3He Concentrations Research Articles

Cosmogenic 3He in alluvial metal and alloy grains: Assessing the potential for quantifying sediment transport times

We have measured the He isotope composition in detrital native gold, copper, silver, Pt3Fe and OsIr alloy grains from placer deposits in order to assess the extent to which cosmogenic He can be quantified and used to determine the exposure history of grains in sedimentary systems. 3He/4He ratios (0.1–273 Ra) exceed values typical of radiogenic He. 4He is dominantly the result of the decay of U and Th, or 190Pt in the case of Pt-rich alloys. 3He concentrations in grains from alluvial deposits range from 0.05 to 2 × 108 at g−1 and appears to be dominantly cosmogenic in origin. The absence of significant nucleogenic or mantle-derived 3He in metals is tentatively confirmed by the analysis of grains from underground mine workings. The implantation of cosmogenic He is not the main source of 3He in alluvial metals but can be relatively important for small grains. New GEANT4-based calculations predict the production rate of 3He (including in precursor 3H) from Au and Pt to be approximately 25 at g−1 yr. Using this we determine that cosmogenic 3He concentrations in the detrital grains record minimum exposure times of between 0.5 and 7 million years that generally increase with distance from the putative source. Paired fragments from three large grains show that there is considerable heterogeneity in cosmogenic He concentrations that may reflect the complex origin of detrital metal grains. This work suggests that cosmogenic He in detrital metals and minerals may find use for providing the chronology of sedimentary processes.

U-Th/He systematics of fluid-rich ‘fibrous’ diamonds – Evidence for pre- and syn-kimberlite eruption ages

The physical characteristics and impermeability of diamonds allow them to retain radiogenic 4He produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic 4He concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped 4He, the in-situ produced radiogenic 4He, α-implantation, α-ejection, diffusion, and cosmogenic 3He production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic 3He component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the 4He and U-Th concentrations approximates the initial 4He concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the 4He and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71 Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.

A Cenozoic record of seawater uranium in fossil corals

We measured U/Ca ratios, ^4He concentrations, ^(234)U/^(238)U, and ^(238)U/^(235)U in a subset of well-preserved aragonitic scleractinian fossil corals previously described by Gothmann et al. (2015). Comparisons of measured fossil coral He/U ages with the stratigraphic age demonstrate that well-preserved coral aragonite retains most or all of its radiogenic He for 10’s of millions of years. Such samples must be largely or entirely free of alteration, including neomorphism. Measurements of ^(234)U/^(238)U and ^(238)U/^(235)U further help to characterize the fidelity with which the original U concentration has been preserved. Analyses of fossil coral U/Ca show that the seawater U/Ca ratio rose by a factor of 4-5 between the Early Cenozoic and today. Possible explanations for the observed increase include (1) the stabilization of U in seawater due to an increase in seawater [CO_3^(2-)], and a resulting increase in UO_2-CO_3 complexation as originally suggested by Broecker (1971); (2) a decrease in the rate of low-temperature hydrothermal alteration from Early Cenozoic to present, leading to a diminished U sink and higher seawater [U]; or (3) a decrease in uranium removal in reducing sediments, again leading to higher seawater [U].

Dating of glacial palaeogroundwater in the Ordovician-Cambrian aquifer system, northern Baltic Artesian Basin

The Ordovician-Cambrian aquifer system in the northern Baltic Artesian Basin contains glacial palaeogroundwater that originates from the Scandinavian Ice Sheet that covered the study area in the Pleistocene. Previously, no absolute dating of this palaeogroundwater has been attempted. In this multi-tracer study, we use 3H, 14C, 4He and stable isotopes of water to constrain the age distribution of groundwater. We apply the geochemical modelling approach developed by van der Kemp et al. (2000) and Blaser et al. (2010) to calculate the theoretical composition of recharge waters in three hypothetical conditions: modern, glacial and interstadial for 14C model age calculations. In the second phase of the geochemical modelling, the calculated recharge water compositions are used to calculate the 14C model ages using a series of inverse models developed with NETPATH. The calculated 14C model ages show that the groundwater in the aquifer system originates from three different climatic periods: (1) the post-glacial period; (2) the Late Glacial Maximum (LGM) and (3) the pre-LGM period. A larger pre-LGM component seems to be present in the southern and north-eastern parts of the aquifer system where the radiogenic 4He concentrations are higher (from ∼3.0·10−5 to 5.5·10−4 cc·g−1) and the stable isotopic composition of water is heavier (δ18O from −13.5‰ to −17.3‰). Glacial palaeogroundwater from the north-western part of the aquifer system is younger and has 14C model ages that coincide with the end of the LGM period. It is also characterized by lower radiogenic 4He concentrations (∼2.0·10−5 cc·g−1) and lighter stable isotopic composition (δ18O from −17.7 to −22.4‰). Relations between radiogenic 4He and 14C model ages and between radiogenic 4He and Cl− concentration show that groundwater in the aquifer system does not have a single well-defined age. Rather, the groundwater age distribution has been influenced by mixing between waters originating from end-members with strongly differing ages. Overall the results suggest, that in the shallower northern part of the aquifer system, significant changes in groundwater composition can be brought about by glacial meltwater intrusion during a single glaciation. However, multiple cycles of glacial advance and retreat are needed to transport glacial meltwater to the deeper parts of the aquifer system.

Proton driven fast ignition of low-radioactivity DT–3He fuel

With the advent of proton fast driven ignition and more efficient energy transport of the ignitor beam, we might be interested to try successful ignition of the targets mixed with advanced fuels where DT main fuel acts simply as a seed. Here, we have examined the ignition and burn conditions of a DT–3He pre-compressed fuel pellet driven by proton ignitor beam generated separately by the well-known laser-accelerated ion mechanism. We have, first, derived the zero-dimensional model of the energy balance equations of one fluid, three temperatures (i.e. electron, ion and radiation temperatures) including the most important gain and loss processes. Then, in order to evaluate the feasibility of the system and its energy gain, these coupled equations are solved numerically. It has been shown that the system energy gain quickly responses and is sensitive to the initial concentration of He-3, proton beam power and total areal density of fuel. A saturation in energy gain for ρR > 8 g/cm2 has been demonstrated which is equivalent to burn fraction of higher than 0.4. Enhancement in energy gain exceeds 33 percent. Moreover, in a complete catalytic regime of both tritium and He-3 ions, as much as an order of magnitude reduction in neutron flux is observed.

Helium and Argon Isotopes in the Fe-Mn Polymetallic Crusts and Nodules from the South China Sea: Constraints on Their Genetic Sources and Origins

In this study, the He and Ar isotope compositions were measured for the Fe-Mn polymetallic crusts and nodules from the South China Sea (SCS), using the high temperature bulk melting method and noble gases isotope mass spectrometry. The He and Ar of the SCS crusts/nodules exist mainly in the Fe-Mn mineral crystal lattice and terrigenous clastic mineral particles. The results show that the 3He concentrations and R/RA values of the SCS crusts are generally higher than those of the SCS nodules, while 4He and 40Ar concentrations of the SCS crusts are lower than those of the SCS nodules. Comparison with the Pacific crusts and nodules, the SCS Fe-Mn crusts/nodules have lower 3He concentrations and 3He/4He ratios (R/RA, 0.19 to 1.08) than those of the Pacific Fe-Mn crusts/nodules, while the 40Ar/36Ar ratios of the SCS samples are significantly higher than those of the Pacific counterparts. The relatively low 3He/4He ratios and high 40Ar concentrations in the SCS samples are likely caused by terrigenous detrital input with high radiogenic 4He and 40Ar contents. The SCS crusts and nodules have shorter growth periods, implying that in situ post-formation radiogenic 3He, 4He and 40Ar produced by decay of U, Th and K have no effect on their isotope compositions. Thus, the SCS crusts/nodules inherited the noble gases characteristics of their sources. Helium and Ar isotope compositions in the SCS Fe-Mn crusts and nodules reflect the product of an equilibrium mixture between air-saturated seawater and radiogenic components during their growth, while the partial 3He excess in some SCS samples may represent a little mantle-derived origin. The different He and Ar isotope compositions of the Fe-Mn crusts and nodules between the South China Sea and the Pacific Ocean are due to their different sources and genetic processes. The characteristics of He and Ar isotope compositions in the SCS polymetallic crusts and nodules are similar to the properties of hydrogenetic Fe-Mn oxide/hydroxide precipitates, which reflects mainly the product of an equilibrium mixture between air-saturated seawater and radiogenic components.

Development of Helium Isotope Reference Materials near Ambient 3He/4He Ratio

The 3He/4He ratio in the atmosphere is known to be 1.34 × 10−6. For accurate measurement of low 3He concentrations, a certified reference material (CRM) with a low 3He/4He ratio is necessary. In this work, we produced a new 3He CRM of helium isotopic ratio standard by the gravimetric method with a 3He concentration similar to that of the atmospheric air. Previously, a helium isotope CRM with a 3He concentration of 3He/4He = 4743 μmol/mol was developed. Because 3He/4He ratio of atmospheric helium is much lower than this, we decided to produce a new 3He CRM of helium isotopic ratio and the previously prepared 3He CRM was diluted by the gravimetric method. The concentration of 3He in the high purity helium gas used for dilution was measured by comparing it with the newly prepared low 3He concentration CRMs.

Determining gas expulsion vs retention during hydrocarbon generation in the Eagle Ford Shale using noble gases

The recent proliferation of tight or unconventional petroleum bearing reservoirs as an energy resource enables a more detailed investigation of the geochemical behaviour of these systems. In addition, it provides an opportunity to improve our understanding of low-permeability crustal fluid systems at depth. Organic-rich shales are not only the source-rocks in conventional petroleum systems, but potential seals, which may be important for the trapping and storage of CO2 and/or nuclear waste. The use of noble gas isotopes as tracers of fluid provenance and physical exchange processes is well established in other crustal fluid systems. Noble gas concentrations and isotopic characteristics were determined in 10 natural gas samples produced from the Eagle Ford shale, Texas, along with the concentration and δ13C and δD of hydrocarbon gases. By sampling gases produced directly from unconventional reservoirs we are able to determine their residence time and the extent of interaction of these fluids with other fluids in the wider hydrogeological system. The large range in thermal maturity exhibited across the basin, as demonstrated by the range in δ13C of methane from −37.8 to −47.5‰ VPDB, allows us to constrain the evolution of the noble gas signature within a source-rock during hydrocarbon generation and expulsion. For the first time, we show that 36Ar concentrations in hydrocarbon gases are not simply controlled by solubility exchange with formation water. Instead, they are shown to decrease dramatically (from 2.6 × 10−7 to 7.0 × 10−9 cm3STPcm−3) with increasing thermal maturity, which we attribute to a dilution effect as more short-chain hydrocarbon compounds are generated through cracking of kerogen and secondary cracking of oil. We develop a model that combines 36Ar concentrations with δ13C data in order to quantify the retention capacity for generated hydrocarbons, and show that between 40 and 80% of the hydrocarbons generated by the Eagle Ford shale are retained within the formation. We also calculate that radiogenic 4He concentrations within the Eagle Ford are well in excess of that which could have accumulated internally since deposition and requires contributions from external 4He sources. The identification of both hydrocarbon expulsion and helium addition to nominally ‘tight’ formations now provides a process driven and quantitative understanding of the fluid migration dynamics and processes controlling these critical formations.

Evolution of noble gas and water isotopes along the regional groundwater flow path of the Konya Closed Basin, Turkey

Noble gas and water isotope compositions of regional groundwater were investigated along two transects in the Konya Closed Basin (KCB) of central Turkey. According to the 3He/4He versus Ne/He plot of samples, crust (up to 86%) and mantle (up to 26%) appear to be the primary and secondary sources of dissolved He in groundwater, respectively. After the beginning of both transects where the flow domain is confined, both 3He and 4He accumulate steadily in groundwater. Thereafter, the intermediate recharge from the surface in the unconfined part of regional flow system disrupts the steady accumulation trend of 3He and 4He. Effect of intermediate recharge on regional groundwater is also indicated by the spatial variations in specific conductance, temperature, water isotopes, and tritium signals. At the last part of the flow domain where the system becomes confined again 3He and 4He start to accumulate in the eastern transect whereas they continue to decline in the western transect probably because of degassing to atmosphere through thinner confining unit. The 4He accumulation rates based on radiocarbon ages and 4He concentrations along both transects between the mountain flank and Obruk Plateau are in agreement with literature values but differ from each other. The accumulation rate along the eastern and western transects are 4.93 × 10-11 cm3 STP/g•year and 1.99 × 10-10 cm3 STP/g•year, respectively. These values reveal that the 4He accumulation rates may differ at least four times at different parts of a single aquifer, which are located 30 to 60 km apart. It is understood that any groundwater age-dating attempt by 4He requires a firm pre-assessment of spatial 4He accumulation rates. He isotope signal in the groundwater of KCB proves the mantle and/or crustal gas input into the carbonate aquifer in which an on-going hypogenic karst development has been suspected.

Helium3 isotope separation and lambda front observation

Abstract We present the method of the liquid helium enrichment with He3 isotope based on a special solution of the helium bath cryostat (25 l LHe) equipped with the heat exchanger and entropy filter to stimulate quantum filtration process in low temperature region. The cryostat design makes it possible to increase the initial concentration of He3 more than one order of magnitude. While the temperature of lambda point was achieved close to the capillary-equipped heat exchanger, the movement of lambda front within the liquid helium is observed. A consistent description of the various stages of the cryostat operation with cooling process above the lambda transition, during the long period of lambda front movement and in a superfluid region, with the acquisitioning of He3 isotope in all of these regions, is devoted to the design of a helium isotope flow-separator on an industrial scale.

Noble gas geochemistry of fluid inclusions in South African diamonds: implications for the origin of diamond-forming fluids

Fibrous diamond growth zones often contain abundant high-density fluid (HDF) inclusions and these provide the most direct information on diamond-forming fluids. Noble gases are incompatible elements and particularly useful in evaluating large-scale mantle processes. This study further constrains the evolution and origin of the HDFs by combining noble gas systematics with δ13C, N concentrations, and fluid inclusion compositions for 21 individual growth zones in 13 diamonds from the Finsch (n = 3), DeBeers Pool (n = 7), and Koffiefontein (n = 3) mines on the Kaapvaal Craton. C isotope compositions range from −2.8 to −8.6‰ and N contents vary between 268 and 867 at.ppm, except for one diamond with contents of <30 at.ppm N. Nine of the thirteen studied diamonds contained saline HDF inclusions, but the other four diamonds had carbonatitic or silicic HDF inclusions. Carbonatitic and silicic HDFs yielded low He concentrations, R/Ra (3He/4Hesample/3He/4Heair) values of 3.2–6.7, and low 40Ar/36Ar ratios of 390–1940. Noble gas characteristics of carbonatitic-silicic HDFs appear consistent with a subducted sediment origin and interaction with eclogite. Saline HDFs are characterised by high He concentrations, with R/Ra mostly between 3.9 and 5.7, and a wide range in 40Ar/36Ar ratios (389–30,200). The saline HDFs likely originated from subducted oceanic crust with low He but moderate Ar contents. Subsequent interaction of these saline HDFs with mantle peridotite could explain the increase in He concentrations and mantle-like He isotope composition, with the range in low to high 40Ar/36Ar ratios dependent on the initial 36Ar content and extent of lithosphere interaction. The observed negative correlation between 4He contents and R/Ra values in saline HDFs indicates significant in situ radiogenic 4He production.

Identifying temporally and spatially changing boundary conditions at an aquifer – aquitard interface using helium in porewater

Helium concentrations and 3He/4He isotope ratios of porewater, groundwater and rock were measured on samples collected from a Jurassic sediment sequence at the Mont Terri underground rock laboratory (Northern Switzerland). Porewater He data of rock samples collected from borehole BDB-1 at high spatial resolution across a karstic limestone unit (Passwang Formation) into the underlying claystone sequence (Opalinus Clay, Staffelegg Formation) describe a continuous profile from the water-conducting zone in the limestone into the clay-rich rocks of low permeability. Concentrations of 4He, 3He and their parent nuclides in the rock allow calculating in-situ production and accumulation terms. Since the time of sedimentation, 90%–97% of the in-situ produced 4He has been released to the porewater. Today only 2.5% of the maximum possible accumulated 4He is still retained in the porewater while the major part of in-situ produced 4He was removed from the system presumably by porewater–groundwater exchange. The porewater 4He concentrations show a diffusion profile from the aquitard towards the aquifer, reflecting a) a transient state between 4He in-situ production and porewater–groundwater exchange, b) a transient state from previously higher 4He concentrations in the porewater, and c) a spatially variable boundary in the karstic limestone unit. Evolutionary models of porewater 4He concentration profiles in combination with constraints from independent chemical and isotopic tracers allow deciphering a complex palaeo-hydrogeological history of the system over about the last 30 ka. A local excursion from the general profile towards higher 4He concentrations and 3He/4He ratio in a limestone layer in the Opalinus Clay cannot be further constrained in time based on the present sample frequency, but appears to represent a hydrogeological signal.

Reconstructing mantle carbon and noble gas contents from degassed mid-ocean ridge basalts

The fluxes of volatile elements from the mantle have long been used to understand mantle structure and evolution, and are critical controls on Earth's climate stability. Because of the ubiquity of magmatic degassing, inferring pre-degassing volatile concentrations from measured basalts requires the application of a degassing model. Such models, including the commonly-applied equilibrium Rayleigh distillation, typically assume equilibrium or solubility-based partitioning between melt and vapor. Here, we demonstrate that ratios of radiogenic isotopes of He, Ne, Ar and, especially, Xe measured in global mid-ocean ridge basalts (MORBs) are inconsistent with equilibrium degassing models, even when not considering He. We conclude that kinetic disequilibrium is a crucial process affecting volatile abundances during degassing. We present a simple disequilibrium Rayleigh distillation model to reconstruct pre-degassing MORB noble gas and carbon concentrations, which predicts that He and Ne achieve nearly equilibrium partitioning between melt and vesicles, but the slower-diffusing heavier noble gases are strongly affected by disequilibrium, resulting in non-equilibrium fractionation of noble gas elemental ratios, and other volatile element ratios like CO2/He.We apply our model to a large set of MORB data, and find average pre-degassing 3He, 22Ne, and 36Ar concentrations of 4.4 ± 0.9 × 10−10, 6.6 ± 1.4 × 10−11, and 6.8 ± 4.5 × 10−10 ccSTP/g (2σ), with variations of approximately 2 orders of magnitude, similar to other highly incompatible elements. Pre-degassing noble gas concentrations imply a mid-ocean ridge 3He flux of 800 ± 170 mol/yr and upper mantle 3He/22Ne and 3He/36Ar ratios of 6.6 ± 2.0 and 0.64 ± 0.44, but substantial variability in these ratios between samples. Applying our model to CO2, we calculate an average mantle CO2/3He molar ratio of 1.67 ± 0.21 × 109, which, when combined with our estimate of 3He flux, implies an upper mantle CO2 flux of 5.9 ± 1.0 × 1013 g/yr and a CO2 concentration of approximately 110 ppm.Our estimate of the mantle 3He flux is the first determined independently of oceanographic 3He measurements, and consequently represents a time-integrated flux substantially longer than ∼1000 years. And although at the high end of the range of previous estimates, our estimate does not resolve the long-standing heat-helium paradox. Additionally, our requirement for heterogeneous pre-degassing 3He/22Ne and 3He/36Ar ratios between samples is contrary to conclusions of previous applications of disequilibrium degassing models, which advocated uniform ratios. Furthermore, we find that CO2/Ba ratios are highly variable in MORB samples, but still consistent with an average mantle mass ratio of ∼100. However, estimating pre-degassing CO2 concentrations and the mantle CO2 flux depend strongly on the poorly-constrained carbon diffusivity. Consequently, our demonstration of the prevalence of disequilibrium during mid-ocean ridge degassing, and the potential for disequilibrium in other volcanic settings, highlights the need for better characterization of the physical parameters associated with volcanic degassing.

Application of combined 81Kr and 4He chronometers to the dating of old groundwater in a tectonically active region of the North China Plain

Groundwater dating by radio-krypton (81Kr; half-life of about 229,000 years) was applied to the sedimentary basin aquifer of the North China Plain (NCP). Krypton gas extracted from deep groundwater in the Coastal Plain was analyzed for 81Kr/Kr ratios by Atom Trap Trace Analysis, which yielded normalized ratios of 0.05 to 0.20, corresponding to groundwater residence times of 0.5–1 million years. Helium isotope compositions were determined on groundwater samples collected from the Central Plain and the Coastal Plain along a flow path of about 200 km. Helium dissolved in the groundwater samples are a mixture of atmospheric, crustal radiogenic and mantle derived sources. Mantle derived 3He contributes up to 30% of the total, and the area of occurrence coincides with zones of previous magmatic/tectonic activities. By contrast, >90% of 4He is derived from crustal reservoirs and correlates with 81Kr ages. The absolute groundwater ages (81Kr) and radiogenic 4He concentrations permit us to calibrate the 4He flux into the aquifer as well as the vertical diffusion rate of 4He to utilize the radiogenic 4He in groundwater as a quantitative age tracer. Previously, groundwater showed 14C activities near the limit of detection (30–40 k yr), in contrast Kr and radiogenic 4He data reveal progressively older ages from the recharge area to the Coastal Plain, from <20,000 yr to 0.5 to 1 Ma along the flow path of the NCP aquifers.

Chemical Fluxes From a Recently Erupted Shallow Submarine Volcano on the Mariana Arc

AbstractHydrothermal discharge from submarine arc volcanoes is thought to be an important contributor to global hydrothermal budgets, but quantitative flux measurements are scarce. Ahyi Seamount, a shallow (&lt;100 m) submarine intraoceanic arc volcano located in the Commonwealth of the Northern Mariana Islands, erupted in May 2014. In May and December 2014, we sampled the hydrothermal plume created by the eruption and estimated chemical fluxes from Ahyi by combining shipboard hull‐mounted Acoustic Doppler Current Profile current vector measurements with continuous and discrete Conductivity, Temperature, and Depth (CTD) data. Towed CTD sections were conducted perpendicular to the mean current direction: a sampling strategy that optimized chemical flux calculations by reducing complexities introduced by temporal variability in the speed and direction of plume dispersion. The Ahyi plume had an elevated optical backscatter signal accompanied by evidence of reduced chemical species and a lowered pH. We found enriched concentrations of H2, 3He, CH4, particulate S, Mn, and Fe, observations consistent with a highly active hydrothermal system. The fluxes of magmatic 3He and Fe from Ahyi were similar to that measured at three slow‐spreading ridge‐crest sites, whereas CH4 and Mn were 100–1,000 times lower. This is the first study to constrain export fluxes of a shallow submarine arc volcano into the euphotic zone. However, our data were collected soon after an eruption and thus may not be fully representative of the longer‐term chemical inputs from Ahyi.

Groundwater flowpaths and residence times inferred by 14C, 36Cl and 4He isotopes in the Continental Intercalaire aquifer (North-Western Africa)

In a semi-arid to arid climate context, dependency on groundwater resources may lead to overexploitation and deterioration of water quality. The Continental Intercalaire (CI) aquifer is one such continental-scale aquifer (more than a million of km2), which is mainly confined, poorly recharged but intensely abstracted. To date, the management of this resource relies on hydrogeological modelling and key parameters such as recharge/discharge rate and groundwater dynamics. We use a combination of residence time indicators (14C, 36Cl, 4He) and stable isotopes of water (2H and 18O) to give greater constraint on the groundwater residence time in the CI. In previous studies, 14C measurements and steady state modelling indicate a residence time of less than 100 ka whereas in others, 36Cl measurements and transient scenarios modelling suggest a longer residence time (>500 ka).In this study, most of the 14C measurements are below the limit of detection, establishing residence times greater than 40 ka and confirming the necessity of strict sampling protocols to exclude all air and AMS measurements when low 14C concentrations are expected.In the Tunisian recharge area, detectable 14C indicate sporadic recharge episodes (3–7 ka and 29–43 ka), whereas 4He and 36Cl concentrations in central areas suggest very old (<2 Ma) groundwaters. In these central areas, chlorine concentration can reach more than 2 g/l. Since 36Cl concentrations are up to 4 time less than the initial input, we are confident there is no excessive deep 36Cl production.We characterise five distinct flowpaths reaching the Tunisian discharge area using their isotopic signatures. According to our mixing model, the average contribution from the main recharge area, the Algerian Atlas Mountains, is around 88%. This value is close to hydrogeological models. Conversely, the contribution from the Dahar Mountains is lower than in the hydrogeological modelling (2% against 10%) whereas the Tinhert shows a greater contribution (10% against 1%).Increase of abstraction from the CI can potentially activate the circulation of old brackish groundwaters and dramatically decrease the water quality in the whole system.

Geochemistry and geochronology of the Banxi Sb deposit: Implications for fluid origin and the evolution of Sb mineralization in central-western Hunan, South China

Central-western Hunan in South China hosts the largest antimony belt in the world with two types of Sb deposits identified: Sb-Au Woxi-type and Sb only Xikuangshan-type. Banxi is the most representative deposit in the region with vein-type Sb mineralization hosted in Neoproterozoic clastic rocks, stibnite developing in ores, and arsenopyrite mainly occurring in altered country rocks.Trace element contents (including rare earth elements; REE) and isotopic ratios (S, Pb, Sr, Nd, He and Ar) in stibnite and/or arsenopyrite were analyzed to determine the timing of ore formation and to elucidate fluid origin and evolution processes. Most sulfides have high ΣREE contents (45–103ppm), moderate fractionation between LREE and HREE (LREE/HREE=6.38–11.56) and slightly negative Eu anomalies (Eu/Eu*=0.53–0.72). The δ34S values are 3.88–5.81‰ for stibnite and 9.25–11.82‰ for arsenopyrite. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios range from 18.617 to 18.635, 15.695 to 15.739, and 38.965 to 38.981 for stibnite, and from 18.594 to 18.609, from 15.587 to 15.698, and from 38.893 to 38.926 for arsenopyrite. The 87Rb/86Sr and 87Sr/86Sr ratios are 0.3016–3.538 and 0.711463–0.717591, respectively for stibnite, and 0.2251–2.214 and 0.711244–0.711565, respectively for arsenopyrite. The 147Sm/144Nd and 143Nd/144Nd ratios are 0.1174–0.9816 and 0.511942–0.512768 for stibnite, with εNd(t) values (t=130Ma) ranging from −12.4 to −6.6, and two-stage model ages (T2DM) ranging from 1457 to 1932Ma. The 40Ar/36Ar and R/Ra ratios (where Ra is the atmospheric 3He/4He ratio of 1.4×10−6) are 409–545 and 0.0088–0.0348, respectively for stibnite. The Rb–Sr and Sm–Nd isotopic analyses of the sulfides yield isochron ages of 129.4±2.4Ma (2σ, MSWD=1.3) and 130.4±1.9Ma (2σ, MSWD=1.6), respectively.The combined trace element and isotopic data-set indicates that the ore-forming fluids were a mixture of dominantly deep basement-derived, solute-rich parent water and a small amount of dilute, heated meteoric water. The decreases in ΣREE, δ34S, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 87Rb/86Sr, 87Sr/86Sr, 147Sm/144Nd and 143Nd/144Nd and increases in 40Ar/36Ar and 4He contents in the stibnite from the earlier quartz-stibnite mineralization stage (Stage II) through to the later stibnite mineralization stage (Stage III) indicate that the influx of meteoric water increased during Sb mineralization. The deep-sourced, relatively high temperature, rock-reacted fluid mixed with increasing amounts of more oxidized, low temperature meteoric fluids, causing the precipitation of arsenopyrite and stibnite sequentially at the Banxi deposit.A two-period genetic model has been proposed for the world-class Sb mineralization in the central-western Hunan region. The Woxi-type Sb–Au deposits were derived from orogenic fluids generated from folding and shearing during the Caledonian and Indosinian Orogeny, whereas the Xikuangshan-type Sb deposits were formed by mixing heated basement-derived hydrothermal fluid circulating during the flare-up of magmatism in South China, with lower temperature meteoric water.

Corrigendum to “Palaeohydrogeology and Transport Parameters Derived from 4He and Cl Profiles in Aquitard Pore Waters in a Large Multilayer Aquifer System, Central Australia”

A study of chloride and 4He profiles through an aquitard that separates the Great Artesian Basin from the underlying Arckaringa Basin in central Australia is presented. The aquitard separates two aquifers with long water residence times, due to low recharge rates in the arid climate. One-dimensional solute transport models were used to determine the advective flux of groundwater across the aquitard as well as establish any major changes in past hydrological conditions recorded by variations of the pore water composition. This in situ study showed that both diffusion and slow downward advection ( mm/yr) control solute transport. Numerical simulations show that an increase in chloride concentration in the upper part of the profile is due to a reduction in recharge in the upper aquifer for at least 3000 years. Groundwater extraction since 2008 has likely increased chloride and 4He concentrations in the lower aquifer by pulling up water from deeper layers; however, there has been insufficient time for upward solute transport into the pore water profile by diffusion against downward advection. The transport model of 4He and chloride provides insight into how the two aquifers interact through the aquitard and how climate change is being recorded in the aquitard profile.

About the nature of 3He isotope content increase in the region of 2.223 MeV gamma line generation in the period of solar flare

The intense solar flare on January 20, 2005 is studied based on the SONG-D/AVSF/ KORONAS-F data. The data on time profiles of the 2.223 MeV neutron line and narrow nuclear lines are involved in the analysis. Model calculations of neutrons propagating from their generation region in the corona to the neutron line formation region in the lower chromosphere and photosphere are performed. Coulomb energy losses of helium-3 ions in the case of their allowed acceleration in the flare are calculated. The increase in the relative 3He concentration during the flare is numerically analyzed. The study performed in combination with independent data on the helium-3 content in the flare region and helium-3 fluxes in interplanetary space lead to the conclusion that the acceleration region is the most probable immediate source of the increased content of accelerated helium-3 isotope ions.

Production of 21Ne in depth-profiled olivine from a 54 Ma basalt sequence, Eastern Highlands (37° S), Australia

In this study we investigate the cosmogenic neon component in olivine samples from a vertical profile in order to quantify muogenic 21Ne production in this mineral. Samples were collected from an 11 m thick Eocene basalt profile in the Eastern Highlands of southeastern Australia. An eruption age of 54.15±0.36 Ma (2σ) was determined from 40Ar/39Ar step-heating experiments (n=6) on three whole-rock samples. A 36Cl profile on the section indicated an apparent steady state erosion rate of 4.7±0.5 m Ma−1. The eruption age was used to calculate in situ produced radiogenic 4He and nucleogenic 3He and 21Ne concentrations in olivine. Olivine mineral separates (n=4), extracted from the upper two metres of the studied profile, reveal cosmogenic 21Ne concentrations that attenuate exponentially with depth. However, olivine (Fo68) extracted from below 2 m does not contain discernible 21Ne aside from magmatic and nucleogenic components, with the exception of one sample that apparently contained equal proportions of nucleogenic and muogenic neon. Modelling results suggest a muogenic neon sea-level high-latitude production rate of 0.02±0.04 to 0.9±1.3 atoms g−1 a−1 (1σ), or <2.5% of spallogenic cosmogenic 21Ne production at Earth’s surface. These data support a key implicit assumption in the literature that accumulation of muogenic 21Ne in olivine in surface samples is likely to be negligible/minimal compared to spallogenic 21Ne.