Oxygen, hydrogen, and helium isotopes for investigating groundwater systems of the Cape Verde Islands, West Africa

Stable isotopes (δ¹⁸O, δ²H), tritium (³H), and helium isotopes (³He, ⁴He) were used for evaluating groundwater recharge sources, flow paths, and residence times of three watersheds in the Cape Verde Islands (West Africa). Stable isotopes indicate the predominance of high-elevation precipitation that undergoes little evaporation prior to groundwater recharge. In contrast to other active oceanic hotspots, environmental tracers show that deep geothermal circulation does not strongly affect groundwater. Low tritium concentrations at seven groundwater sites indicate groundwater residence times of more than 50 years. Higher tritium values at other sites suggest some recent recharge. High ⁴He and ³He/⁴He ratios precluded ³H/³He dating at six sites. These high ³He/⁴He ratios (R/R_a values of up to 8.3) are consistent with reported mantle derived helium of oceanic island basalts in Cape Verde and provided end-member constraints for improved dating at seven other locations. Tritium and ³H/³He dating shows that São Nicolau Island’s Ribeira Fajã Basin has groundwater residence times of more than 50 years, whereas Fogo Island’s Mosteiros Basin and Santo Antão Island’s Ribeira Paul Basin contain a mixture of young and old groundwater. Young ages at selected sites within these two basins indicate local recharge and potential groundwater susceptibility to surface contamination and/or salt-water intrusion.     

The tritium-helium-3 method and its application to groundwater dating by the example of the Kirovsk mine region,Murmansk oblast

The measurement of ³H, ³He, ⁴He, and ²⁰Ne concentrations in waters at the Tsentralny pumping station (southern Khibiny massif, Kola Peninsula) showed that they are a mixture of young (>90%) and old (<10%) waters. The excess noble gas component from the young water is caused by the dissolution of air bubbles trapped during recharge in the unsaturated zone. The ³H-³He(³H) age of the young water is 21 ± 1.5 yr. The U-Th-4He age of the old water is about 50 ka. The high concentrations of helium and some toxic elements (e.g., aluminum) in this old water are caused by dissolution of the alkaline rocks of the Khibiny massif as a result of water-rock interaction.     

稀有气体同位素地球化学在矿床学研究中的应用进展

稀有气体因其化学惰性以及在不同来源地质体中的同位素组成差异很大,在研究成矿流体来源、演化和壳-幔相互作用过程中具有非常重要的意义。另外,由于~4He、⁴⁰Ar是放射成因子体同位素,具有年代积累效应,因此,它们常被用于同位素测年。本文简要回顾了流体包裹体中稀有气体同位素的后生影响和样品、分析方法选择注意事项,以及近年来稀有气体同位素在成矿流体示踪,⁴⁰K-⁴⁰Ar、⁴⁰Ar-39Ar定年及(U-Th)/He定年方面的研究进展。已有研究证实流体包裹体中的稀有气体可能受后期扩散丢失、后生叠加和同位素分馏的影响,要根据目的选择不同的分析方法;稀有气体同位素可以示踪不同类型矿床的流体来源、演化及壳-幔相互作用、稀有气体同位素与卤素联合运用可以用来指示流体和盐度来源、演化过程以及矿物沉淀机制等,~3He/热的研究可以追溯流体的热源及其运移方式;流体包裹体⁴⁰Ar-³⁹Ar可以用于矿床直接定年,表生含钾矿物的⁴⁰K-⁴⁰Ar、⁴⁰Ar-³⁹Ar定年以及锆石、磷灰石和铁氧化物(U-Th)/He定年可为矿床及氧化带的形成时间、矿床形成后的抬升、剥露历史、古气候演化等重大地质问题讨论提供大量有意义的信息。     

A New Single Vacuum Furnace Design for Cosmogenic 3He Dating

Helium‐3 is a stable cosmogenic isotope that can be used to determine the time interval during which a rock sample has been at or close to the Earth’s surface. As a result of the high production rate of ‘cosmogenic’³He (≈ 130 at g^?1 year^?1) and the low detection limit of modern mass spectrometers, it is possible to date exceptionally young surfaces (≈ 1000 years). The precision and accuracy of cosmogenic ³He measurements depend critically on the passive helium blank (produced by the metalwork of the extraction furnace) which can be significant relative to the sample signals. We have developed and constructed, at the CRPG (Nancy, France), a new high temperature furnace (< 1500 °C) to extract helium in minerals such as apatite, pyroxene and olivine at 1050, 1350 and 1450 °C, respectively. The furnace demonstrated an excellent helium extraction yield (> 99% for olivine and pyroxene for heating times of 20–30 min and temperatures in the range 1050–1450 °C) and low residual helium contributions (the blank, obtained under the same analytical conditions as for sample extraction: 1 × 10^?15 mole ⁴He and < 4 × 10^?21 mole ³He). This is approximately an order of magnitude lower than those reported by other laboratories using conventional furnaces.     

Tritium corrected 14C and atmospheric noble gas corrected 4He applied to deduce ages of mixed groundwaters: Examples from the Baden region,Switzerland

Mixing of recent water (post-1953) and old water (up to a limit of 25,000 years) can be identified by tritium and ¹⁴C values. The age of an old endmember may be deduced, applying a tritium correction to the measured ¹⁴C value.⁴He may be used as a second semi-quantitative age indicator of old groundwaters, overlapping the ¹⁴C range and extending beyond it. Tritium may be used to deduce the radiogenic ⁴He concentration in the old endmember. In addition, the atmospheric noble gases may be used to check He retention and to correct for losses.The corrected ¹⁴C and ⁴He values may be used to check concordance between computed ages in mixed and unmixed groundwaters. Concordant ages indicate that the basic dating premises were not violated, whereas discordant ages indicate that secondary processes were active.The suggested approach is demonstrated for a number of springs and wells studied in the regions of Baden and Yverdon, Switzerland.     

Helium,neon, and argon in the iron meteorites Dongling,Nantan and Ningbo

The light noble gases He, Ne and Ar have been measured in the iron meteorites Dongling, Nantan and Ningbo. Dongling and Ningbo show a deficit of cosmic-ray that produced³He of ca. 30% and 10%, respectively, which is argued to be caused by the loss of³H (tritium) from the meteoroids during the time of their exposure to the cosmic radiation. Nantan has the lowest content of noble gases as yet reported for any iron meteorite. Cosmogenic³He and³⁸Ar are only about 1/5000 of those in Dongling which has particularly interesting implications if the two meteorites belong to the same fall^[2]. In addition, Nantan contains nonspallogenic⁴He which we believe to be of radiogenic origin. This radiogenic⁴He, together with a U-content of 2.6×10⁻¹¹ g/g^[20] yields a⁴He retention age close to the cosmic-ray exposure age of Dongling. If Dongling and Nantan were part of the same meteoroid^[2], this result would indicate that He retention in the meteoroid age were 4,500 Ma, a U-content of less than 7.2×10⁻¹³ g/g is required to explain the non-cosmogenic₄He present. An upper limit to the number of transuranium or superheavy-element atoms which have decayed by α-emission in Nantan since onset of He retention is 2×10¹⁰ per gram.     

Noble gas study of HIMU and EM ocean island basalts in the Polynesian region

Noble gas isotopes of HIMU and EM ocean island basalts from the Cook-Austral and Society Islands were investigated to constrain their origins. Separated olivine and clinopyroxene (cpx) phenocrysts were used for noble gas analyses. Since samples are relatively old, obtained from the oceanic area and showing chemical zoning in cpx phenocrysts, several tests on sample preparation and gas extraction methods were performed. First, by comparing heating and crushing methods, it has been confirmed that the crushing method is suitable to obtain inherent magmatic noble gases without radiogenic and cosmogenic components which were yielded after eruption, especially for He and Ne analyses. Second, noble gas compositions in the core and the rim of cpx phenocrysts were measured to evaluate the zoning effect on noble gases. The result has been that noble gas concentrations and He and Ne isotope ratios are different between them. The enrichment of noble gases in the rim compared to the core is probably due to fractional crystallization. Difference of He and Ne isotope ratios is explained by cosmogenic effect, and isotope ratios of the trapped component seem to be similar between the rim and the core. Third, leaching test reveals no systematic differences in noble gas compositions between leached and unleached samples.³He/⁴He ratios of HIMU samples in the Cook-Austral Islands are uniform irrespective of phenocryst type (olivine and cpx) and age of samples (10–18 Ma), and lower (average 6.8 R_A) than those of the Pacific MORB. On the other hand, ³He/⁴He of EM samples in the Cook-Austral Islands are similar to MORB values. EM samples in the Society Islands show rather higher ³He/⁴He than MORB. Ne, Kr and Xe isotope ratios are almost atmospheric within analytical uncertainties. ⁴⁰Ar/³⁶Ar are not so high as those of MORB. Anomalous noble gas abundance pattern such as He and Ne depletion and Kr and Xe enrichment relative to atmospheric abundances was observed. Furthermore, Ne/Ar and Kr/Ar show correlation with some trace elemental ratios like La/Yb.Lower ³He/⁴He of HIMU than MORB values requires relatively high time-integrated (U + Th)/³He for the HIMU source, which suggests that the HIMU source was produced from recycled materials which had been once located near the Earth’s surface. Moreover, extreme noble gas abundance pattern and strong correlation of Ne/Ar and Kr/Ar with La/Yb indicate that the HIMU endmember is highly depleted in light noble gases and enriched in heavy noble gases. Such feature is not common to mantle materials and is rather similar to the noble gas abundance patterns of the old oceanic crust and sediment, which supports the model that the HIMU source originates from subducted oceanic crust and/or sediment.If the HIMU source corresponds to the oceanic crust which subducted at 1–2 Ga as suggested by Pb isotope studies, however, the characteristic ³He/⁴He of HIMU (6.8 R_A) would be too high because radiogenic ⁴He produced by U and Th decay should dramatically decrease ³He/⁴He. To overcome this problem, the He open system model is introduced which includes the effects of ⁴He production and diffusion between the HIMU source material and the surrounding mantle. This model favors that the HIMU source resides in the upper mantle, rather than in the lower mantle. Furthermore, this model predicts the thickness of the HIMU source to be in the order of 1 km.In contrast to low and uniform ³He/⁴He character of HIMU, ³He/⁴He of EM are rather variable. Entrainment of upper mantle material and/or a less-degassed component are required to explain the observed ³He/⁴He of EM in the Polynesian area. Participation of the less-degassed component would be related to the “superplume” below the Polynesian region.     

Dissolved noble gases in the porewater of lacustrine sediments as palaeolimnological proxies

This paper presents results from the numerical modelling of the transport of atmospheric noble gases (He, Ne, Ar, Kr, Xe), tritiated water and ³He produced by radioactive decay of ³H, in unconsolidated lacustrine sediment. Two case studies are discussed: (1) the evolution of ³H and ³He concentrations in the sediment porewater of Lake Zug (Switzerland) from 1953 up to the present; and (2) the response of dissolved atmospheric noble gas concentrations in the sediment porewater of a subtropical lake to an abrupt climatic change that occurred some 10 kyr before the present. (1) Modelled ³H and ³He porewater concentrations are compared with recent data from Lake Zug. An estimate of the effective diffusion coefficients in the sediment porewater is derived using an original approach which is also applicable also to lakes for which the historical ³H and ³He concentrations in the water column are unknown. (2) The air/water partitioning of atmospheric noble gases is sensitive to water temperature and salinity, and thus provides a mechanism by which these environmental variables are recorded in the concentrations of atmospheric noble gases in lakes. We investigate the feasibility of using noble gas concentrations in the porewater of lacustrine sediments as a proxy for palaeoenvironmental conditions in lakes. Numerical modelling shows that heavy noble gases in sediment porewater, because of their comparatively small diffusion coefficients and the strong temperature sensitivity of their equilibrium concentrations, can preserve concentrations corresponding to past lake temperatures over times on the order of 10 kyr. Noble gas analysis of sediment porewaters therefore promises to yield valuable quantitative information on the past environmental states of lakes.     

Diffusion kinetics of proton-induced Ne, He, and He in quartz

A natural quartz sample free of mineral and fluid inclusions was irradiated with a 200 MeV proton beam to produce spallogenic ²¹Ne, ³He and ⁴He. Temperature-dependent diffusivities of these three nuclides were then determined simultaneously by high precision stepped-heating and noble gas mass spectrometry. The outward mobility of proton-induced nuclides reflects diffusion through the quartz lattice. In the studied range of 70 to 400°C the helium diffusion coefficients exceed those of neon by 5-7 orders of magnitude. The implied diffusion parameters E_a = 153.7 ± 1.5 (kJ/mol) and ln(D_o/a²) = 15.9 ± 0.3 (ln(s⁻¹)) and E_a = 84.5 ± 1.2 (kJ/mol) and ln(D_o/a²) = 11.1 ± 0.3 (ln(s⁻¹)) for proton-induced ²¹Ne and ³He, respectively, indicate that cosmogenic neon will be quantitatively retained in inclusion-free quartz at typical Earth surface temperatures whereas cosmogenic helium will not. However, the neon diffusion parameters also indicate that diffusive loss needs to be considered for small (<1 mm) quartz grains that have experienced elevated temperatures. Since natural quartz often contains fluid inclusions which may enhance noble gas retentivity, these parameters likely represent an end-member case of purely solid-state diffusion. The ∼70 kJ/mol higher activation energy for neon diffusion compared to helium diffusion likely represents an energy barrier related to its ∼13% greater diameter and provides a fundamental constraint with which to test theories of solid state diffusion. The diffusion parameters for proton-induced ⁴He are indistinguishable from those for ³He, providing no evidence for the commonly expected inverse square root of the mass diffusion relationship between isotopes. We also find preliminary indication that increased exposure to radiation may enhance neon and helium retentivity in quartz at low temperatures.     

Improving noble gas based paleoclimate reconstruction and groundwater dating using Ne/Ne ratios

The interpretation of noble gas concentrations in groundwater with respect to recharge temperature and fractionated excess gas leads to different results on paleo-climatic conditions and on residence times depending on the choice of the gas partitioning model. Two fractionation models for the gas excess are in use, one assuming partial re-equilibration of groundwater supersaturated by excess air (PR-model, Stute et al., 1995), the other assuming closed-system equilibration of groundwater with entrapped air (CE-model, Aeschbach-Hertig et al., 2000). In the example of the Continental Terminal aquifers in Niger, PR- and CE- model are both consistent with the data on elemental noble gas concentrations (Ne, Ar, Kr, and Xe). Only by including the isotope ratio ²⁰Ne/²²Ne it can be demonstrated that the PR-model has to be rejected and the CE-model should be applied to the data. In dating applications ³He of atmospheric origin (³He_atm) required to calculate ³H-³He water ages is commonly estimated from the Ne excess presuming that gas excess is unfractionated air (UA-model). Including in addition to the Ne concentration the ²⁰Ne/²²Ne ratio and the concentration of Ar enables a rigorous distinction between PR-, CE- and UA-model and a reliable determination of ³He_atm and of ³H-³He water ages.     

A New Reference Material for Tritium Organic Molecules in Sediment: Results of an International Intercomparison Exercise

Solid matrix ³H reference materials are challenging to prepare given the volatile nature of ³H and are often unrepresentative of the range of ³H forms that may be encountered during routine analysis. As a result, few ³H reference materials are currently available, undermining verification of analytical techniques for environmental ³H measurement. To address this, an International Working Group on Organically‐Bound Tritium Analysis determined to produce a tritium natural matrix reference material (NMRM). The reference material comprises marine sediment blended with sewage sludge contaminated with ³H‐organic species arising from authorised discharges from a radiopharmaceutical manufacturing site. Previous studies have demonstrated that the ³H species have persisted in the environment over three decades providing valuable supporting data to underpin the characterisation of the NMRM. The preparation and characterisation of the NMRM are described along with the subsequent application of the reference material in an international intercomparison exercise involving nineteen laboratories from nine countries. A reference value of 168 ± 22 Bq kg^?1 was derived from the data arising from the proficiency test.     

Age structure and recharge conditions of a coastal aquifer (northern Germany) investigated with 39Ar, 14C, 3H, He isotopes and Ne

A tide-influenced two-layer aquifer system in northern Germany was investigated using environmental dating tracers (³H, ³⁹Ar, ¹⁴C), the noble gas isotopes ³He, ⁴He and Ne. The study area is a marshland at the River Ems estuary, exposed to regular flooding until?AD 1000. The construction of dykes, artificial land drainage and groundwater abstraction define the hydraulic gradient. The aquifer depicts a pronounced age stratification with depth. Tritium concentrations above 0.03 TU are found only in the top 30?m. Two tritium-free samples between 20 and 30?m depth show ³⁹Ar ages of 130 and 250?years. Below a clay layer?Cabout 50?m below surface level (mbsl)?Call analysed samples are ³⁹Ar free and, thus, older than 900?years. The initial ¹⁴C activities were about 70 pmC. Resulting ¹⁴C ages increase with depth and increase up to 9,000?years, in agreement with minimal ³⁹Ar ages. Concentrations of radiogenic ⁴He correlate with ¹⁴C ages. Samples in a mid-depth range (20?C70 mbsl) show significant gas loss. The gas loss is assigned to recharge in a methane producing environment. Deduced ⁴He ages were used to assign this water to a infiltration period of about AD 1000.     

Heterogeneous lithospheric mantle metasomatism in the eastern North China Craton: He–Ar isotopes in peridotite xenoliths from Cenozoic basalts

The abundances and isotopic compositions of Helium and Argon have been analyzed in a suite of fresh spinel peridotite xenoliths in Cenozoic basalts from the eastern North China Craton (NCC) by step-wise heating experiments, to investigate the nature of noble gas reservoirs in the subcontinental lithospheric mantle beneath this region. The xenoliths include one harzburgite collected from Hebi in the interior of the NCC, two lherzolites from Hannuoba at the northern margin of the craton, and three lherzolites from Shanwang and Nushan on the eastern margin. ³He/⁴He ratios in most of the xenoliths are similar to those of mid-ocean ridge basalts (MORB) or slightly lower (2–10.5 Ra, where Ra is the ³He/⁴He ratio of the atmosphere), suggesting mixing of MORB-like and radiogenic components. One olivine separate from Nushan has a helium value of 25.3 Ra, probably suggesting cosmogenic ³He addition. The ⁴⁰Ar/³⁶Ar ratios vary from atmospheric value (296) to 1625, significantly lower than the MORB value. Available data of the peridotite xenoliths indicate the He and Ar isotopic systematics of the mantle reservoirs beneath the NCC can be interpreted as mixtures of at least three end-members including MORB-like, radiogenic and atmospheric components. We suggest that the MORB-like noble gases were derived from the underlying asthenosphere during mantle upwelling, whereas the radiogenic and recycled components probably were incorporated into the lithospheric mantle during circum-craton subduction of oceanic crust. Available data suggest that the MORB-like fluids are better preserved in the interior of the NCC, whereas the radiogenic ones are more prevalent at the margins. The Paleo-Asian ocean subduction system probably was responsible for the enriched and recycled noble gas signatures on the northern margin of the craton, while the Pacific subduction system could account for the observed He–Ar isotopic signatures beneath the eastern part. Therefore, integration of helium and argon isotopes reflects heterogeneous metasomatism in the lithospheric mantle and demonstrates the critical importance of lithospheric mantle modification related to both circum-craton subduction of oceanic crust and asthenospheric upwelling beneath the eastern NCC.     

Residence time of Chalk groundwaters in the Paris Basin and the North German Basin: a geochemical approach

The comparative geochemical and isotopic study of confined and unconfined Chalk groundwaters of the Paris Basin and the N German Basin proves a significant chemical evolution during groundwater flow from the recharge zones to the deep confined aquifer. Different time dependent geochemical parameters have been tested as dating tools: Cation ratios (Sr²⁺/Ca²⁺, Mg²⁺/Ca²⁺), N–NO⁻₃, noble gas contents as paleotemperature indicators (Ne, Ar, Kr, Xe), radiogenic He, ¹³C, ¹⁴C, ¹⁸O, ²H, ³H. Cation ratios and ¹³C show the importance of incongruent dissolution processes in the Chalk aquifer. Water–rock interactions were taken into account in a multi-step dissolution model to determine radiocarbon groundwater ages. The oldest waters in the confined part of the Paris basin Chalk with maximum ¹⁴C ages of 14,000 a B.P. contain pleistocene recharge components as can be shown by a stable isotope depletion and noble gas temperatures significantly lower than in recent groundwaters. Chalk waters at the Lägerdorf site in Northern Germany show a distinct stratification with respect to residence times and hydrochemistry.     

Dating groundwater in the Bohemian Cretaceous Basin: Understanding tracer variations in the subsurface

The northern section of the Bohemian Cretaceous Basin has been the site of intensive U exploitation with harmful impacts on groundwater quality. The understanding of groundwater flow and age distribution is crucial for the prediction of the future dispersion and impact of the contamination. State of the art tracer methods (³H, ³He, ⁴He, ⁸⁵Kr, ³⁹Ar and ¹⁴C) were, therefore, used to obtain insights to ageing and mixing processes of groundwater along a north–south flow line in the centre of the two most important aquifers of Cenomanian and middle Turonian age. Dating of groundwater is particularly complex in this area as: (i) groundwater in the Cenomanian aquifer is locally affected by fluxes of geogenic and biogenic gases (e.g. CO₂, CH₄, He) and by fossil brines in basement rocks rich in Cl and SO₄; (ii) a thick unsaturated zone overlays the Turonian aquifer; (iii) a periglacial climate and permafrost conditions prevailed during the Last Glacial Maximum (LGM), and iv) the wells are mostly screened over large depth intervals.Large disagreements in ⁸⁵Kr and ³H/³He ages indicate that processes other than ageing have affected the tracer data in the Turonian aquifer. Mixing with older waters (>50 a) was confirmed by ³⁹Ar activities. An inverse modelling approach, which included time lags for tracer transport throughout the unsaturated zone and degassing of ³He, was used to estimate the age of groundwater. Best fits between model and field results were obtained for mean residence times varying from modern up to a few hundred years. The presence of modern water in this aquifer is correlated with the occurrence of elevated pollution (e.g. nitrates).An increase of reactive geochemical indicators (e.g. Na) and radiogenic ⁴He, and a decrease in ¹⁴C along the flow direction confirmed groundwater ageing in the deeper confined Cenomanian aquifer. Radiocarbon ages varied from a few hundred years to more than 20 ka. Initial ¹⁴C activity for radiocarbon dating was calibrated by means of ³⁹Ar measurements. The ¹⁴C age of a sample recharged during the LGM was further confirmed by depleted stable isotope signatures and near freezing point noble gas temperature. Radiogenic ⁴He accumulated in groundwater with concentrations increasing linearly with ¹⁴C ages. This enabled the use of ⁴He to validate the dating range of ¹⁴C and extend it to other parts of this aquifer. In the proximity of faults, ³⁹Ar in excess of modern concentrations and ¹⁴C dead CO₂ sources, elevated ³He/⁴He ratios and volcanic activity in Oligocene to Quaternary demonstrate the influence of gas of deeper origin and impeded the application of ⁴He, ³⁹Ar and ¹⁴C for groundwater dating.     

Multiple sources for mineralizing fluids in the Charmitan gold(-tungsten) mineralization (Uzbekistan)

Mineral assemblages present within the Charmitan gold(-tungsten) quartz-vein mineralization have been investigated for their cathodoluminescence behaviour, chemical composition and noble gas isotope systematics. This inventory of methods allows for the first time a systematic reconstruction of the paragenetic relationships of quartz, scheelite, sulphides and native gold within the gold mineralization at Charmitan and provides the basis to utilise noble gas data in the discussion of sources and evolution of ore-forming fluids. The vein quartz is classified into four generations based on microscopic and cathodoluminescence investigations. Quartz I shows intense brittle deformation as associated scheelite I. Undeformed scheelite II overgrows scheelite I and has lower light rare earth element and higher intermediate rare earth element contents as well as higher strontium concentrations. Scheelite II is associated with the economic gold mineralization and formed during re-crystallisation and re-precipitation of material which was partly re-mobilised from early scheelite I during infiltration of gold-bearing fluids. Early stage native gold inclusions are often associated with stage 2 sulphides, scheelite II and bismuth tellurides and contain Ag (3.6–24.4 wt.%), Hg (≤1.0 wt.%) and Bi (≤0.2 wt.%). Later stage electrum grains occur in association with stage 3 sulphides and sulphosalts and contain Hg (<0.8 wt.%) and elevated Sb concentrations (up to 3.0 wt.%). Noble gas isotope data (³He/⁴He: 0.2-0.4 Ra) for hydrothermal ore fluids trapped in the gold-related sulphides and sulphosalts (stage 2 pyrite and arsenopyrite; stage 3 pyrite, sphalerite, galena and lead sulphosalts) suggest that diverse fluid sources were involved in the formation of the Charmitan gold deposit. These data are indicative of a small, but significant input of fluids from external, deep-seated (mantle and possibly lower crust) sources. A decrease in the input of mantle helium and an increasing role of crustal helium from early to later stages of the mineralization is suggested by the measured ³He/⁴He and ⁴⁰Ar*/⁴He ratios. Sulphides from ore veins in meta-sedimentary rocks contain higher portions of meteoric fluids than those in intrusive rock types as indicated by their lower ³He/³⁶Ar ratios. The ³He/³⁶Ar ratios in the meta-sedimentary rocks agree well with ratios typical of gold mineralizations in the Tien Shan gold province completely hosted by meta-sedimentary sequences, indicating intense fluid-wall rock interaction.     

Apparent decoupling of the He and Ne isotope systematics of the Icelandic mantle: The role of He depletion, melt mixing, degassing fractionation and air interaction

We present new He-Ne data for geothermal fluids and He-Ne-Ar data for basalts from throughout the Icelandic neovolcanic zones and older parts of the Icelandic crust. Geothermal fluids, subglacial glasses, and mafic phenocrysts are characterized by a wide range in helium isotope ratios (³He/⁴He) encompassing typical MORB-like ratios through values as high as 36.8 R_A (where R_A = air ³He/⁴He). Although neon in geothermal fluids is dominated by an atmospheric component, samples from the northwest peninsula show a small excess of nucleogenic ²¹Ne, likely produced in-situ and released to circulating fluids. In contrast, geothermal fluids from the neovolcanic zones show evidence of a contribution of mantle-derived neon, as indicated by ²⁰Ne enrichments up to 3% compared to air. The neon isotope composition of subglacial glasses reveals that mantle neon is derived from both depleted MORB-mantle and a primordial, ‘solar’ mantle component. However, binary mixing between these two endmembers can account for the He-Ne isotope characteristics of the basalts only if the ³He/²²Ne ratio of the primordial mantle endmember is lower than in the MORB component. Indeed, the helium to neon elemental ratios (⁴He/²¹Ne∗ and ³He/²²Ne_s where ²¹Ne∗ = nucleogenic ²¹Ne and ²²Ne_s = ‘solar’-derived ²²Ne) of the majority of Icelandic subglacial glasses are lower than theoretical values for Earth’s mantle, as observed previously for other OIB samples. Helium may be depleted relative to neon in high-³He/⁴He ratio parental melts due to either more compatible behavior during low-degree partial melting or more extensive diffusive loss relative to the heavier noble gases. However, Icelandic glasses show higher ⁴He/⁴⁰Ar∗ (⁴⁰Ar∗ = radiogenic Ar) values for a given ⁴He/²¹Ne∗ value compared to the majority of other OIB samples: this observation is consistent with extensive open-system equilibrium degassing, likely promoted by lower confining pressures during subglacial eruptions of Icelandic lavas. Taken together, the He-Ne-Ar systematics of Icelandic subglacial glasses are imprinted with the overlapping effects of helium depletion in the high-³He/⁴He ratio parental melt, binary mixing of two distinct mantle components, degassing fractionation and interaction with atmospheric noble gases. However, it is still possible to discern differences in the noble gas characteristics of the Icelandic mantle source beneath the neovolcanic zones, with MORB-like He-Ne isotope features prevalent in the Northern Rift Zone and a sharp transition to more primitive ‘solar-like’ characteristics in central and southern Iceland.     

Weathering geochronology by (U-Th)/He dating of goethite

Nine samples of supergene goethite (FeOOH) from Brazil and Australia were selected to test the suitability of this mineral for (U-Th)/He dating. Measured He ages ranged from 61 to 8 Ma and were reproducible to better than a few percent despite very large variations in [U] and [Th]. In all samples with internal stratigraphy or independent age constraints, the He ages corroborated the expected relationships. These data demonstrate that internally consistent He ages can be obtained on goethite, but do not prove quantitative ⁴He retention. To assess possible diffusive He loss, stepped-heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic ³He. The ³He release pattern indicates the presence of at least two diffusion domains, one with high helium retentivity and the other with very low retentivity at Earth surface conditions. The low retentivity domain, which accounts for ∼ 5% of ³He, contains no natural ⁴He and may represent poorly crystalline or intergranular material which has lost all radiogenic ⁴He by diffusion in nature. Diffusive loss of ³He from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate, so probably represents diffusion from individual micrometer-size goethite crystals. The ⁴He/³He evolution during the incremental heating experiments shows that the high retentivity domain has retained 90%-95% of its radiogenic helium. This degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to Earth surface temperature and held for the appropriate duration. Considering both the high and low retentivity domains, these data indicate that one of the samples retained 90% of its radiogenic ⁴He over 47.5 Ma and the other retained 86% over 12.3 Ma. Thus while diffusive-loss corrections to supergene goethite He ages are required, these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton-irradiation ⁴He/³He method.     

浅层地下水3H-3He法测年技术在石家庄市应用研究

3H-3He测年法是利用母体3H衰变成子体3He这一对核素的衰减和累积原理实现的,它能够很好地取代氚法测年。根据石家庄地区地质和水文地质实际及原有研究基础,选取有代表性的井点取样、测试并计算了各井点浅层水年龄。计算结果与1982年的氚法测年结果对比,具有一致的变化规律,石家庄市浅层地下水年龄从北到南逐渐变大,东南部最大;滹沱河沿岸一带由于近年来黄壁庄水库每年或隔年向滹沱河放水,其浅层地下水年龄较小。总之,3H-3He测年法在石家庄市浅层地下水测年应用中取得了比较理想的结果。     

Noble gas evidence for two fluids in the Baca (Valles Caldera) geothermal reservoir

Noble gas elemental and isotopic abundances were measured in steam from four wells in the Baca geothermal reservoir located in the Valles Caldera, New Mexico. The ${}^{40}\text{Ar}{}^{36}\text{Ar}$ ratio and noble gas elemental abundances relative to ³⁶Ar are all strongly correlated with 1/³⁶Ar, the inverse of the argon content. Ratios of (α,n)-produced ²¹Ne1 and radiogenic ⁴⁰Ar1 to total ⁴He (dominantly radiogenic) are nearly constant at 2.1 × 10?8 and 0.20, respectively. The ${}^3\text{He}{}^4\text{He}$ ratio covers a restricted range of 3.9 to 4.8 times atmospheric. The high ³He content of the gas indicates the presence of a helium component ultimately derived from the mantle. Kr and Xe isotopic compositions are close to atmospheric; excess ¹²⁹Xe1 is <0.25% of the total ¹²⁹Xe.The high degree of linear correlation among the various noble gas results strongly suggests that the Baca reservoir contains two distinct fluids that are produced in varying proportions from individual wells. The noble gases in fluid A (~2900 mg/1 C1) are air-like, but with lighter gases and isotopes preferentially enriched. The fluid A ³⁶Ar content is low, only 13% that of 10°C air-saturated water (ASW). The second fluid, B (~ 1700 mg/1 C1), is the dominant carrier of the radiogenic and mantle-derived gases. The heavier non-radiogenic gases are preferentially enriched in fluid B, and its ³⁶Ar content is very low, only 5–7% ASW. The source of the noble gases in fluid A is tentatively ascribed to leaching of the relatively young (<1.4 m.y.) volcanic Bandelier Tuff. The radiogenic gases and mantle-derived helium in fluid B suggest a deeper source, possibly including gases escaping from a magma.