Radiogenic Helium Research Articles

Quantifying the Contribution and Mobility of In Situ-Produced Helium in He-Retentive Minerals: A Case Study of the Salla-Kuolajarvi Metasomatic Rocks (Russia)

An alternative approach to separating trapped and radiogenic helium, and assessing the mobility of the latter in He-retentive minerals implemented in the present work, has been developed on the basis of helium extraction patterns obtained via incremental heating of mineral samples. We used these data both to estimate helium mobility in concentrates of various ore minerals (pyrite, magnetite, and hematite) and to assess the contribution of in situ-produced 4He in the total helium budget. The rocks of uranium-ore and gold mineralization of the Salla-Kuolajarvi belt (northern Karelia, Fennoscandian Shield) were used as test materials for the new approach. The method allowed such parameters as diffusivity, activation energy, closure temperature, and the contribution of the trapped helium to be obtained for all samples. The latter was used for the correction of apparent U-Th-He bulk ages. The interpretation of the calculated values was performed taking into account the closure temperatures of the U-Th-He system, as well as the peculiarities of each individual sample.

Deep CO2 emissions facilitated by localized shear deformation: A case study of the Karakoram fault system, western Tibet

Strike-slip faults play a significant role in creating deeply penetrating fractures with high permeability, thus promoting rapid migration of CO2-rich fluids to the surface. However, there are rare observations regarding how strike-slip movement could affect deep CO2 emissions. Here, we focus on the Karakoram fault system (KKFS), western Tibet, to estimate diffuse soil CO2 fluxes and to unravel potential controlling factors for CO2 emissions. Average CO2 fluxes of geothermal fields range in 22–2475 g m−2 d−1, significantly higher than the across-fault profiles (6–116 g m−2 d−1). A mass balance model based on δ13C-CO2 and CO2 concentration of soil gases reveals that deep carbon constitutes 49.1–91.5 % (average = 73.9 %) and 0.2–40.5 % (average = 25.5 %) of soil-gas carbon released from geothermal fields and across-fault profiles, respectively. Deep carbon could be produced by thermal decomposition of crustal rocks considering CO2-rich fluids with radiogenic helium isotopes. Strikingly, higher CO2 fluxes preferentially occur in geothermal fields along a bending segment of the KKFS, where localized shear deformation is prominent as documented by high slip rates over geological timescales, dense splay faults, clustering of earthquake events, and elevated strain rates. We suggest that high stress acting on the KKFS bend could enhance the deformation and fracturing of fault zone rocks, leading to production of metamorphic CO2 and efficient release of CO2-rich fluids through the highly permeable fault system. Our results could shed new light on CO2 origins and fluxes of strike-slip faults that are characterized by spatially heterogeneous strain partitioning and thus localized enhanced shear deformation.

Probing thermodynamics of radiogenic helium and defects in δ-plutonium alloys and interactions with adsorbed environmental gases

Differential scanning calorimetry coupled with simultaneous evolved gas analysis (DSC-EGA) on aged δ-Pu samples shows that most radiogenic helium remains trapped within the Pu matrix at temperatures very close to, or slightly above, the melting temperature. Our results indicate that helium release from 50-year-old δ-Pu occurs as a burst just below the melting temperature (>0.994 Tm), with subsequent pressure oscillations as temperature increases. Subordinate quantities of H2 were also released along with helium. The helium emission tails off and ceases above ∼720 – 750°C. In a δ-Pu alloy aged 6 years, the initial helium burst occurs slightly above melting (∼1.015 – 1.042 Tm), with a discrete, larger helium spike occurring between 670 and 686°C. The proximity of helium release to the liquidus transition presented challenges in the deconvolution of overlapping process enthalpies, the liquidus endotherm, and the exotherm resulting from bubble collapse, annealing and gas expulsion. Helium’s strong affinity for vacancy binding in a 2He-vac configuration is predicted by Density Functional Theory (DFT) modeling. The measured stored energy associated with the He release events in a 50-year-old δ alloy is on the order of ∼10–11 J/g, which is significantly higher than stored energies measured in the sub-solidus regimes (∼2 J/g) that are related to the solid-state annealing of processing- and radiation-induced defects. This implies that aged δ Pu alloys have a remarkable resilience to accommodate the lattice strain produced by the internal pressure of the helium bubbles and provides further insight into the thermodynamic behavior of aged δ Pu.

A North Pacific Meridional Section (U.S. GEOTRACES GP15) of Helium Isotopes and Noble Gases I: Deep Water Distributions

AbstractThe noble gas signature of incoming Pacific Bottom Water (PBW), when compared to North Atlantic Deep Water, indicates the addition of 450 ± 70 GT a−1glacial melt water to form AABW and subsequently PBW. The downstream evolution of this signature between the southern (20°S to equator) and northern (25°–45°N) bottom waters indicates a decrease in sea level pressure around Antarctica over the past two millennia. Vertical profiles of noble gases in the deep Pacific show exponential relationships with depth with scale heights identical to temperature and salinity. Unlike the other noble gases, helium isotopes show evidence of mid‐depth injection of non‐atmospheric helium. Using observed deviations from exponential behavior, we quantify its magnitude and isotope ratio. There is a clear latitude trend in the isotope ratio of this added helium that decreases from a high exceeding 9 RA(atmospheric3He/4He ratio) in the south to around 8 RAnear the equator. North of 30–40°N, it systematically decreases northward to a low of ∼2 RAnorth of 50°N. This decline results from a combination of northward decline in seafloor spreading, release of radiogenic helium from increased sediment thickness, and the possible emission of radiogenic helium through cold seeps along the Alaskan and North American margins. Finally, we derive an improved method of computing the excess helium isotope concentrations and that the distributions of bottom water3HeXS/4HeXSare consistent with what is known about bottom water flow patterns and the input of low3He/4He sedimentary helium.

On the parameterisation of dual-continuum models for reactive transport modelling in fractured media

In sparsely fractured rocks, the rock matrix is an important geochemical buffer and provides significant retardation to contaminants advected through the flowing fractures. Accounting for geochemical reactions and mass exchange between these two regions is key to properly capture the overall buffering capacity and the related hydrogeochemical evolution of a fractured medium. Reactive transport modelling in these kinds of fractured media is routinely performed using Equivalent Continuous Porous Media (ECPM) models: i.e. continuum models based on permeability and porosity fields that somehow preserve the underlying fracture properties, which are in turn described by companion Discrete Fracture Network (DFN) models. However, the proper parameterisation of these models, in terms of mass exchange between fractures and the bordering matrix, is still a largely unresolved issue. Here, we leverage the Dual Continuum Disconnected Matrix Model (DCDMM) formulation included in the massively parallel code PFLOTRAN to propose a novel parameterisation approach that honours the local volumetric fracture density (P32). The proposed approach is first benchmarked against a semi-analytical solution with a problem that entails flow and transport along two different and consecutive fractures. Two demonstrative large-scale reactive transport problems are also presented and discussed: the first is related to the generation and migration of radiogenic helium and the second assesses the buffering capacity of a realistic fractured medium against the infiltration of acidic water. The latter simulation, which includes more than three hundred million transport degrees of freedom is one of the largest subsurface reactive transport models ever formulated and solved. This simulation was made possible by the highly efficient implementation of the DCDMM in PFLOTRAN, which makes the solution of the secondary continuum equations embarrassingly parallel.

Late Cenozoic deepening of Yosemite Valley, USA

Although Yosemite Valley, USA, catalyzed the modern environmental movement and fueled foundational debates in geomorphology, a century of investigation has failed to definitively determine when it formed. The non-depositional nature of the landscape and homogeneous bedrock have prevented direct geological assessments. Indirect assumptions about the age of downcutting have ranged from pre-Eocene to Pleistocene. Clarity on this issue would not only satisfy public interest but also provide a new constraint for contentious debates about the Cenozoic tectonic and geomorphologic history of the Sierra Nevada in California. Here we use thermochronometric analysis of radiogenic helium in apatite crystals, coupled with numerical models of crustal temperatures beneath evolving topography, to demonstrate significant late Cenozoic deepening of Tenaya Canyon, Yosemite’s northeastern branch. Approximately 40%−90% of the current relief has developed since 10 Ma and most likely since 5 Ma. This coincides with renewed regional tectonism, which is a long-hypothesized but much debated driver of Sierran canyon development. Pleistocene glaciation caused spatially variable incision and valley widening in Yosemite Valley, whereas little contemporaneous erosion occurred in the adjacent upper Tuolumne watershed. Such variations probably arise from glacial erosion’s dependence on opographic focusing of ice discharge into zones of rapid flow, and on the abundance of pre-existing fractures in the substrate. All available data, including those from our study, are consistent with a moderately high and slowly eroding mid-Cenozoic Sierra Nevada followed by significant late Cenozoic incision of some, but not all, west-side canyons. A likely driver of this event was range-crest uplift accompanied by fault-induced beheading of some major drainages, although other mechanisms such as drainage reorganization following volcanic deposition are plausible.

Atomistic simulation of helium diffusion and clustering in plutonium dioxide.

This study uses molecular dynamics and barrier searching methods to investigate the diffusion and clustering of helium in plutonium dioxide. Such fundamental understanding of helium behaviour is required because radiogenic helium generated from the alpha decay of Pu nuclei can accumulate over time and storage of spent nuclear fuel needs to be safe and secure. The results show that in perfect PuO2, interstitial He is not mobile over nanosecond time scales at temperatures below 1500 K with the lowest diffusion barrier being 2.4 eV. Above this temperature O vacancies can form and diffusion increases. The He diffusion barrier drops to 0.6 eV when oxygen vacancies are present. High temperature simulations show that the key He diffusion mechanism is oxygen vacancy assisted inter-site hopping rather than the direct path between adjacent interstitial sites. Unlike oxygen vacancies, plutonium vacancies act as helium traps. However, isolated substitutional He at Pu sites can be easily ejected through displacement by neighbouring interstitial Pu atoms. High temperature MD simulations show that helium can diffuse into clusters with the majority of helium clusters which form over nanosecond time scales having a He : vacancy ratio below 1 : 1. Further static calculations show that a ∼3.5 : 1 He : vacancy ratio is the largest possible for an energetically stable helium cluster. Schottky defects act as seed points for He cluster growth and a high local concentrations of He can create such defects which then pin the growing He cluster.

Accommodation of helium in PuO2±x and the role of americium.

The high alpha-activity of plutonium dioxide (PuO2) results in significant ingrowth of radiogenic helium (He) in the aged material. To safely store/dispose PuO2 or use in applications such as space exploration, the impact of He accumulation needs to be understood. In this work, defect energies obtained using a density functional theory (DFT) + U + D3 scheme are used in a point defect model constructed for PuO2 to predict the method of He incorporation within the PuO2 lattice. The simulations predict that the preferred incorporation site for He in PuO2 is a plutonium vacancy, however, the point defect model indicates that helium will be accommodated as an interstitial irrespective of He concentration and across a wide stoichiometric range. By considering the charge imbalance that arises due to incorporation of Am3+ ions it is shown that He accommodation in oxygen vacancy sites will dominate in PuO2-x as the material ages.

Unlocking the Potentials of Helium in Nigeria

With the dwindling price of crude oil globally and its negative consequences on the economy and environment, there is an urgent need to diversify in the search for alternative sources of revenue for the country particularly in the Oil and Gas sector.  The discovery of radiogenic helium in commercial quantity serves to augment the gross domestic product of any nation due to the various roles it plays in our everyday life; from entertainment to the medical field, where it is used for cooling superconducting magnets in medical Magnetic Resonance Imaging (MRI) scanners and Nuclear Magnetic Resonance (NMR) spectrometers to industrial applications as an agent for in maintaining controlled atmosphere and in detecting leakages. It is also useful in supersonic wind tunnels because of its stability, thermal and caloric perfect nature as well as its high speed of sound and high heat capacity ratio. Just recently, it was discovered that helium is being used to create hard drives in computers. Helium has no known substitution in cryogenic uses. It is obvious that there is a great need for the indigenous sourcing of this valued resource which is non-renewable. The discovery of large quantities of helium in Tanzania has heightened the search for this noble gas in Nigeria.  This paper is therefore aimed at reviewing the generation, migration and reservoir accumulation of the gas from both biogenic and abiogenic sources.in order to support and likewise make vital contribution towards sourcing for helium in Nigeria.

Mobility of Radiogenic Helium in Amphibole

Recently experiments on He extraction from an amphibole by the incremental heating unexpectedly revealed that the He release pattern depends on the heating rate. During slow heating (~4 K·min−1) of the amphibole grains, one smooth peak of the He flux from the mineral was observed; in contrast, during fast heating (~40 K·min−1) an additional sharp peak appeared at a temperature about 750 °C. In order to explain these observations, we developed a model of He diffusion from the amphibole, which allowed the calculated He fluxes from the mineral to be reconciled with those observed. From the modelling we derived: (i) the helium diffusion domain size distribution, and evolution of the distribution in the course of incremental heating; (ii) occurrence of the tensile stresses, operating under enhanced temperatures above 700 °C. The stresses are different in sites with the different local thermal expansion of the crystalline lattice and they increase the He diffusion flux. The model can be applied to other minerals (materials).

Mantle helium in Southern Quebec groundwater: A possible fossil record of the New England hotspot

The Monteregian Hills are an alignment of magmatic intrusions of Cretaceous age located in the St. Lawrence Lowlands, Quebec, Canada. Their origin is controversial and numerous studies have failed to decipher between a hotspot trail or sub-continental magmatism related to the opening of the North Atlantic Ocean. Here, we show that 17.7±9.6% of the helium of the modern to Holocene-aged groundwater from the regional aquifer is of mantle origin, with a 3He/4He (R) of up to 1.42 times the atmospheric ratio (Ra). It suggests that a fossil Monteregian Hills magmatic signal, diluted by local radiogenic helium and preserved in the Monteregian Hills intrusions, is leached locally by flowing modern or sub-modern groundwater. Helium isotopic measurements by pyrolysis in Monteregian Hills bulk rocks and clinopyroxene separates show R/Ra values of up to 4.96, suggesting that fossil mantle helium has been partially preserved in these rocks and their mineral phases. Monte Carlo simulations of a magma aging model shows that the initial 3He/4He ratio in these Cretaceous intrusions could have been between 21±10Ra and 33±28Ra (2σ), favoring the hypothesis that the Monteregian Hills are the product of the passage of the North American plate over the New England hotspot. This study raises the prospect of using modern groundwater as an archive of mantle He over a hundreds of millions of years timescale.

Radiogenic helium concentration and isotope variations in crustal gas pools from Sichuan Basin, China

Helium is an exhaustible natural resource and the main commercial helium was separated from helium-rich natural gas. The helium in the sedimentary basin is mainly the radiogenic origin of crustal rocks with high level of uranium and thorium. Data from 242 natural gas samples were used to investigate the distribution and formation mechanism of helium-rich gases in the Sichuan Basin of China. All gas samples show remarkably low 3He/4He ratios with ranging from 0.002 to 0.05 Ra, which represent typical crustal helium. The Weiyuan gas pool in the southern Sichuan Basin possesses high levels of helium (1500–18770 ppm, average 2785.7 ppm) in Precambrian and Cambrian strata. A positive relationship were observed in the helium and nitrogen contents increased with reservoir layer age for all Weiyuan gas samples. Presinian granite was considered as the main helium source for the Weiyuan gas pool. Gaoshiti–Moxi and Weiyuan gas pools belonged to the same ancient gas pool before the Late Cretaceous, but the helium abundance of Gaoshiti–Moxi gas pool is relatively low (200–891 ppm). The change of pressure and fracture system caused by the uplift of Weiyuan structure in the Himalayan orogenesis led to the helium accumulation and enrichment of Weiyuan gas pool. The shale of the Silurian Longmaxi Formation has high contents of uranium and thorium and generated a large amount of helium while forming large quantities of hydrocarbon gases. Generally, the helium abundance of shale gases and conventional gases generated from the Longmaxi Formation ranges from 63 to 1500 ppm and some of the gas pool are sufficient for commercial helium resource exploitation. Therefore, the relative quantities of helium and hydrocarbon gases are act as a key factor influencing the formation of helium-rich natural gas reservoirs.

Groundwater dating tools (3H, 3He, 4He, CFC-12, SF6) coupled with hydrochemistry to evaluate the hydrogeological functioning of complex evaporite-karst settings

The hydrogeological functioning of four different areas in a complex evaporite-karst unit of predominantly aquitard behavior in S Spain was investigated. Environmental dating tracers (3H, 3He, 4He, CFC-12, SF6) and hydrochemical data were determined from spring samples to identify and characterize groundwater flow components of different residence times in the media. Results show a general geochemical evolution pattern, from higher (recharge areas) to lower positions (discharge areas), in which mineralization rises as well as the value of the rCl−/SO42−, evidencing longer water-rock interaction. Ne values show degassing of most of the samples, favored by the high salinity of groundwater and the development of karstification so that the concentration of all the considered gases were corrected according to the difference between the theoretical and the measured Ne. The presence of modern groundwater in every sample was proved by the detection of 3H and CFC-12. At the opposite, the higher amount of radiogenic 4He in most samples also indicates that they have an old component. The 3H/3He dating method does not give reliable ages as a consequence of degassing and the large uncertainty of the 3He/4He ratios of the sources for the radiogenic Helium. The large SF6 concentrations suggest terrigenic production related to halite and dolomite. Binary Mixing and Free Shape Models were created based on 3H and CFC-12 data to interpret the age distribution of the samples. Two parameters (GA50 and >70%) were proposed as an indicator of that distribution, as they provide further information than the mean age. Particularly, GA50 is derived from the median groundwater age and is presented as a new way of interpreting mixed groundwater age data. A greater fraction of old groundwater (3H and CFC-12 free) was identified in discharge areas, while the proportion and estimated infiltration date of the younger fractions in recharge areas were higher and more recent, respectively. The application of different approaches has been useful to corroborate previous theoretical conceptual model proposed for the study area and to test the applicability of the used environmental tracer in dating brine groundwater and karst springs.

Fluid geochemistry of the Cuopu high temperature geothermal system in the eastern Himalayan syntaxis with implication on its genesis

High-temperature geothermal fluids dissolve constituents pertinent to water-rock interaction and magmatic volatile absorption, resulting in high total dissolved solid (TDS) values. However, this study focuses on the hydrochemical evolution of the low-salinity HCO3–Na type high-temperature geothermal fluid in Cuopu, eastern Himalayas. The geothermal water is recharged by local precipitation and glacier water from surrounding mountains. The TDS values are below 834 mg/L and the constituents are mainly products of the water-carbon dioxide-rock interactions lacking magmatic volatile dissolved in the fluid. The geothermal water reaches an almost complete chemical equilibrium with the feldspar or plagioclase-enriched reservoir rock in a reducing condition. The reservoir temperature is between 175 °C–200 °C, while the temperature could reach up to 400 °C in the deep crustas indicated by the carbon isotopic exchange equilibrium between CO2 and CH4. The infiltrated glacier water was heated during its circulation within the hot thickened crust and continued dissolving the crustal metamorphic gas, such as radiogenic helium and limestone metamorphic CO2, until the junction of the two sets of faults provided an ascending channel for it. Upon rising along the conduit, the geothermal water mixed with cold groundwater to different degrees. Approximately 0.015 mol/L CO2 escaped from the geothermal fluid when it scattered as bubbling hot springs on the surface of the anisotropic porous Quaternary sediments. Such kind ofhydrochemical evolution of lowsalinity alkaline HCO3–Na type water represents a typical formation mechanism of the high-temperature geothermal systems along the Himalayas.

Contrasting noble gas compositions of peridotitic and eclogitic monocrystalline diamonds from the Argyle lamproite, Western Australia

He-Ne-Ar compositions were determined in diamonds from the Argyle lamproite, Western Australia, to assess whether subducted material affects the noble gas budget and composition of stable old sub-continental lithospheric mantle (SCLM). Twenty diamonds (both peridotitic and eclogitic) were characterized for their carbon isotopic compositions and N abundance and aggregation from which 10 eclogitic growth zones and 5 peridotitic growth zones were analysed for their He-Ne-Ar compositions. The eclogitic diamonds have δ13C values of −4.7 to −16.6‰ indicating a subduction signature, whereas the peridotitic diamonds have mantle-like compositions of −4.0 to −7.8‰. Mantle residence temperatures based on N-in-diamond thermometry showed that the eclogitic diamonds were mainly formed at 1260–1270 °C or above 1300 °C near the base of the lithosphere, whereas the peridotitic diamonds generally formed at lower temperatures (mostly 1135–1230 °C). A noble gas subduction signature is present to various extents in the eclogitic diamonds and is inferred from a hyperbolic mixing relationship between R/Ra and 4He and δ13C values concentrations with a predominance of low R/Ra values (<0.5; R/Ra = 3He/4Hesample/3He/4Heair). In addition, low 40Ar/4He and 40Ar/36Ar ratios, high nucleogenic 21Ne/4He and low 3He/22Ne ratios are characteristic of subducted material and were found in the eclogitic diamonds. The peridotitic diamonds show generally higher R/Ra values (median 1.1 ± 1.1) and lower 4He/40Ar ratios compared to eclogitic diamonds (median 0.1 ± 0.8 R/Ra; with 7/10 samples having an average of 0.13 ± 0.14 R/Ra). The studied peridotitic diamond growth zones showed a negative correlation between R/Ra and 4He concentrations over 2 orders of magnitude and limited variation in 3He, that can be largely explained by radiogenic 4He ingrowth. At low 4He concentrations the R/Ra value is around 2.8 for both paragenesis of diamonds and is significantly lower than present-day SCLM values, suggesting (1) a more radiogenic helium isotope composition beneath the Halls Creek Orogen than those for typical SCLM from other cratons and/or (2) that the peridotitic diamonds are formed from fluids that also had a subduction input. The high mantle residence temperature and low R/Ra value in the core and low temperature and higher R/Ra value in the rim of a single peridotitic diamond indicate multiple growth events and that part of the lherzolitic diamond population may be genetically related to the eclogitic diamonds. Combining the diamond mantle residence temperatures with noble gas compositions shows that noble gas subduction signatures are present at the base of the lithosphere below 180 km depth beneath Argyle and that fluid migration and interaction with the SCLM occurred over scales of at least 15 km, between 180 and 165 km depth.

A palaeoclimatic record from the Ledo-Paniselian Aquifer in Belgium – Indications for groundwater recharge and flow in a periglacial environment

Understanding the mechanisms of groundwater recharge and flow in periglacial environments remains enigmatic. Methane releases from melting permafrost in a warming world are a hot topic in contemporary climate research. Noble gas analyses have been proposed to study methane-driven degassing in lakes as well as in permafrost terrains. The Ledo-Paniselian aquifer is shown to provide an analogue for such processes, from the last period of global warming after the LGM. It is postulated that recharge from methane bubbling thermokarst lakes to groundwater took place, and can be recognized by a specific signature in the groundwater. The Ledo-Paniselian aquifer offers unique opportunities to study periglacial groundwater recharge during the LGM, as it was located in the permanent permafrost area to the south of the ice sheet at that time. It contains Holocene and Pleistocene groundwaters, which are separated by a recharge gap, referring to permafrost during the LGM. This is confirmed by noble gas temperature (NGT) data. In the samples immediately upstream of this gap, several potential indicators for the specific methanogenic recharge conditions from thawing permafrost are being put forward, based on noble gases, chemical and isotopic data.Immediately upstream of the age gap, groundwaters show strikingly deviating HCO3−, SO42−, δ13C, δ18O and δ2H compared to the regional trend, while their 14C model ages are too young, and NGTs seem too high. These samples are affected by degassing, which is not an artefact of sampling, and which, according to their radiogenic helium concentrations, occurred early on, during recharge or shortly thereafter. The origin of these samples could be explained by the conditions prevailing as the permafrost thawed, and groundwater recharge could resume. It is inferred that the degassed samples were infiltrated during or shortly after the LGM, when recharge from methane bubbling thermokarst lakes to groundwater took place, associated to the melting permafrost.

Tracing the migration of mantle CO2 in gas fields and mineral water springs in south-east Australia using noble gas and stable isotopes

Abstract Geochemical monitoring of CO2 storage requires understanding of both innate and introduced fluids in the crust as well as the subsurface processes that can change the geochemical fingerprint of CO2 during injection, storage and any subsequent migration. Here, we analyse a natural analogue of CO2 storage, migration and leakage to the atmosphere, using noble gas and stable isotopes to constrain the effect of these processes on the geochemical fingerprint of the CO2. We present the most comprehensive evidence to date for mantle-sourced CO2 in south-east Australia, including well gas and CO2-rich mineral spring samples from the Otway Basin and Central Victorian Highlands (CVH). 3He/4He ratios in well gases and CO2 springs range from 1.21 to 3.07 RA and 1.23–3.65 RC/RA, respectively. We present chemical fractionation models to explain the observed range of 3He/4He ratios, He, Ne, Ar, Kr, Xe concentrations and δ13C(CO2) values in the springs and the well gases. The variability of 3He/4He in the well gases is controlled by the gas residence time in the reservoir and associated radiogenic 4He accumulation. 3He/4He in CO2 springs decrease away from the main mantle fluid supply conduit. We identify one main pathway for CO2 supply to the surface in the CVH, located near a major fault zone. Solubility fractionation during phase separation is proposed to explain the range in noble gas concentrations and δ13C(CO2) values measured in the mineral spring samples. This process is also responsible for low 3He concentrations and associated high CO2/3He, which are commonly interpreted as evidence for mixing with crustal CO2. The elevated CO2/3He can be explained solely by solubility fractionation without the need to invoke other CO2 sources. The noble gases in the springs and well gases can be traced back to a single end-member which has suffered varying degrees of radiogenic helium accumulation and late stage degassing. This work shows that combined stable and noble gas isotopes in natural gases provide a robust tool for identifying the migration of injected CO2 to the shallow subsurface.

Geochemical and isotopic characteristics of geothermal discharges in the Emet Basin, Western Anatolia, Turkey

The geochemical and isotopic compositions of low-temperature thermal waters in the N–S-oriented Emet Basin, located in Western Anatolia, Turkey, were comprehensively investigated. Three distinct physicochemical processes affecting the chemical composition of the thermal waters were distinguished at this site: dissolution of CaCO3, gradual transformation of Ca–HCO3-type waters to Ca–SO4-type waters by the addition of SO4, and mixing of deep Na–HCO3-type waters with Ca–SO4-type waters. The first two processes are associated with the dissolution of carbonates and evaporites, respectively, and predominantly control the chemistry of thermal waters in the basin. The third process is observed locally and is manifested as CO2-rich thermal waters with relatively higher total ionic concentration.The majority of the thermal waters lie close to the local meteoric water line with a deuterium excess of +17‰. The single CO2-rich thermal water in the basin, located in the Yeniceköy area, shows a slight enrichment in 18O and a shift from the local meteoric water line. The δ34SSO4 and δ18OSO4 values of the dissolved sulfate range from 14.49‰ to 22.46‰ (vs. V-CDT) and 11.9‰–17.8‰ (vs. V-SMOW), respectively, which indicates that the source of sulfate in the thermal waters is evaporites such as gypsum and anhydrite in the basin. The δ13CDIC values of the thermal waters vary between −0.63‰ and −10.36‰ (vs. V-PDB). By using a binary mixing model, the δ13C value of the deep carbon source associated with the formation of CO2-rich thermal waters was calculated as 0‰, which is a typical value of marine limestones. Thermal decomposition of marine limestones and dissolution of carbonates dominate the carbon budgets of thermal waters, whereas the organic carbon contribution is generally less than 20% and the mantle-derived carbon contribution was found to be insignificant.Air-corrected 3He/4He ratios of the thermal waters ranging from 0.34 RA to 1.57 RA indicate that radiogenic (crustal) helium is dominant in the region, although up to 19.4% of the total helium is of mantle origin. The 3He/heat ratios vary from 0.4 × 10−15 cm3 STP/J to 13.2 × 10−15 cm3 STP/J, which are very close to the ratios in the continental environment. This indicates that the dominant heat source in the region is radiogenic.

Noble Gases: A Record of Earth's Evolution and Mantle Dynamics

Noble gases have played a key role in our understanding of the origin of Earth's volatiles, mantle structure, and long-term degassing of the mantle. Here we synthesize new insights into these topics gained from high-precision noble gas data. Our analysis reveals new constraints on the origin of the terrestrial atmosphere, the presence of nebular neon but chondritic krypton and xenon in the mantle, and a memory of multiple giant impacts during accretion. Furthermore, the reservoir supplying primordial noble gases to plumes appears to be distinct from the mid-ocean ridge basalt (MORB) reservoir since at least 4.45 Ga. While differences between the MORB mantle and plume mantle cannot be explained solely by recycling of atmospheric volatiles, injection and incorporation of atmospheric-derived noble gases into both mantle reservoirs occurred over Earth history. In the MORB mantle, the atmospheric-derived noble gases are observed to be heterogeneously distributed, reflecting inefficient mixing even within the vigorously convecting MORB mantle. ▪ Primordial noble gases in the atmosphere were largely derived from planetesimals delivered after the Moon-forming giant impact. ▪ Heterogeneities dating back to Earth's accretion are preserved in the present-day mantle. ▪ Mid-ocean ridge basalts and plume xenon isotopic ratios cannot be related by differential degassing or differential incorporation of recycled atmospheric volatiles. ▪ Differences in mid-ocean ridge basalts and plume radiogenic helium, neon, and argon ratios can be explained through the lens of differential long-term degassing.

U–Th–He Dating of Pyrite from the Uzelga Copper-Zinc Massive Sulfide Deposit (South Urals, Russia): First Application of a New Geochronometer

Based on a study of pyrite from the Uzelga Cu–Zn volcanogenic massive sulfide (VMS) deposit (South Urals) the age of ore mineralization was first determined with the direct age dating method, based on the fraction of radiogenic helium, incorporated into the pyrite crystal lattice from submicron inclusions of U and Th minerals. Taking into account the measurement errors, the obtained age of 377 ± 8 Ma (MSWD = 1.2) is quite consistent with the independent age dates available for the ore mineralization (Late Eifelian–Early Givetian, 385–390 Ma).