Mantle 3He Research Articles

Hydrogeochemical investigation of shallow aquifers before and after the 2012 Emilia seismic sequence (northern Italy)

Soon after the MW = 6.0 main event of May 20, 2012 that struck the central part of the Emilia-Romagna region (Italy), several geochemical surveys were carried out on groundwater within the epicentral area. A total of 20 water samples were periodically collected, from May 2012 to July 2014, from shallow (up to 6 m depth) to deep (down to 175 m depth) wells within both unconfined and confined aquifers and analyzed for major ions, trace elements, dissolved gases and stable isotopes (δ2H–H2O, δ18O–H2O and δ13C-TDIC). Geochemical data were compared with previous data collected and analyzed in 2006 in a phase of absence of significant seismic activity.Monitored waters showed concentration variations in post-earthquake sampling on a large number of geochemical parameters. Many of these variations were recorded during the co-seismic phase and were transient as the geochemical parameters returned towards pre-earthquake values over time.The most significant transient variations involved trace elements, which generally show high sensitivity even to small variations in the surrounding environment due to their usually low concentrations in groundwater. Physical-chemical parameters (water temperature, pH, TDS) and major ions provided less unambiguous indications, whereas among dissolved gases CH4 and CO2 showed a general post-seismic increase within the unconfined aquifer. Increased contents of such gas species in both aquifers (this study) and soils (previous studies) suggest that the seismic-induced overpressure on the ground had enhancing effects on soil permeability and porosity and triggered their co-seismic migration upwards from deeper reservoirs. Water isotopes showed the systematic post-seismic change in δ2H toward heavier compositions with no significant changes in the δ18O, which was interpreted as due to isotopic exchange between water and a H2-bearing gas phase (e.g., H2S, CH4, H2) entering the system. Calculated δ13C data of CO2 suggested a relatively shallow production from both plant-root respiration and microbial-driven degradation of organic matter while the almost pure crustal origin of He (R/Ra values = 0.04–0.16 from the bubbling gas phase emitted by one of the monitored wells) reasonably excludes any evidence of both primary mantle 3He degassing and ascent of heavier CO2 from deep (mantle, decarbonation) inorganic sources.Monitored waters which showed the most significant transient variations are aligned in the same E-W direction along which the seismicity and soil gas anomalies were distributed, at about 5 km S from the epicenter of the May 20th seismic event and along the main direction of the May 29th (MW = 5.8) event. This confirms that the transient variations have been activated by the seismic sequence in a sector of the crust where the presence of a fault/fracture system favors the intensification of processes affecting sediments and groundwater (variations of porosity/permeability of soils, the groundwater level, redox state, etc.) and which are able to explain the observed geochemical variations.Only one sample monitored, the one closest to the epicenter of the May 20th event, showed clear geochemical evidence suggesting the hypothesis of mixing between superimposed water bodies.

Primordial Helium‐3 Exchange Between Earth's Core and Mantle

AbstractVolatiles from the solar nebula are known to be present in Earth's deep mantle. The core also may contain solar nebula‐derived volatiles, but in unknown amounts. Here we use calculations of volatile ingassing and degassing to estimate the abundance of primordial 3He now in the core and track the rate of 3He exchange between the core and mantle through Earth history. We apply an ingassing model that includes a silicate magma ocean and an iron‐rich proto‐core coupled to a nebular atmosphere of solar composition to calculate the amounts of 3He acquired by the mantle and core during accretion and core formation. Using experimentally determined partitioning between core‐forming metals and silicate magma, we find that dissolution from the nebular atmosphere deposits one or more petagrams of 3He into the proto‐core. Following accretion, 3He exchange depends on the convective history of the coupled core‐mantle system. We combine determinations of the present‐day surface 3He flux with estimates of the present‐day mantle 3He abundance, mantle and core heat fluxes, and our ingassed 3He abundances in a convective degassing model. According to this model, the mantle 3He abundance is evolving toward a statistical steady state, in which surface losses are compensated by enrichments from the core.

Determining the role of diffusion and basement flux in controlling 4He distribution in sedimentary basin fluids

The transport of helium from the crystalline continental basement and overlying Phanerozoic sedimentary formations to the near surface can be controlled by both diffusive and advective processes. The relative role of each is vital to helium resource prediction, and important in quantifying the residence times of fluids relevant to groundwater resources, hydrocarbon systems, geologic repositories for nuclear waste and carbon sequestration. The Williston Basin, North America, is a prominent sedimentary basin, providing an excellent natural laboratory to assess these processes. Here, we report noble gas isotopic and composition data for 28 gas samples from natural gas wells that sample different stratigraphic horizons down to the basement (Cretaceous to the Cambrian). Helium isotope ratios show a resolvable mantle 3He component (up to 4.7%) in most samples. Neon isotopic compositions of the Cambrian samples are consistent with a crystalline basement gas contribution. Both helium and neon isotopic observations provide evidence for the contribution of conservative noble gases from the crystalline basement or deeper into the overlying sedimentary basin. 4He groundwater concentrations in the sedimentary formations, calculated from 4He/20Ne values in gas samples, are in excess of in situ U+Th 4He production in some shallow units and depleted in others, providing further evidence of cross formation gas contributions.The highest 4He groundwater concentrations can be compared with the results obtained from a fully-coupled vertical scale transport model characterising diffusive-dominated transport through a static groundwater column. The model includes the 4He flux into the basin from the Precambrian basement and quantifies the apparent basement 4He flux to be between 0.8 - 1.6 × 10−6 mol 4He/m2 yr, comparable to the steady-state flux estimated for the average continental crust (1.47 × 10−6 mol 4He/m2 yr) (Torgersen, 2010). The lithologies in which 4He concentrations are significantly lower than the reference model predictions are consistent with a history of water flooding and produced water disposal in those formations over decades of hydrocarbon production. While an advective component cannot be ruled out, this work demonstrates the importance of both diffusion and the basin architecture development in controlling 4He flux into and out of different lithologies. The assumption of negligible 4He loss from the top surface of a lithology is often made when determining the 4He age of its groundwater. In the Williston Basin, this study shows that deeper lithologies may reach steady state at different stages of basin development, with shallower lithologies sometimes also showing significant 4He loss from their top surface. In the Williston Basin, 4He diffusive loss from the target lithology must be considered to accurately interpret 4He groundwater residence times and accumulation potential.

Apparent oxygen utilization rates based on tritium-helium dating in the South China Sea: Implications for export production

We present tritium (3H) and helium isotope (δ3He) data from the South China Sea (SCS), to estimate apparent oxygen utilization rates (AOURs). The observed δ3He values are close to the theoretical solubility equilibrium value of -1.7% in the upper mixed layer, followed by an increase with depth down to ~1500 m. Below 1500 m depth, δ3He is homogenously distributed with a value of 20.9% ± 1.0%. The distribution of δ3He reveals that a significant fraction of 3He throughout the water column over the entire SCS basin is allochthonous, derived from mantle sources originated from the Pacific Ocean. By using the salinity-normalized “potential” alkalinity (NPA) as a conservative mixing tracer, 3He produced by tritium decay (tritiogenic) was separated from mantle 3He, and the apparent 3H–3He ages of the SCS water masses were subsequently calculated to be 11 ± 5 years at 100 m depth and 50 ± 4 years at 1000 m. Together with the observed dissolved oxygen concentrations, we estimated the mean AOURs and obtained values of 4.15 ± 0.27 μmol kg−1·yr−1 for the depth range of 100–500 m, and 0.81 ± 0.23 μmol kg−1·yr−1 for that between 800 and 1000 m depth. The depth-integrated AOURs between 100 and 1000 m depth, yield a spatially and temporally averaged export flux of organic carbon of 1.96 ± 0.16 mol-C·m−2·yr−1, comparable with previous estimates based on either 238U/234Th disequilibrium or nutrient and oxygen mass balance calculations in the SCS.

Mantle degassing of primordial helium through submarine ridge flank basaltic basement

The degassing of primordial gases from Earth's interior is evidenced by the high He3/4He ratios in submarine hydrothermal plumes, vent fluids, and rock samples with mantle origin from active hydrothermal systems in mid-ocean ridges (MOR) and subduction zone volcanism. As the largest aquifer on Earth, the uppermost 40-500 m of permeable submarine ridge flank basement (1-65 million-years-old, Myr) holds ∼2% of the ocean volume and accounts for 70% of the seafloor hydrothermal heat flux. However, the degassing of primordial gases through the oceanic ridge flank crust has not yet been directly quantified. Here, we show that high integrity hydrothermal (65°C) fluids from the sediment-buried 3.5 Myr basaltic crust from the eastern flank of the Juan de Fuca Ridge (JdFR) contain elevated 3He. The He3/4He for the basaltic fluid is 4.5 ± 0.1 Ra (relative to the air ratio), which is greatly elevated when compared to deep seawater (1.05 Ra), but is half of that observed for high-temperature vent fluids (∼8 Ra) emitting from MOR. Only a small fraction of the 3He in ridge flank fluids is derived from the entrainment of high-temperature ridge-axis fluids and is better explained by degassing of the mantle through the mantle-crust boundary. The lower than MOR He3/4He ratios indicate that radiogenic 4He originates from aged uranium and thorium decay within the mantle as well as from the ridge-flank basalts. The 3He outgassing through warm ridge flanks (4.9 to 36 mol/yr) accounts for 0.7-6% of the global 3He outgassing, exceeded only by degassing through mid-ocean ridges and subduction volcanism. The presence of mantle 3He suggests that the abiogenic methane present in the ridge flank fluids might be mantle-derived. Based on the 3He outgassing flux, a possibly mantle-derived abiotic methane production rate at the ridge flank is estimated to be 0.3–35×108mol/yr.

Helium and carbon isotopic signatures of thermal spring gases in southeast Yunnan, China

Late Palaeozoic and Cenozoic magmatism, including Pingbian Quaternary basaltic volcanoes, occurs in southeast Yunnan, China, where seismic tomography evidence indicates magma activity in the uppermost mantle. However, the characteristics of mantle fluid release remain poorly understood. Here, we report the chemical compositions of hot spring gases in SE Yunnan and measure the 3He/4He ratios, 4He/20Ne ratios, and δ13C-values (CO2, CH4) to determine their origins and the factors influencing their migration. Except for five samples contaminated by the atmosphere (4He/20Ne ratios, 0.39 ̶2.39), the Rc/Ra ratios (Rc is the air-corrected 3He/4He ratio, Ra is the air ratio with 1.4 × 10−6) range from 0.042 Ra to 0.470 Ra, indicating that the helium is mostly of crustal origin with minor addition of mantle-derived helium. The calculated maximum proportion of mantle helium was ~5% of total helium, indicating that mantle 3He leakage correlates with the ductile lower crust. The hot spring gases are fractionated by the partial dissolution of CO2 in groundwater, and the initial δ13CCO2 values are modelled by Rayleigh fractionation within a range of −26.5‰ to −6.14‰. The modelled CO2 in samples estimated by the HeC isotope coupling model originates from the mixing of limestone and organic sediment end-members. The isotopic signature of CH4 (−69.2‰ to 35.9‰) suggests a thermogenic source and microbial oxidation in some samples. Furthermore, the apparent isotopic temperatures are estimated using fractionation factors for CH4 and CO2, suggesting a lack of magmatism in the crust beneath the Red River fault zone (188–318 °C), but partial melting under the Bozhushan granitoid plutons (572–688 °C). The low emission of mantle-derived helium is attributed to the low fault permeability and minor active crustal deformation in present-day SE Yunnan, which restrain the upward migration of mantle fluids.

Reversible scavenging traps hydrothermal iron in the deep ocean

Recent studies suggest that seafloor hydrothermal vents could be an important source of iron (Fe) to the surface ocean, stimulating plankton growth and biological carbon export. However, quantifying the supply of hydrothermal Fe to the surface ocean requires accurately modeling its stabilization and removal processes, which are poorly known. Here, we determine the physical speciation of dissolved Fe along an oceanographic transect following a coherent hydrothermal plume that emanates from the East Pacific Rise (EPR) and persists westward over 4,000 km in the Tropical South Pacific. Our observations show that the plume persists horizontally, but descends vertically, and consists primarily of very large Fe colloids. Guided by these observations, we develop a new size-resolved mechanistic model of hydrothermal Fe dispersion in this region, in which the stabilization of hydrothermal Fe is explained by a reversible particulate exchange process. This model accurately captures the lateral dispersion, downward settling and physical speciation of hydrothermal Fe along this transect. An alternate model that uses a hydrothermal source of Fe-binding ligands to facilitate Fe transport within the deep ocean can reproduce the long-range transport of hydrothermal Fe, but does not reproduce the vertical descent of the plume. Our model shows that hydrothermal Fe vented from the EPR is trapped in the deep ocean, and only 1% of this iron ever makes it to the surface where it can stimulate biological productivity. At the global scale, 3-5% of hydrothermal Fe makes it to the surface ocean, the vast majority of which originates from Southern Ocean vents and upwells in the Southern Ocean. Our best estimate of the global supply of hydrothermal Fe to the surface ocean, based on data-constrained estimates of ocean circulation, mantle 3He venting, and the hydrothermal Fe:3He ratio from the EPR, is 0.12±0.07 Gmolyr−1. This is about 60-70 times lower than the supply of Fe from aerosol dust deposition, but could be regionally important in the Antarctic zone of the Southern Ocean.

The use of noble gas isotopes to constrain subsurface fluid flow and hydrocarbon migration in the East Texas Basin

The migration of hydrocarbons from source-rock to reservoir is a critical stage in petroleum system evolution. However, as migration occurs along confined pathways over geologically rapid timescales it is challenging to place quantitative constraints on migration behaviour in terms of interaction with other basinal fluids, flux, and lengthscales associated with this process. Due to their inert nature, noble gases are ideal tracers of fluid provenance and physical processes in a variety of subsurface systems, and thus can serve as a potential geochemical indicator of migration distance. The recent proliferation of unconventional hydrocarbon production (i.e., production directly from source-rocks) allows for the analysis of fluids from these strata. Here we present noble gas data from natural gas samples produced from 27 related conventional and unconventional wells within the East Texas basin. These data enable characterisation of fluids during migration from source-rock to conventional reservoir. Unconventional gases and fluids are produced from the Haynesville shale (n = 8), and conventional gases and fluids are sourced from the overlying Cotton Valley (n = 5), Travis Peak (n = 9), and James Lime (n = 5) formations. Samples consist primarily of methane (>70%), with small contributions from longer-chain hydrocarbons, and other gases including CO2 and N2. Atmosphere-derived 36Ar, which is introduced into natural gas accumulations via interaction with groundwater during hydrocarbon generation and migration, is consistently higher in samples that have undergone a greater migration distance from the source interval. Thus, calculated volume ratios of gas-water interaction (Vg/Vw) show a far greater incidence of groundwater interaction in migrated samples. Radiogenically-produced 40Ar is also consistently elevated in migrated samples, despite these reservoirs being located in geologically younger strata. We derive the representative volume of rock required to produce the observed radiogenic 40Ar in each sample and show that samples that have migrated further have acquired radiogenic isotopes from larger volumes of rock. Furthermore, our volumetric parameters derived from atmospheric and radiogenic isotopes are consistent with a simple conceptual model of migration. We quantitatively apply this model to demonstrate that migration occurs along relatively localised pathways. Helium (3He/4He) and argon (40Ar/36Ar) isotopes are correlated and show elevated amounts of mantle-derived 3He and radiogenic 40Ar in migrated samples. We interpret this to represent mixing between a pristine source-rock signature and an endmember characterised by elevated 3He/4He and 40Ar/36Ar, likely representative of mantle-enriched groundwater circulating in the wider hydrogeological basin. Finally, we use an advection-diffusion model to show that enriched mantle 3He in the shallower strata and towards the southern edge of the basin can be explained by influx of mantle helium along the fault-bounded southern edge of the basin over timescales of millions of years. Together these approaches represent the first complete noble gas based characterization of a hydrocarbon system, sampling migration from source-rock to trap. We show conclusively that both atmospheric and radiogenic noble gases are entrained in the hydrocarbon phase during migration and that concentrations scale proportionally with migration distance, as greater volumes of groundwater and host rock are encountered.

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.

Content and Isotope Ratios of Noble Gases in Congelation Ice of Lake Vostok

Isotope ratios of noble gases (He, Ne, Ar) were studied in samples collected by degassing of cores of water frozen over a glacier of Lake Vostok. The gases were collected into glass retorts during three days of degassing of cores, which have just been extracted from the borehole. Within the error, the isotope 3He/4He ratios of 0.28 ± 0.08 RA (RA = 1.38 × 10–6 is the ratio for air) correspond to those from [1]. The 4He/20Ne and 40Ar/36Ar ratios (12.4 ± 4.6 RA and 1.0074 ± 0.0023 RA, respectively) exceed their contents in air (4He/20NeA = 0.29; 40Ar/36ArA = 298.6) and may indicate some contribution of terrigenous gas to the gaseous balance of the lake, as well as the high content of ancient ground waters in the lake. The 3He/4He ratio of 0.28 RA means low mantle 3He flux typical of continental platforms far from active rift zones.

Quantifying He fluxes from the mantle using multi-tracer data assimilation.

A global, coarse-resolution ocean model previously fitted to geostrophic shear estimates and to data of 10 hydrographic parameters and tracers has been used to simulate the 3He and 4He distributions resulting from the release of mantle helium from mid-ocean ridges. The model is in very good agreement with 14C and chlorofluorocarbon data and has realistic global ocean overturning strength as well as water mass formation and transport rates. It is found that previously published global mantle 3He fluxes are too high by a factor of about 2. In the model, optimal agreement of modelled δ3He with data is achieved for a global flux of 450 ± 50 mol 3He yr−1. The formulation of He source strengths proportional to ridge spreading rates appears compatible with data. A model/data misfit analysis shows significant and large-scale δ3He underestimation in the southwestern Pacific centred over the Lau Backarc Basin (approx. 179° W/20° S). These misfits disappear in a set-up with 30 of the 450 mol yr−1 global total 3He flux released in the Lau Basin over a depth range between 1250 and 2500 m. Such He flux contributions are missing in present mantle He source compilations. Hydrothermal fluxes of other trace elements and isotopes (TEI) can be calculated from He fluxes on the basis of TEI : He ratios measured close to the sources.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Objective estimates of mantle 3He in the ocean and implications for constraining the deep ocean circulation

Hydrothermal vents along the ocean's tectonic ridge systems inject superheated water and large amounts of dissolved metals that impact the deep ocean circulation and the oceanic cycling of trace metals. The hydrothermal fluid contains dissolved mantle helium that is enriched in 3He relative to the atmosphere, providing an isotopic tracer of the ocean's deep circulation and a marker of hydrothermal sources. This work investigates the potential for the 3He/4He isotope ratio to constrain the ocean's mantle 3He source and to provide constraints on the ocean's deep circulation. We use an ensemble of 11 data-assimilated steady-state ocean circulation models and a mantle helium source based on geographically varying sea-floor spreading rates. The global source distribution is partitioned into 6 regions, and the vertical profile and source amplitude of each region are varied independently to determine the optimal 3He source distribution that minimizes the mismatch between modeled and observed δ3He. In this way, we are able to fit the observed δ3He distribution to within a relative error of ∼15%, with a global 3He source that ranges from 640 to 850 mol yr−1, depending on circulation. The fit captures the vertical and interbasin gradients of the δ3He distribution very well and reproduces its jet-sheared saddle point in the deep equatorial Pacific. This demonstrates that the data-assimilated models have much greater fidelity to the deep ocean circulation than other coarse-resolution ocean models. Nonetheless, the modelled δ3He distributions still display some systematic biases, especially in the deep North Pacific where δ3He is overpredicted by our models, and in the southeastern tropical Pacific, where observed westward-spreading δ3He plumes are not well captured. Sources inferred by the data-assimilated transport with and without isopycnally aligned eddy diffusivity differ widely in the Southern Ocean, in spite of the ability to match the observed distributions of CFCs and radiocarbon for either eddy parameterization.

Recent mantle degassing recorded by carbonic spring deposits along sinistral strike-slip faults, south-central Australia

The interior of the Australian continent shows evidence for late Quaternary to Recent fault-controlled mantle 3He and CO2 degassing. A series of interconnected NW-striking sinistral faults, the Norwest fault zone (NFZ), in south-central Australia are associated with travertine mounds, the latter show a regular spacing of 50–70 km. U-series ages on 26 samples range from 354±7 to 1.19±0.02ka (2σ errors) and suggest a clustering every ∼3–4 ka since ∼26 ka. Geochemical data demonstrate a remarkable mantle-to-groundwater connection. Isotopic data indicate that the groundwater is circulating to depths >3 km and interacting with Neoproterozoic/Cambrian basement and mantle volatiles. 3He/4He isotope ratios show that the He comes in part from the mantle. This demonstrates that the NFZ cuts through the entire crust and provides pathways for mantle degassing. Scaling relationships suggest that the series of sinistral faults that make up the NFZ are interconnected at depths and have a significant strike length of 60–70 km or more. The NFZ occurs where a major compositional boundary and a significant heat flow anomaly occurs, and a major step in lithospheric thickness has been mapped. We discuss a tectonic model in which recent stress field, heat flow and lithospheric structure in central Australia reactivated a set of steeply dipping Neoproterozoic faults, which may now be growing into a crustal/lithospheric-scale structure.

Mapping modern CO2 fluxes and mantle carbon content all along the mid-ocean ridge system

Quality criteria have been used to select ∼400 vesicularity measurements on zero-age mid-ocean ridge glasses from ∼600 data available in the literature published over the past ∼30 years. At face value, observations show that for a given depth of sampling, enriched basalts from slow spreading ridge segments are more vesicular than those from depleted and intermediate or fast spreading ridges. A shallower depth of eruption enhances these effects because lower hydrostatic pressure favours bubble expansion. In order to get an insight into these complex and intermingled processes, we used empirical and semi-quantitative approaches based on a limited number of inputs (segment depth D, spreading rate τ and K2O/TiO2 ratios). Both models give equivalent results and predict vesicularities within ±50%. From these calculations, we compute the equivalent CO2 concentration at the depth of eruption all along the oceanic ridge system. The total calculated CO2 fluxes are low ranging from 6.5±1.8 to 8.7±2.8×1011 mol/yr and the CO2 mantle content displays large variabilities from 66−19+27 to 78−40+82 ppm. In order to test these results, the mantle 3He fluxes have been evaluated using the calculated CO2 fluxes and a CO2/3He ratio of 2.2×109. These fluxes range from 295±82 to 395±127 mol/yr and are close to the values reported by Jean-Baptiste (1992) (267–534 mol/yr) and the most recent estimate (Bianchi et al., 2010, ∼527±102 mol/yr) using box-model of the three main ocean basins constrained by measurements of 3He and radiocarbon data. As these independent methods give similar helium fluxes at regional and global scales, it provides strong support to a low and heterogeneous mantle carbon concentration and distribution. Finally, the calculated volcanic CO2 emissions at oceanic ridges correspond to ∼30 seconds of anthropogenic emissions, at current rates.

Mantle 3He and CO2 degassing in carbonic and geothermal springs of Colorado and implications for neotectonics of the Rocky Mountains

Helium isotope ( 3 He/ 4 He) data from geothermal springs in the Colorado Rocky Mountains (western United States) provide unequivocal evidence for a remarkable mantle-to-groundwater connection, with contributions of up to 27% mantle-derived helium. Hydrochemical modeling of springs shows the mantle helium is associated with high p CO 2 with 76 ± 20% of the CO 2 also derived from endogenic (deep geologic) sources. These springs occur preferentially along faults, have highest 3 He/ 4 He values above domains of low mantle velocity, and demonstrate unexpectedly widespread neotectonic mantle degassing. Total CO 2 flux through these springs is 3 × 10 8 mol/yr, a small but persistent contribution to the CO 2 budget and an important baseline for carbon sequestration/leakage studies.

Noble gas and carbon isotope ratios in Argyle diamonds, Western Australia: Evidence for a deeply subducted volatile component

The Argyle lamproite pipe of Western Australia contains diamonds formed at depths exceeding 150 km. We undertook noble gas and carbon isotope ratio (δ13C) analyses of three diamonds (likely of eclogitic paragenesis) from the Argyle lamproite to test for the possible presence of deeply subducted volatile components, and to further constrain the noble gas evolution of the Earth's mantle. The Argyle diamonds are characterised by mantle 3He (with 3He/4He ratios of 0.79 R A to 0.25 R A, where R A is the atmospheric 3He/4He ratio of 1.4 × 10–6), small excess Ar and Xe isotope anomalies relative to atmospheric components, and δ13C values of –11.6 to –10.2‰ VPDB. These observations indicate that noble gas and carbon isotopic compositions of the mantle where the Argyle diamonds formed, represent mixtures of an intrinsic mantle component with sedimentary and atmospheric components that may have been introduced through subduction processes.

Fossil brines preserved in the St-Lawrence Lowlands, Québec, Canada as revealed by their chemistry and noble gas isotopes

Brines in Cambrian sandstones and Ordovician dolostones of the St-Lawrence Lowlands at Bécancour, Québec, Canada were sampled for analysis of all stable noble gases in order to trace their origin and migration path, in addition to quantifying their residence time. Major ion chemistry indicates that the brines are of Na–Ca–Cl type, possibly derived from halite dissolution. 87Sr/ 86Sr ratios and Ca excess indicate prolonged interactions with silicate rocks of the Proterozoic Grenville basement or the Cambrian Potsdam sandstone. The brines constrain a 2–3% contribution of mantle 3He and large amounts of nucleogenic 21Ne ∗ and 38Ar ∗ and radiogenic 4He and 40Ar ∗. 4He/ 40Ar ∗ and 21Ne ∗/ 40Ar ∗ ratios, corrected for mass fractionation during incomplete brine degassing, are identical to their production ratios in rocks. The source of salinity (halite dissolution), plus the occurrence of large amounts of 40Ar ∗ in brines constrain the residence time of Bécancour brines as being older than the Cretaceous. Evaporites in the St-Lawrence Lowlands likely existed only during Devonian–Silurian time. Brines might result from infiltration of Devonian water leaching halite, penetrating into or below the deeper Cambrian–Ordovician aquifers. During the Devonian, the basin reached temperatures higher than 250 °C, allowing for thermal maturation of local gas-prone source rocks (Utica shales) and possibly facilitating the release of radiogenic 40Ar ∗ into the brines. The last thermal event that could have facilitated the liberation of 40Ar ∗ into fluids and contributed to mantle 3He is the Cretaceous Monteregian Hills magmatic episode. For residence times younger than the Cretaceous, it is difficult to find an appropriate source of salinity and of nucleogenic/radiogenic gases to the Bécancour brines.

Combined Li–He isotopes in Iceland and Jan Mayen basalts and constraints on the nature of the North Atlantic mantle

Lithium (Li) isotopes are thought to provide a powerful proxy for the recycling of crustal material, affected by low temperature alteration, through the mantle. We present Li isotope compositions for basaltic volcanic rocks from Hengill, Iceland, and Jan Mayen in order to examine possible links between ocean island volcanism and recycled oceanic crust and to address recent suggestions that mantle 3He/ 4He is also related to recycling of ancient slabs. Basaltic glasses spanning a range of chemical enrichment from the Hengill fissure system define an inverse correlation between δ 7Li (3.8–6.9‰) and 3He/ 4He (12–20 R A). The high- 3He/ 4He basalts have low δ 18O as well as excess Eu and high Nb/U, but carry no Li isotope evidence of being the product of recycling of altered slab or wedge material. In fact, there is no clear correlation between Li or He isotopes on the one hand and any of the other fingerprints of recycled slab components. The low- 3He/ 4He samples do have elevated Nb/U, Sr/Nd, positive Eu anomalies and high δ 7Li (∼6.9‰), providing evidence of a cumulate-enriched source that could be part of an ancient altered ocean floor slab. Basalts from Jan Mayen are characterized by large degrees of enrichment in incompatible trace elements typical of EM-like basalts but have homogeneous δ 7Li typical of depleted mantle (3.9–4.7‰) providing evidence for a third mantle source in the North Atlantic. It appears that oceanic basalts can display a wide range in isotope and trace element compositions associated with recycled components whilst exhibiting no sign of modern surface-altered slab or wedge material from the Li isotope composition.

The 40Ar/39Ar and U/Pb dating of young rhyolites in the Kos‐Nisyros volcanic complex, Eastern Aegean Arc, Greece: Age discordance due to excess 40Ar in biotite

High‐precision dating of Quaternary silicic magmas in the active Kos‐Nisyros volcanic center (Aegean Arc, Greece) by both 40Ar/39Ar on biotite and U/Pb on zircon reveals a complex geochronological story. U/Pb ID‐TIMS multi and single‐grain zircon analyses from 3 different units (Agios Mammas and Zini domes, Kefalos Serie pyroclasts) range in age from 0.3 to 0.5 to 10–20 Ma. The youngest dates provide the maximum eruption age, while the oldest zircons indicate inheritance from local continental crust (Miocene and older). Step‐heating 40Ar/39Ar experiments on 1–3 crystals of fresh biotite yielded highly disturbed Ar‐release patterns with plateau ages typically older than most U/Pb ages. These old plateau ages are probably not a consequence of inheritance from xenocrystic biotites because Ar diffuses extremely fast at magmatic temperatures and ratios are reset within a few days. On the basis of (1) elevated and/or imprecise 40Ar/36Ar ratios, (2) shapes of the Ar release spectra, and (3) a high mantle 3He flux in the Kos‐Nisyros area, we suggest that biotite crystals retained some mantle 40Ar that led to the observed, anomalously old ages. In contrast, sanidine crystals from the only sanidine‐bearing unit in the Kos‐Nisyros volcanic center (the caldera‐forming Kos Plateau Tuff) do not appear to store any excess 40Ar relative to atmospheric composition. The eastern edge of the Aegean Arc is tectonically complex, undergoing rapid extension and located close to a major structural boundary. In such regions, which are characterized by high fluxes of mantle volatiles, 40Ar/39Ar geochronology on biotite can lead to erroneous results due to the presence of excess 40Ar and should be checked either against 40Ar/39Ar sanidine or U/Pb zircon ages.

Low helium flux from the mantle inferred from simulations of oceanic helium isotope data

The high 3He/ 4He isotopic ratio of oceanic helium relative to the atmosphere has long been recognized as the signature of mantle 3He outgassing from the Earth's interior. The outgassing flux of helium is frequently used to normalize estimates of chemical fluxes of elements from the solid Earth, and provides a strong constraint to models of mantle degassing. Here we use a suite of ocean general circulation models and helium isotope data obtained by the World Ocean Circulation Experiment to constrain the flux of helium from the mantle to the oceans. Our results suggest that the currently accepted flux is overestimated by a factor of 2. We show that a flux of 527 ± 102 mol year − 1 is required for ocean general circulation models that produce distributions of ocean ventilation tracers such as radiocarbon and chlorofluorocarbons that match observations. This new estimate calls for a reevaluation of the degassing fluxes of elements that are currently tied to the helium fluxes, including noble gases and carbon dioxide.