based on the radiogenic He component, ⁴He ages of the groundwater in the central plain were calculated by assuming either pure in situ production or an external helium flux J₀ of 4.7 × 10⁻⁸ cm³STPcm⁻²a⁻¹. The estimated ⁴He ages fall between 9.5 and 51.4 ka and are comparable to the ¹⁴C ages, suggesting that the results of ⁴He dating are reasonable and can be an effective tool to estimate groundwater residence times under suitable conditions.     

Rn,He and CO2 soil gas geochemistry for the study of active and inactive faults

Two Italian areas, characterized by different seismological histories, were investigated to enhance the basic knowledge of gas migration mechanisms during earthquakes. Sharp variations occur in the movement and concentration of some gaseous species due to the evolution of the local stress regime. The first area (Colpasquale) is located in the central Italian region of Marche and provided a good location to study gas migration in a seismically active region. The area was devastated by a sequence of shallow earthquakes over a 3 month-long period (September–December, 1997). The occurrence of this catastrophic event, as well as the long duration of the “seismic sequence”, presented a unique opportunity to study gas migration in a zone undergoing active displacement. Soil gas surveys were performed 1 day, 1 week, 1 year and 2 years after the main shock (Ms 5.6) in the Colpasquale area. In particular, results highlight a change in the Rn distribution during the three monitoring years indicating a variation of gas migration that may be linked to the evolution of the stress regime.     

On the calculation of cosmic-ray exposure ages of stone meteorites

Abundances of cosmic ray-produced noble gases and ²⁶Al, including some new measurements, have been compiled for some 23 stone meteorites with exposure ages of < 3 × 10⁶ yr. Concentrations of cosmogenic He, Ne, and Ar in these meteorites have been corrected for differences in target element abundances by normalization to L-chondrite chemistry. Combined noble gas measurements in depth samples of the Keyes and St. Séverin chondrites are utilized to derive equations for normalizing the production rates of cosmogenic ³He, ²¹Ne, and ³⁸Ar in chondrites to an adopted ‘average’ shielding: ${}^{22}\text{Ne}{}^{21}\text{Ne}\text{= 1.114.}$ The measured unsaturated ²⁶Al concentrations and the calculated equilibrium ²⁶Al for these meteorites are combined to estimate exposure ages. These exposure ages are statistically compared with chemistry- and shielding-corrected concentrations of cosmogenic He, Ne, and Ar to derive absolute production rates for these nuclides. For L-chondrites, at ‘average’ shielding, these production rates (in 10^?8 cm³/g 10⁶ yr) are: ³He = 2.45,²¹Ne = 0.47, and ³⁸Ar = 0.069, which are ~ 25% higher than production rates used in the past. From these production rates and relative chemical correction factors, production rates for other classes of stone meteorites are derived.     

Comparison of 4He ages and 14C ages in simple aquifer systems: implications for groundwater flow and chronologies

⁴He concentrations in excess of the solubility equilibrium with the atmosphere by up to two to three orders of magnitude are observed in the Carrizo Aquifer in Texas, the Ojo Alamo and Nacimiento aquifers in the San Juan Basin, New Mexico, and the Auob Sandstone Aquifer in Namibia. A simple ⁴He accumulation model is applied to explain these excess ⁴He concentrations in terms of both in situ production and a crustal flux across the bottom layer of the aquifer. Results from the model simulations suggest variability in the ⁴He fluxes, ranging from 6×10⁻⁶ cm³ STP cm⁻² yr⁻¹ for the Auob Sandstone Aquifer to 3.6×10⁻⁷ cm³ STP cm⁻² yr⁻¹ for the Carrizo aquifer. For the Ojo Alamo and Nacimiento aquifers an intermediate value of 3×10⁻⁶ cm³ STP cm⁻² yr⁻¹ was estimated. The contribution of in-situ produced ⁴He to the measured concentrations was also estimated. This contribution is negligible for the Auob Sandstone Aquifer as compared with both the concentrations measured at the top and bottom of the aquifer for most of the pathway. In the Carrizo aquifer, in-situ produced ⁴He contributes 27.5% and 15.4%, to the total ⁴He observed at the top and bottom of the aquifer, respectively. For both aquifers of the San Juan Basin in-situ production almost entirely dominates the ⁴He concentrations at the top of the aquifer for most of the pathway. In contrast, the internal production is negligible as compared with the measured concentrations at the bottom of these aquifers, reaching, at most, 1.1%. The model simulations require an exponential decrease in the horizontal velocity of the water with increasing recharge distance to reproduce the distribution of ⁴He in these aquifers. For the Auob Sandstone Aquifer the highest range in the velocity values is obtained (25 to 0.4 m yr⁻¹). The simulations for the Carrizo aquifer and both aquifers located in the San Juan Basin require velocities varying from 4 to 0.1 m yr⁻¹, and from 2 to 0.3 m yr⁻¹, respectively. For each aquifer, average permeability values were also estimated. They are generally in agreement with results obtained from pumping tests, hydrodynamic modeling and previous ¹⁴C measurements. On the basis of the results obtained by calibrating the model with the measured ⁴He concentrations, the mean water residence times were estimated. They agree reasonably well with ¹⁴C ages. When applied as chronologies for noble gas temperatures in the same aquifers, the calculated ⁴He ages allow the identification of three different climate periods similar to those previously identified using ¹⁴C ages: (1) the Holocene period (0–10 Ka BP), (2) the Last Glacial Maximum (≈18 Ka BP), and (3) the preceeding period (30–150 Ka BP).     

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.     

Glacial and volcanic history of Icelandic table mountains from cosmogenic 3He exposure ages

Basaltic table mountains in the neovolcanic zones of Iceland have been interpreted as subglacial volcanoes that emerged through an ice sheet. Using their distinctive morphological and lithostratigraphic characteristics, the approximate surface elevation and thickness of the ice sheet at the time of eruption can be determined. We measured cosmogenic ³He concentrations in olivine phenocrysts from subaerially erupted basaltic lava caps of table mountains to determine their exposure ages. We argue that these exposure ages closely approximate eruption ages; the possibility of past snow cover is the main uncertainty. The resulting ³He exposure (eruption) ages, calculated using a locally derived ³He production rate calibration, and comprising 42 individual ages from 13 table mountains, allow reconstruction of ice sheet surface profiles through time. The new ³He chronology indicates that 12 of the 13 dated table mountains experienced their final eruptive phase during the last deglaciation. This eruptive chronology is broadly consistent with the hypothesis that melt production in Iceland is enhanced by pressure release from ice sheet unloading during deglaciation. The clustered distribution of table mountain ages suggests that distinct episodes of ice sheet thinning may have coincided with, or closely followed, the two strongest warming events in the North Atlantic region during the last deglaciation: the Bølling warming (ca 14.5 ka) and the warming at the end of the Younger Dryas.     

The dynamics of shale compaction and evolution of pore-fluid pressures

A mathematical model of sedimentation and compaction of fine-grained rocks such as shale has been constructed. Water is considered to flow upward or downward out of a compacting rock according to Darcy's law until the pore-water pressure within the rock is normal for the depth in question. The porosity decreases during compaction until a minimum porosity, determined by the difference between total vertical stress (overburden pressure) and pore-water pressure, is obtained. The model takes into account the dependence of permeability on porosity for a given rock type, and the dependence of water viscosity on salinity, temperature, and pressure. The derived equations have been computer programmed to obtain the time dependence of porosity, pressure, water velocity, permeability, and other factors within a compacting shale during (a) shale sedimentation, (b) a time lapse following shale deposition, (c) the deposition of normally pressured sediments over the shale, and (d) a second time lapse following deposition of the normally pressured unit. Solutions to these problems are given for the situation when the unit underlying the shale is normally pressured, and for the situation when the underlying unit is impermeable. The calculations show that a portion of a thick shale adjacent to a normally pressured unit may have a considerably reduced porosity and permeability, and act as a seal for the remainder of the shale. High fluid pressures may persist for many millions of years in thick shales with low permeability. The computations can be extended to cover more complicated cases of interbedded shales, sands, and other lithologies.     

The effect of pore-fluid compressibility on failure loads in elasto-plastic soil

The influence of pore-fluid compressibility on failure loads in certain soil mechanics problems is investigated. The results are tested using a finite element approach treating the soil as an elasto-plastic material. A limiting value for the pore-fluid compressibility to give satisfactory solutions and numerical stability is suggested.     

Intercomparison of tritium and noble gases analyses, 3H/3He ages and derived parameters excess air and recharge temperature

Groundwater age dating with the tritium–helium (³H/³He) method has become a powerful tool for hydrogeologists. The uncertainty of the apparent ³H/³He age depends on the analytical precision of the ³H measurement and the uncertainty of the tritiogenic ³He component. The goal of this study, as part of the groundwater age-dating interlaboratory comparison exercise, was to quantify the analytical uncertainty of the ³H and noble gas measurements and to assess whether they meet the requirements for ³H/³He dating and noble gas paleotemperature reconstruction.Samples for the groundwater dating intercomparison exercise were collected on 1 February, 2012, from three previously studied wells in the Paris Basin (France). Fourteen laboratories participated in the intercomparison for tritium analyses and ten laboratories participated in the noble gas intercomparison. Not all laboratories analyzed samples from every borehole.The reproducibility of the tritium measurements was 13.5%. The reproducibility of the ³He/⁴He ratio and ⁴He, Ne, Ar, Kr and Xe concentrations was 1.4%, 1.8%, 1.5%, 2.2%, 2.9%, and 2.4% respectively.The uncertainty of the tritium and noble gas measurements results in a typical ³H/³He age precision of better than 2.5 years in this case. However, the measurement uncertainties for the noble gas concentrations are insufficient to distinguish the appropriate excess air model if the measured helium concentration is not included. While the analytical uncertainty introduces an unavoidable source of uncertainty in the ³H/³He apparent age estimate, other sources of uncertainty are often much greater and less well defined than the analytical uncertainty.     

随机输入情况下汇流计算方法的探讨

大量的实测资料表明,在汇流过程中存在着许多难以预料和控制的不确定性因素。为了更合理的描述汇流过程,建模时应用随机微分方程替代确定性常微分方程。初步探讨了有白噪声输入情况下的线性随机汇流系统,在获得初始条件的随机特性后可以得到汇流模型的解析解和数值解,计算实例表明,在汇流模型建模中引进随机微分方程理论值得深入研究。     

~3He/~4He比值质谱计的研制与应用

本文介绍了本课题组研制的国内第一台用于地学气体样品测定的NG 10 0 0型3 He/ 4He比值质谱计。该质谱计具有小的静态容积 ,分辨率和灵敏度高及重现性等良好的分析性能。利用该质谱计测定了我国一些地区气体样品中的3 He/ 4He、4He/ 2 0 Ne比值。所分析的3 He/ 4He结果与VG 5 40 0型、MM 5 40 0型质谱计外检结果相符。     

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).     

3He/4 He比值质谱计的研制与应用

本文介绍了本课题组研制的国内第一台用于地学气体样品测定的NG-1000型3He/4He比值质谱计.该质谱计具有小的静态容积,分辨率和灵敏度高及重现性等良好的分析性能.利用该质谱计测定了我国一些地区气体样品中的3He/4He、4He/20Ne比值.所分析的3He/4He结果与VG-5400型、MM-5400型质谱计外检结果相符.     

^3He／^4He Ratios and Heat Flow in the Continental Rift—valley,Eastern China

Heat flow and the origin of helium in natural gases from fault basins of the continental rift-valley in eastern China are discussed in terms of helium isotope geochemistry.³He/⁴He ratios in natural gases from the rift-valley range from 2.23 × 10⁻⁷ to 7.21 × 10⁻⁶, which are directly proportional to the concentration of helium and ΣNHC/ΣHC ratio in natural gases. Geological and isotope geochemical data suggest that helium in natural gases consists predominantly of crustal radio-genic and upper mantle-derived helium. In a simple mixing pattern between crustal He and mantle-derived He, mantle-derived helium in natural gases would account for 10–60%. Calculated values for heat flow (Q) range from 59.7 to 82.4mWm⁻², of which about 60 percent in the rift-valley is derived from the upper mantle. Natural helium reservoirs would be found in the areas where the upper mantle uplifted greatly and heat flow is large in the continental rift-valley. The project is financially supported by the National Natural Science Foundation of China.     

Controls of variation in suspended sediment concentration in the River Rother, West Sussex, England

Suspended sediment concentrations were determined for samples collected from the River Rother, West Sussex, and rating loops constructed for several hydrographs. The rating loops often exhibit hysteresis with a greater suspended sediment concentration for a given discharge occurring on the rising limb than on the falling limb. A comparison of these loops indicates that the hysteresis, and the suspended sediment concentration (for a given discharge) become progressively reduced when storm events occur in rapid succession. Various types of rating curves can be identified, and a model of suspended sediment concentration and discharge constructed. Controlling factors of suspended sediment concentration are river discharge, duration and frequency of the storm event, the length of time between successive events, and the time of year.     

塔里木盆地与南天山的耦合关系:来自(U-Th)/He年龄的新证据

塔里木盆地与南天山构造-沉积耦合关系是目前国内地质研究的热点之一。文中首次利用盆地内钻井样品的磷灰石和锆石(U-Th)/He年龄探讨了塔里木盆地与南天山构造-沉积耦合关系。塔北隆起He年龄为15～3Ma的磷灰石和锆石来自于南天山,热史模拟结果揭示了南天山在晚中新世开始快速抬升的时间约为15Ma,一直持续到5Ma左右。在此基础上,建立了南天山与塔里木盆地北缘新近纪的构造-沉积耦合关系演化模式。晚中新世,南天山开始快速隆升遭受剥蚀,而塔里木盆地北缘剧烈沉降接受来自南天山的沉积物。盆地内的磷灰石和锆石He年龄有效地记录了这些地质信息,为盆-山耦合关系研究提供了新证据。     

Measurements of ^222Rn and ^220Rn with a Large Size Collector of Radon Progeny

Radon concentrations in high background radiation areas in the south are higher than those of others in China, especially ^220Rn concentration is significantly high. Therefore, measurements of ^222Rn and ^220Rn concentrations should be carried out there. This paper introduces a large size collector of radon progeny and its applications. The collector is a sheet of polyvinyl chloride fiber with electrostatics of (-500 V)-(-700 V). Its size (60 mm in diameter) is larger than those of others (26 mm in diameter) that work with the same principle. The collector is more effective to adsorb radon progeny than most of others. The equipment of ZnS(Ag) Scintillation Counting System is available for large size collectors to detect radon progeny. Therefore, its sensitivity of measurement is higher than that of others. According to the different half lives of radon progeny, and based on both theory and experiments, a formula for discrimination and calculation of ^222Rn and ^220Rn concentrations is deduced. The ^222Rn and ^220Rn concentrations were surveyed with electrostatic collectors of radon progeny on the campus of commercial school and some other areas in Hainan, southern China. Neither ^222Rn nor ^220Rn concentration was found significantly high. However, several faults underground were delineated. The collector is also used to study radon transportation. Results indicate that radon changes regularly with date when it has transported for a certain distance. Velocities of radon migration in the four media are quite different. Radon migrates more quickly in vertical tube than in the horizontal tube.     

α-Emitting mineral inclusions in apatite, their effect on (U-Th)/He ages, and how to reduce it

U-Th rich mineral inclusions in apatite are often held responsible for erroneously old (U-Th)/He ages, because they produce “parentless” He. Three aspects associated with this problem are discussed here. First, simple dimensional considerations indicate that for small mineral inclusions, the parentless helium problem might not be as serious as generally thought. For example, a mineral inclusion that is 10% the length, width and height of its host apatite needs to be a thousand times more concentrated in U and Th to produce an equal amount of He. Therefore, single isolated inclusions smaller than a few μm are unlikely to contribute significant helium. For larger or more abundant inclusions, the parentless helium problem can be solved by dissolution of the apatite and its inclusions in hot HF. Second, besides creating parentless helium, inclusions also complicate α-ejection corrections. Mathematical exploration of this latter problem for spherical geometries reveals that for randomly distributed inclusions, the probability distribution of single-grain ages is predicted to have a sharp mode at the mean age, with tails towards younger and older ages. Multiple-grain measurements will yield accurate and precise age estimates if 10 or more randomly distributed α-emitting mineral inclusions are present in a sample. Third, thermal modeling indicates that mineral inclusions have a non-trivial but minor (<5 °C) effect on the closure temperature. These predictions were tested on apatites from rapidly cooled migmatites of Naxos (Greece) which contain abundant U-rich zircon inclusions. Thirty-seven samples were subjected to two kinds of treatment. The “pooled” age (i.e., the synthetic multi-grain age computed from a number of single-grain analyses) of four inclusion-free samples (13 apatites), prepared in HNO₃ is 10.9 Ma, close to apatite and zircon fission-track ages from the same rock. (U-Th)/He ages of 14 inclusion-bearing samples dissolved in HNO₃ range between 9 and 45 Ma, with a pooled age of 22.6 Ma. The ages of 19 HF-treated samples range between 5 and 16 Ma, with 10 of 14 single-grain samples between 9 and 13 Ma and a pooled age of 10.9 Ma. These observations agree with the theoretical predictions and support the addition of HF-treated apatite (U-Th)/He dating to the thermochronological toolbox.     

扎格罗斯前陆盆地晚中新世快速剥露证据——来自磷灰石(U-Th)/He的约束

伊朗扎格罗斯造山带是世界上最年轻的造山带之一,前人对其构造演化历史的研究尚存争议。前陆盆地蕴藏着丰富的造山带热史信息,为研究造山带的隆升和剥露历史提供了重要途径。本研究选取位于扎格罗斯前陆盆地前缘中部的Kuh-E Bedush向斜和Kuh-E Murdeh向斜翼部出露的第三纪陆相红层(Agha Jari组)为研究对象,共采集11个粗砂岩样品进行磷灰石(U-Th)/He测试分析。11个样品共得到38个单颗粒年龄,年龄区间为0.8～79.9Ma。大部分样品的单颗粒年龄比较分散,表明这些样品没有发生完全热重置。相反,位于Murdeh向斜剖面最底部的样品,(U-Th)/He年龄小于其对应的地层年龄,并且集中分布在8.3～6.8Ma,表明该样品发生了完全重置并记录了最后一次剥露事件的时间。因此,我们认为扎格罗斯前陆盆地于晚中新世～7.5 Ma经历了一期快速剥露事件。此外,我们发现未重置的(U-Th)/He年龄大致分布在四个时间段:晚白垩纪—早古新世、早—中始新世、渐新世、早—中中新世。根据所得热年龄并结合前人研究设定了三种不同的热史,正演模拟结果与实际样品拟合较好,由此推断出前陆中部Agha J...