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.     

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.     

New geochronometer for the direct isotopic dating of native platinum minerals (190Pt-4He method)

A new method of isotope geochronology was proposed for dating native platinum minerals on the basis of the ??-decay of the natural isotope ¹⁹⁰Pt. The analysis of the thermal desorption of helium in the crystal lattice of native metals, including platinum, allowed us to predict a very high thermal stability (retentivity) of radiogenic ⁴He in native platinum minerals up to their melting temperatures. In order to validate the proposed ¹⁹⁰Pt-⁴He method, direct isotopic dating was performed for isoferroplatinum from the Galmoenan dunite-clinopyroxenite and Kondyor alkaline ultramafic massifs. The results of dating obtained by this method for primary ore platinum from the Galmoenan Massif (70 ± 5 Ma) are consistent with geological observations and mean Sm-Nd and Rb-Sr isotopic age estimates. The ¹⁹⁰Pt-⁴He age obtained for placer isoferroplatinum from the Kondyor Massif (112 ± 7 Ma) also agrees with geological observations and is close to the K-Ar and Rb-Sr ages of koswites (phlogopite-magnetite pyroxenites, gabbros, nepheline syenites, and metasomatic rocks after dunites). Our experimental data demonstrated that the ¹⁹⁰Pt-⁴He method is a promising tool for dating native platinum minerals.     

About possibility of isotope dating of native gold by the (U-Th)/He method

For investigation of helium in native gold, a new measuring complex was created and used: the high sensitivity mass spectrometer MSU-G (ZAO SKB “SPECTRON”). The sensitivity of measuring ⁴He was 5.3 × 10^?13 cm³/g per impulse. Experiments in stepwise heating of samples have been carried out, and the kinetics of radiogenic ⁴He emanation from native gold was investigated. Migration parameters (activation energy and frequency factor) were determined. Model calculations of stability (closure temperature) of radiogenic ⁴He in the native gold structure with a given time and temperature of thermal influences were made using the data received. The concentration of ⁴He in native gold from the original deposit Nesterovskoe is (4.7 ± 0.1) × 10^?5 cm³/g in the sample from the placer; from Chudnoe deposit, it is (3.8 ± 0.1) × 10^?5 cm³/g; from sulfide deposits of Kitoiskii knot of Eastern Sayani, it is (1.9 ± 0.1) × 10^?5 cm³/g; and from the South Muiskii ore region it is (8.7 ± 0.5) × 10^?7 cm³/g. The received curve lines of kinetics of ⁴He emanation from native gold show that radiogenic helium is well bonded in the native gold structure: in all the examined samples, most ⁴He emanates only by reaching the temperature of 950–1000°C. A specific feature of the kinetics of radiogenic ⁴He emanation in all examined samples is an outburstlike emanation in the form of a peak of large amplitude in the area of temperatures near the melting temperature point of gold. This is stipulated by the existence of helium bubbles released by metals only while they melt. The spectrum of helium thermal desorption from native gold has a complicated form and is a result of superposition of several peaks. This proves the migration of groups of atoms located in the gold structure in different energy states. Very large values of the activation energy of helium migration from native gold were received: up to 161–176 kcal/mol. Extremely large values of the frequency factor, from 2 × 10¹⁸ to 3 × 10³², correspond to such values of activation energies. This is caused probably by helium migration in the form of gas bubbles. The received data indicate the very high stability of the (U-Th)/He isotope system in native gold. Using the (U-Th)/He method of isotope geochronology seems to be very promising for isotope dating of these strategic raw materials.     

Rare gas isotopes and parent trace elements in ultrabasic-alkaline-carbonatite complexes, Kola Peninsula: identification of lower mantle plume component

During the Devonian magmatism (370 Ma ago) ∼20 ultrabasic-alkaline-carbonatite complexes (UACC) were formed in the Kola Peninsula (north-east of the Baltic Shield). In order to understand mantle and crust sources and processes having set these complexes, rare gases were studied in ∼300 rocks and mineral separates from 9 UACC, and concentrations of parent Li, K, U, and Th were measured in ∼70 samples. ⁴He/³He ratios in He released by fusion vary from pure radiogenic values ∼10⁸ down to 6 × 10⁴. The cosmogenic and extraterrestrial sources as well as the radiogenic production are unable to account for the extremely high abundances of ³He, up to 4 × 10⁻⁹ cc/g, indicating a mantle-derived fluid in the Kola rocks. In some samples helium extracted by crushing shows quite low ⁴He/³He = 3 × 10⁴, well below the mean ratio in mid ocean ridge basalts (MORB), (8.9 ± 1.0) × 10⁴, indicating the contribution of ³He-rich plume component. Magnetites are principal carriers of this component. Trapped ³He is extracted from these minerals at high temperatures 1100°C to 1600°C which may correspond to decrepitation or annealing primary fluid inclusions, whereas radiogenic ⁴He is manly released at a temperature range of 500°C to 1200°C, probably corresponding to activation of ⁴He sites degraded by U, Th decay.Similar ⁴He/³He ratios were observed in Oligocene flood basalts from the Ethiopian plume. According to a paleo-plate-tectonic reconstruction, 450 Ma ago the Baltica (including the Kola Peninsula) continent drifted not far from the present-day site of that plume. It appears that both magmatic provinces could relate to one and the same deep-seated mantle source.The neon isotopic compositions confirm the occurrence of a plume component since, within a conventional ²⁰Ne/²²Ne versus ²¹Ne/²²Ne diagram, the regression line for Kola samples is indistinguishable from those typical of plumes, such as Loihi (Hawaii). ²⁰Ne/²²Ne ratios (up to 12.1) correlate well with ⁴⁰Ar/³⁶Ar ones, allowing to infer a source ⁴⁰Ar/³⁶Ar ratio of about 4000 for the mantle end-member, which is 10 times lower than that of the MORB source end-member. In (³He/²²Ne)_PRIM versus (⁴He/²¹Ne)_RAD plot the Kola samples are within array established for plume and MORB samples; almost constant production ratio of (⁴He/²¹Ne)_RAD ≅ 2 × 10⁷ is translated via this array into (³He/²²Ne)_PRIM ∼ 10. The latter value approaches the solar ratio implying the non-fractionated solar-like rare gas pattern in a plume source.The Kola UACC show systematic variations in the respective contributions of in situ-produced radiogenic isotopes and mantle-derived isotopes. Since these complexes were essentially plutonic, we propose that the depth of emplacement exerted a primary control on the retention of both trapped and radiogenic species, which is consistent with geological observations. The available data allow to infer the following sequence of processes for the emplacement and evolution of Kola Devonian UACC: 1) Ascent of the plume from the lower mantle to the subcontinental lithosphere; the plume triggered mantle metasomatism not later than ∼700 to 400 Ma ago. 2) Metasomatism of the lithosphere (beneath the central part of the Kola Peninsula), including enrichment in volatile (e.g., He, Ne) and in incompatible (e.g., U, Th) elements. 3) Multistage intrusions of parental melts, their degassing, and crystallisation differentiation ∼370 Ma ago. 4) Postcrystallisation migration of fluids, including loss of radiogenic and of trapped helium. Based on model compositions of the principle terrestrial reservoirs we estimate the contributions (by mass) of the plume material, the upper mantle material, and the atmosphere (air-saturated groundwater), into the source of parent melt at ∼2%, 97.95%, and ∼0.05%, respectively.     

Effect of alpha-damage annealing on apatite (U–Th)/He thermochronology

Recent data on He diffusion challenge the temperature sensitivity of apatite (U–Th)/He thermochronology: the damage induced by recoil of U and Th decay series during emission of α particles (α-recoil damage) has been proposed to modify He-diffusion properties through time. However, we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly-cooled or reheated basement rocks. To test this hypothesis, we developed a quantitative model including an explicit treatment of α-recoil damage, annealing, and their effect on He-diffusion kinetics, and calibrate it against literature data. Our model is based on two hypotheses: (1) helium is in equilibrium between an apatite crystal and its defects and (2) alpha-recoil damage annealing can be described analogously to fission-track annealing. This model has been embedded into a Monte Carlo simulation of helium production/ejection/diffusion and applied to data from the French Massif Central; a complex slowly-cooled terrain with burial reheating, where the thermal history has been constrained by previous fission-track (FT) data including FT length distributions. (U–Th)/He ages are close to the FT ages from the same samples and are generally reproducible among replicates, but some samples present He-age dispersion that is not correlated with crystal size. Our model reproduces the Massif Central data very well except for three samples where He ages are older than corresponding FT ages. We show that annealing of irradiation damage has an important impact on retentivity of helium and that the He content, [He] is only a rough approximation of the damage level. In particular our results show that independence of He ages on crystal sizes, in case of reheated samples, is a clear indication of the higher He retentivity induced by α-recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of (U–Th)/He ages in such a case. More generally a correlation with the crystal size can bring information on the thermal path only if the age of defects, well represented by the fission-track age, is available, due to the dependence of the partial retention zone on damages. Conversely, in case of rapid cooling or for samples having low U and Th contents, damage effects can be ignored without significant effects on He ages.     

深部物质运动的气体地球化学特征

根据氦同位素地球化学资料讨论了中国东部和云南腾冲地区上地幔的脱气。尽管地球脱气作用主要发生在地球形成时的十亿年间，但是后期的脱气作用仍是影响大气圈演化的主要因素。在两种力学性质不同的构造带──中国东部大陆裂谷和位于欧亚板块与印度板块缝合带的腾冲火山区，采集了天然气样，并分析了气体组分和氦同位素组成，较高的３Ｈｅ／４Ｈｅ值和地质、地球物理资料表明天然气和温泉气中的氦相当一部分是来自上地幔。来自上地幔的氦和其他气体自第三纪以来不断在气藏中聚集或向大气中逃逸。伴有源于上地幔的岩浆活动的地幔脱气是深部物质运动的具体表现形式，它对新生代气候演变可能有直接影响。     

Helium and neon isotope geochemistry of some ground waters from the Canadian Precambrian Shield

Ground waters in a Precambrian granitic batholith at the Whiteshell Nuclear Research Establishment (WNRE) in Pinawa, Manitoba contain between 5 × 10^?5 and 10^?1 cc STP/g_H2O of radiogenic helium-4 but have relatively uniform ³He/⁴He ratios of between 0.6 × 10^?8 and 2.3 × 10³. The highest helium samples also contain radiogenic ^21,22Ne produced by (α,n) or (n,α) reactions with other isotopes. As much as 1.8 × 10^?9ccSTP/g_H2O of excess ²¹Ne and 3.8 × 10^?9ccSTP/gH₂O of excess ²²Ne have been measured. Helium and ²¹Ne ages of these ground waters, calculated on the basis of known crustal production rates of ⁴He and ²¹Ne, are unreasonably high (up to 2 × 10⁵ years) and incompatible with the ¹⁴C ages and other isotopic and hydrogeologic data. Uranium enrichment in the flow porosity of the granite may dominate ⁴He and ^21,22Ne production in this granite and mask the contributions from more typical U and Th concentrations in the rock matrix.At the Chalk River Nuclear Laboratories in Ontario helium concentrations in ground waters in a Precambrian monzonitic gneiss range from 1.5 × 10^?7 to 8.7 × 10^?4ccSTP/g_H2O with the ³He/⁴He ratios ranging from 2.0 × 10^?3 to 1.5 × 10^?7. The highest helium concentrations may be attributable to the presence of a thick uraniferous pegmatite vein and yield helium ages more than two orders of magnitude higher than the ¹⁴C ages. Application of He age dating equations to ground waters from Precambrian granitic rocks requires knowledge of the nature of uranium and thorium enrichment in the subsurface in order to select appropriate values for porosity and uranium and thorium concentration in the rock.     

4He diffusion in specular hematite

In order to test the chronometer qualities of speculante for the (U + Th)/He dating method, ⁴He release experiments by stepwise heating of two specularites from the Rimbach mineralization locality in the southern Vosgues (France) have been carried out. The diffusion coefficients define linear Arrhenius plots within a temperature interval of 250 to 830 °C, which is suggestive of volume diffusion. Extrapolation of the diffusion behavior to 20° C yields diffusion coefficients (D₂₀ values) smaller than 10^?26 [cm² s^?1] for both hematites with activation energies at 116 [kJ/mole]. The results of our study suggest that specularite is a very helium retentive hematite variety which is capable of quantitatively retaining radiogenic helium over geologic periods of time.     

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.     

Helium isotopes in ferromanganese crusts from the central Pacific Ocean

Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. ⁴He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High ⁴He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced ⁴He. ³He/⁴He of VA13/2 samples varies from 18.5 to 1852 R_a due almost entirely to variation in the extraterrestrial He contribution. The highest ³He/⁴He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial ³He concentration of the crusts rules out whole, undegassed 4-40 μm diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.     

Spatial variations in 3He/4He ratios along a high strain rate zone,central Japan

A linear zone with high strain rates along the Japan Sea coast, the Niigata-Kobe Tectonic Zone (NKTZ), is considered to be associated with rheological heterogeneities in the lower crust and/or upper mantle. Helium isotope variations along the NKTZ reveal a close association with the geophysical evidence for rheological heterogeneities in the crust and mantle. In the southern NKTZ, the ³He/⁴He ratios lower than 3.4 Ra (Ra denotes the atmospheric ³He/⁴He ratio of 1.4 × 10⁻⁶) could be interpreted as a two-component mixture of helium stored in aqueous fluids driven off the subducting oceanic crust and radiogenic crustal helium. Higher ³He/⁴He ratios are observed in the central NKTZ where Quaternary volcanoes and high-temperature hot springs are concentrated, suggesting that the ³He emanation manifest in the central NKTZ results from the effective transfer of mantle helium by intrusion and degassing of mantle-derived magma in the crust. In the northern NKTZ where two large inland earthquakes occurred recently, there appears to be many samples with ³He/⁴He ratios significantly higher than those observed in the fore-arc side of northeast Japan. A plausible source of mantle helium could be attributed to upward mobilization of aqueous fluids generated by dehydration of the subducting Pacific Plate slab.     

Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia

⁴⁰Ar/³⁹Ar laser incremental heating analyses of individual grains of supergene jarosite, alunite, and cryptomelane from weathering profiles in the Dugald River area, Queensland, Australia, show a strong positive correlation between a sample’s age and its elevation. We analyzed 125 grains extracted from 35 hand specimens collected from weathering profiles at 11 sites located at 3 distinct elevations. The highest elevation profile hosts the oldest supergene minerals, whereas progressively younger samples occur at lower positions in the landscape. The highest elevation sampling sites (three sites), located on top of an elongated mesa (255 to 275 m elevation), yield ages in the 16 to 12 Ma range. Samples from an intermediate elevation site (225 to 230 m elevation) yield ages in the 6 to 4 Ma range. Samples collected at the lowest elevation sites (200 to 220 m elevation) yield ages in the 2.2 to 0.8 Ma interval.Grains of supergene alunite, jarosite, and cryptomelane analyzed from individual single hand specimens yield reproducible results, confirming the suitability of these minerals to ⁴⁰Ar/³⁹Ar geochronology. Multiple samples collected from the same site also yield reproducible results, indicating that the ages measured are true precipitation ages for the samples analyzed. Different sites, up to 3 km apart, sampled from weathering profiles at the same elevation again yield reproducible results. The consistency of results confirms that ⁴⁰Ar/³⁹Ar geochronology of supergene jarosite, alunite, and cryptomelane yields ages of formation of weathering profiles, providing a reliable numerical basis for differentiating and correlating these profiles.The age versus elevation relationship obtained suggest that the stepped landscapes in the Dugald River area record a progressive downward migration of a relatively flat weathering front. The steps in the landscape result from differential erosion of previously weathered bedrock displaying different susceptibility to weathering and contrasting resistance to erosion. Combined, the age versus elevation relationships measured yield a weathering rate of 3.8 m. Myr⁻¹ (for the past 15 Ma) if a descending subhorizontal weathering front is assumed. The results also permit the calculation of the erosion rate of the more easily weathered and eroded lithologies, assuming an initially flat landscape as proposed in models of episodic landscape development. The average erosion rate for the past 15 Ma is 3.3 m. Myr⁻¹, consistent with erosion rates obtained by cosmogenic isotope studies in the region.     

Rare gas isotopic composition of natural gases

One hundred natural gas samples of different geological periods collected from various localities were determined for their nitrogen, helium and argon contents as well as the isotopic composition of argon. The ratio Ar⁴⁰/Ar³⁶ was found to be from 305 to 9,255. A current heliumargon chronological formula for calculating natural gas ages was corrected. Instead, based upon the chronological accumulating effect of radiogenic Ar⁴⁰, an Ar⁴⁰-Ar³⁹ empirical formula for calculating natural gas ages has been proposed. It can be seen from the ratio of nitrogen to atmospheric argon that most of the nitrogen in these natural gases could not be derived from atmospheric nitrogen. The range of radiogenic He/Ar⁴⁰ is from 5.19 to 23. 18, and that for most samples from 7 to 14. The He/Ar⁴⁰ ratios of Sinian samples are from 5.39 to 6.14, indicating their derivation from the deep interior of the earth. It should be noted that the Ar⁴⁰/Ar³⁶ ratio shows a general trend of approaching the atmospheric value from the Sinian to the Tertiary period, or from old to young in geological age, mirroring, to a great extent, the evolutionary tendency of the rare gas isotopic composition of natural gases with continuous degassing of the earth.     

He,Ne, Ar,and C isotope systematics of geothermal emanations in the Lesser Antilles Islands Arc

We present He, Ne, Ar, and C isotope analyses of hydrothermal brines and gases from fumaroles, hot springs, mofettes and hydrothermal exploration drillings on the major islands of the Lesser Antilles Arc. The origin of hydrothermal brines, which have been analyzed also for O and H isotopes, is essentially meteoric-hydrothermal. Air-corrected isotope compositions of helium (2.2 Rc/Ra < ³He/⁴He < 8.6 Rc/Ra) and carbon (−20 < δ¹³C_PDB < +0.5) are variable and require a variety of crustal and magmatic sources. The diversity of δ¹³C_PDB and ³He/CO₂ ratios within individual volcanic centres suggests that crustal sources (e.g., limestone) contaminate magmatic CO₂ en route from high-level magma reservoirs (depth < 15 km) to the surface. A similar contamination may be found for magmatic helium on distal springs. The ³He/⁴He signature of summit fumaroles, thought to reflect the ³He/⁴He signature of high-level magmas, shows a remarkable systematic variation along the arc. In addition, there is a correlation throughout the arc between published Sr, Pb, and Nd isotope signatures of lavas and the ³He/⁴He signatures of summit fumaroles. On the northern islands (Nevis, Montserrat, Guadeloupe, and Dominica) summit fumaroles have the N-MORB signature (³He/⁴He = 8 ± 1 R/Ra), and the isotope signature of lavas is not dissimilar from comparable intra-oceanic arc tholeiites elsewhere. Variable enrichments in radiogenic Sr and Pb have been reported for lavas of individual volcanic centres of the Southern Islands (Martinique, St.Lucia, and Grenada), and summit fumaroles on these centres match these variations by variable radiogenic He-enrichments, i.e., lower ³He/⁴He ratios. This correlation suggests that radiogenic Sr and Pb enrichments of lavas and low ³He/⁴He signatures on summit fumaroles have a common origin, i.e., a terrigenous contaminant derived from the Orinoco depositionary fan. Crustal assimilation is thought to decouple the He isotope system from any other radiogenic isotope system and, therefore, we argue that the observed correlation of He, Sr, Pb, and Nd isotope systems is related to a terrigenous contaminant derived from subducted sediments. Support for this scenario also comes from the matching of low ³He/⁴He ratios and tectonic features of the forearc thought to favor the subduction of forearc sediments.The present study offers a first clue that, under suitable conditions, crustal helium from oceanic sediments might be subducted to the depth of arc magma sources and, possibly, even recycled into the deeper mantle.     

Processes controlling 234U and 238U isotope fractionation and helium in the groundwater of the St. Lawrence Lowlands,Quebec: The potential role of natural rock fracturing

The goal of this study is to explain the origin of ²³⁴U–²³⁸U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with ³He/⁴He ratios, to gain insight regarding the evolution of groundwater in the region. (²³⁴U/²³⁸U) activity ratios, or (²³⁴U/²³⁸U)_act, were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest (²³⁴U/²³⁸U)_act, 1.14 ± 0.01, was measured in Ca–HCO₃-type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little ²³⁴U–²³⁸U isotopic fractionation. The (²³⁴U/²³⁸U)_act increases to 6.07 ± 0.14 in Na–HCO₃–Cl-type groundwater. Preferential migration of ²³⁴U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between (²³⁴U/²³⁸U)_act and ³He/⁴He ratios has been observed. This relationship reflects the mixing of newly recharged water, with (²³⁴U/²³⁸U)_act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water (¹⁴C ages of 6.6 kyrs), with (²³⁴U/²³⁸U)_act of ≥6.07 and large amounts of radiogenic ⁴He, in excess of the steady-state amount produced in situ. The simultaneous fractionation of (²³⁴U/²³⁸U)_act and the addition of excess ⁴He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced ⁴He and ²³⁴U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic ⁴He excesses measured in groundwater.     

The influence of artificial radiation damage and thermal annealing on helium diffusion kinetics in apatite

Recent work [Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249(3-4), 148-161] revealing a correlation between radiogenic ⁴He concentration and He diffusivity in natural apatites suggests that helium migration is retarded by radiation-induced damage to the crystal structure. If so, the He diffusion kinetics of an apatite is an evolving function of time and the effective uranium concentration in a cooling sample, a fact which must be considered when interpreting apatite (U-Th)/He ages. Here we report the results of experiments designed to investigate and quantify this phenomenon by determining He diffusivities in apatites after systematically adding or removing radiation damage.Radiation damage was added to a suite of synthetic and natural apatites by exposure to between 1 and 100 h of neutron irradiation in a nuclear reactor. The samples were then irradiated with a 220 MeV proton beam and the resulting spallogenic ³He used as a diffusant in step-heating diffusion experiments. In every sample, irradiation increased the activation energy (E_a) and the frequency factor (D_o/a²) of diffusion and yielded a higher He closure temperature (T_c) than the starting material. For example, 100 h in the reactor caused the He closure temperature to increase by as much as 36 °C. For a given neutron fluence the magnitude of increase in closure temperature scales negatively with the initial closure temperature. This is consistent with a logarithmic response in which the neutron damage is additive to the initial damage present. In detail, the irradiations introduce correlated increases in E_a and ln(D_o/a²) that lie on the same array as found in natural apatites. This strongly suggests that neutron-induced damage mimics the damage produced by U and Th decay in natural apatites.To investigate the potential consequences of annealing of radiation damage, samples of Durango apatite were heated in vacuum to temperatures up to 550 °C for between 1 and 350 h. After this treatment the samples were step-heated using the remaining natural ⁴He as the diffusant. At temperatures above 290 °C a systematic change in T_c was observed, with values becoming lower with increasing temperature and time. For example, reduction of T_c from the starting value of 71 to ∼52 °C occurred in 1 h at 375 °C or 10 h at 330 °C. The observed variations in T_c are strongly correlated with the fission track length reduction predicted from the initial holding time and temperature. Furthermore, like the neutron irradiated apatites, these samples plot on the same E_a − ln(D_o/a²) array as natural samples, suggesting that damage annealing is simply undoing the consequences of damage accumulation in terms of He diffusivity.Taken together these data provide unequivocal evidence that at these levels, radiation damage acts to retard He diffusion in apatite, and that thermal annealing reverses the process. The data provide support for the previously described radiation damage trapping kinetic model of Shuster et al. (2006) and can be used to define a model which fully accommodates damage production and annealing.     

Granite is an Effective Helium Source Rock: Insights from the Helium Generation and Release Characteristics in Granites from the North Qinling Orogen,China

Global helium(He) shortage is a challenging problem; however, the types of helium source rock and the mechanisms of He generation and release therein remain still poorly understood. In this study, in order to evaluate the potential of granite as an effective helium source rock, we collected granitic samples from the North Qinling Orogen, Central China, in the south of the helium-rich Weihe Basin. The helium generation and release behaviors in granite were studied through analysis of U and Th concentrations, EMPA images, and He and Ar concentrations and isotopic ratios extracted by crushing and stepwise heating. The results indicate that Ar has a better retention and a lower mobility than He. 3 He/4 He ratios released by crushing and stepwise heating are 0.016–0.056 RA and 0.003–0.572 RA, respectively, where RA is the atmospheric 3 He/4 He of 1.4×10-6, reflecting a crustal and radiogenic source. Helium concentrations extracted by the two ways are 0.13–0.95 ucm3 STP/g and 7.82–115.62 ucm3 STP/g, respectively, suggesting that matrix-sited He accounts for more than 98% of total helium preserved in granite. In addition, the total generated He amounts in granites are calculated based on the measured U and Th concentrations in granitic samples. Dividing the preserved He quantities by the generated He amounts, it turned out that less than 10% of He produced since the formation of the granite is preserved in the rock over geological time, suggesting that more than 90% generated He can be transferred to the Weihe Basin. Temperature and fracture are the two critical factors controlling He release. Based on the relationship between He diffusivity of granites and temperature and the He closure temperatures of a variety of U-and Th-rich minerals(27–250°C), we estimate that He can be partially released out of granite at the depths 400 m and totally released at the depths 7800 m. Fractures provide effective transfer of free He from deep source rocks to shallow reservoirs. Finally, a model on granite as an effective helium source rock is established. We suggest exploring He resources in hydrocarbon basins with granitic basement(or adjacent to granite bodies), high geothermal field, and young active fractures.     

Argon retentivity and argon excess in amphiboles from the garbenschists of the Western Tauern Window,Eastern Alps

22 hornblende K-Ar ages and 10 ³⁹Ar-⁴⁰Ar spectra were obtained for hornblende garbenschists from the Western Tauern Window. The post-kinematic amphiboles were produced during the late Alpine prograde metamorphism (6–10 kb and 500–570° C). Two nearly potassiumfree cummingtonites rimming hornblende yield K-Ar ages of 120 Ma, while the 20 tschermakitic hornblendes scatter between 17 and 37 Ma. The reason for this scatter is excess Ar, possibly incorporated into amphiboles during healing of fractures, now traceable by trails of fluid inclusions. Excess Ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with K-Ar isochrons. It can be quantified in 4 out of 10 hornblendes by ³⁹Ar-⁴⁰Ar stepwise heating experiments. Ages of 18–20 Ma result for corrected hornblendes. The retentivity of ⁴⁰Ar, after correction for excess, shows no correlation with chemistry within the narrow compositional range observed; rather, it shows intriguing correlations with irregularities in Ca/K spectra, pointing to a microstructurally controlled mechanism for Ar loss. This observation leads to a critical evaluation of the closure temperature constant, which apparently depends on an incompletely known number of mineralogical and environmental parameters. In particular those ³⁹Ar-⁴⁰Ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure. This gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media.     

Release of mantle and crustal helium from a fault following an inland earthquake

Static stress changes caused by megathrust slip of the 2011 M_w 9.0 Tohoku-Oki earthquake considerably affected the seismicity patterns in inland areas, resulting in the occurrence of numerous earthquakes along several active faults in Japan. On June 30, 2011, the M_j 5.4 central Nagano earthquake occurred at a shallow depth of 5 km, indicating the reactivation of the Gofukuji fault in Central Japan. This study was undertaken to elucidate spatial and temporal changes of ³He/⁴He ratios around a source region before and after an inland earthquake using both existing and new and helium isotope data from hot spring and drinking water wells. Gas samples near the Gofukuji fault and its surrounding active faults are characterized by an increase in postseismic ³He/⁴He ratios. In contrast, the postseismic ratios decreased by up to about 30% away from the mainshock epicenter. Episodic faulting could either release stored crustal (radiogenic) helium from host rocks, or enhance the transfer of mantle volatiles through permeable fault zones, such that subsequent fluid flow near to the source region could then explain the spatio-temporal variations in ³He/⁴He ratios.