Helium Isotope Measurements Research Articles

Discrepant mass accumulation rates of sediments in the South Pacific Ocean from 230Th and 3He measurements

A common technique for determining the mass accumulation rates of marine sediments is through measurements of constant flux proxies. These proxies, like unsupported 230Th and extraterrestrial 3He, are buried at a known rate at the seafloor, and thus their concentration in sediments is inversely proportional to the vertical mass accumulation rate of ambient sediments. These two proxies have varied assumptions regarding their behavior that have been difficult to test, particularly in regions of very low sedimentation. We present new measurements of helium and thorium isotopes in coretop sediments from the South Pacific Gyre. Our main finding is that 230Th- and 3He-derived mass accumulation rates of these sediments differ by a factor of 2–10, with 230Th-based mass accumulation rates systematically higher than those derived from 3He. While the final cause of the discrepancy is difficult to elucidate, we suggest that up to a factor of 2 worth of the sedimentary 230Th deficit can be explained by lateral transport of 230Th, while the remainder must be explained by other processes, such as ancient coretops undercorrected for post-depositional 230Th decay. Regardless of the mechanism, our findings have critical implications for the application of constant flux proxies in the South Pacific Gyre, with one example being the calibration of dust deposition models in this region. Ultimately, additional testing is required to determine the accuracy of the 3He and 230Th techniques for estimating sediment accumulation rates in regions of the open ocean with extremely low sedimentation rates.

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.

A comprehensive global oceanic dataset of helium isotope and tritium measurements

Abstract. Tritium and helium isotope data provide key information on ocean circulation, ventilation, and mixing, as well as the rates of biogeochemical processes and deep-ocean hydrothermal processes. We present here global oceanic datasets of tritium and helium isotope measurements made by numerous researchers and laboratories over a period exceeding 60 years. The dataset's DOI is https://doi.org/10.25921/c1sn-9631, and the data are available at https://www.nodc.noaa.gov/ocads/data/0176626.xml (last access: 15 March 2019) or alternately http://odv.awi.de/data/ocean/jenkins-tritium-helium-data-compilation/ (last access: 13 March 2019) and includes approximately 60 000 valid tritium measurements, 63 000 valid helium isotope determinations, 57 000 dissolved helium concentrations, and 34 000 dissolved neon concentrations. Some quality control has been applied in that questionable data have been flagged and clearly compromised data excluded entirely. Appropriate metadata have been included, including geographic location, date, and sample depth. When available, we include water temperature, salinity, and dissolved oxygen. Data quality flags and data originator information (including methodology) are also included. This paper provides an introduction to the dataset along with some discussion of its broader qualities and graphics.

Minimising12C3+interference on4He+measurements in a noble gas mass spectrometer

New insights into interferences on helium isotope measurements, using a noble gas mass spectrometer, as a function of ion source trap current and filament voltage.

Changes in the atmospheric helium isotope ratio over the past 40 years

AbstractWe have compared the helium isotope ratio in five samples of Pacific marine air spanning the 40 year period between 1973 and 2013 against a secondary gas standard. In a separate experiment we directly compared the 3He/4He ratio in air samples collected in 1973 and 2013 at the same location in La Jolla, California, eliminating any geographical bias. Both experiments are consistent with zero time rate of change for atmospheric 3He/4He. Our best estimate for the rate of change of the 3He/4He ratio in Pacific marine air is −0.0014 ± 0.0045%/yr (2σ), indicating that air helium is still a valid standard for terrestrial helium isotope measurements.

Mass-spectrometric measurements of helium isotopes in structural materials of the Globus-M tokamak

Nuclear synthesis reactions involving formation of protons and tritium nuclei or neutrons and helium-3 nuclei can proceed in deuterium plasma of tokamaks even at fairly low ion temperatures with practically the same probability. Therefore determination of an amount of any of these components forming in a known time provides the information on the rate of synthesis reactions. Measurement of small amounts of elementary particles (protons and neutrons) and calibration of instruments involve extremely complicated procedures and will not be treated here. We are going to discuss here the methods and difficulties associated with measurement of radioactive tritium and present briefly the technique employed for extraction and purification of gases from solid specimens, as well as the technique of mass-spectrometric measurements of helium isotopes by means of MI-9301 magnetic resonance mass spectrometer. The measured 3He/4He isotopic ratios and the extremely large amounts of 3He in samples suggested a conclusion that a significant part of the light isotope helium-3 revealed in construction materials is largely a product of nuclear synthesis proceeding in the deuterium plasma of the Globus-M spherical reactor. The same data enable to estimate of the rate of generation of helium-3 nuclei (and, hence, of tritium, neutrons and protons). We believe that such mass spectrometric measurements provide a possibility for a detailed study of the processes involving interaction of deuterium plasma with materials of the first wall, divertor and other elements of tokamaks.

High-precision helium isotope measurements in air

Helium has two natural isotopes which have contrasted, and variable sources and sinks in the atmosphere (3He/4Heair = 1.382 ± 0.005 × 10−6). Variations in the atmospheric helium isotopic composition may exist below typical measurement precision thresholds (0.2 to 0.5%, 2σ). In order to investigate this possibility, it is necessary to be able to consistently measure helium isotopes in air with high precision (below 0.2% 2σ). We have created an air purification and measurement system that improves the helium isotope measurement precision. By purifying a large quantity of air at the start of a measurement cycle we can make rapid standard-bracketed measurements. Controlling the amount of helium in each measured aliquot minimizes pressure effects. With this method we improve the standard errors by 2× over measuring the same amount of gas in a single step. Individual measurements have standard errors of 0.2 to 0.3% (2σ), with three repeat samples needed to reach 0.1% or better errors. The long-term reproducibility of our calibration sample is 0.033% (2σ).

Helium distribution in a mantle shear zone from the Josephine Peridotite

A previous study of oceanic mylonites suggested that peridotite helium concentrations are correlated with the degree of high-temperature ductile deformation in the mantle. In order to test this result, this study combines helium measurements with characterization of the deformation state of harzburgite samples in a small (6m width) ductile mantle shear zone from the Josephine Peridotite, Oregon, USA. All measurements were made by coupled in vacuo crushing and melting, demonstrating that most of the helium (>80%) resides within the solid phases rather than fluid or melt inclusions. The present study confirms the influence of deformation on helium contents, but only at the highest shear strain (γ>20) are helium contents significantly higher. The highest helium concentration, by roughly a factor of two, is found in the center-most sample, which also has grain size reduction by a factor of ∼4. Dislocations and sub-grain boundaries are present in all samples and do not correlate with helium concentrations. Mineralogy also appears to have a negligible influence in this shear zone, as modal mineralogy is relatively homogeneous, with all samples being harzburgites. These observations suggest that the increase in helium concentration is related to grain size reduction, with grain boundaries proposed as an additional storage site for helium in the mantle.The present data also characterize the isotopic composition of the Josephine Peridotite: 3He/4He=6.7±0.2Ra (n=33, between 6.3 and 7.1Ra). The presence of cosmogenic 3He in the matrix is indicated by the helium isotopic composition released by melting: 3He/4He=8.5±0.3Ra (n=10; from 7.9 to 10.9). This corresponds to an exposure age of 10Kyr, which is approximately concordant with the end of the last glacial maximum. Very little radiogenic helium is present in the samples, suggesting extremely low uranium and thorium contents ([U]<0.3ppb). Helium isotope measurements in four samples outside the shear zone suggest that mineralogy and melt infiltration are also important factors for understanding helium storage in the mantle.

Inaccuracies in helium isotope measurements

The results of mass-spectrometric measurements of the helium isotope content in aluminum samples obtained at different times and under different conditions and also of the helium isotope content in iron-manganese concretions are presented. The measurement accuracy of the helium isotope content in aluminum depends on various measuring-technique-related factors: vacuum conditions, gas inleakage and escape in the mass analyzer and/or gas extraction system, the device memory effect as applied to any of the helium isotopes, preparation accuracy of reference samples, their persistence, etc. In the given case, the statistical processing of measurement data with different criteria indicates that the variances diverge considerably but does not discover the sources of divergence. To measure the helium isotope content in iron-manganese concretions, a powdered sample is placed in a metallic capsule, which is then thrown into a heated crucible in vacuum, where gases to be tested liberate. Our statistical analysis of measuring data confirms the supposition that, if the capsule is made of low-melting aluminum and the capsule with a fine powder is thrown into a heated crucible, the variance grows and measuring data are underestimated. This seems to be associated with rapid melting of the capsule, emission of fine particles of the sample out of the hot zone of the reactor, and their loss. When the capsule is made of nickel, a much higher melting material than aluminum, it does not melt and the test material is not ejected from the hot zone. That is what follows from the results of the statistical analysis in our opinion.

Origin of 3He/ 4He ratios in HIMU-type basalts constrained from Canary Island lavas

New helium isotope and abundance measurements are reported for olivine and clinopyroxene phenocrysts from HIMU-type (high- μ = elevated 238U/ 204Pb) lavas and xenoliths spanning the stratigraphies of El Hierro and La Palma, Canary Islands. Some pyroxene phenocrysts have suffered post-eruptive modification, either by less than 1% assimilation of crustal-derived He, or by closed-system ageing of He. Olivine phenocrysts record mantle source 3He/ 4He compositions, with the average 3He/ 4He for La Palma olivine (7.6 ± 0.8R A, where R A is the atmospheric 3He/ 4He ratio of 1.38 × 10 −6) being within uncertainty of those for El Hierro (7.7 ± 0.3R A), and the canonical mid-ocean ridge basalt range (MORB: 8 ± 1R A). The new helium isotope data for El Hierro and La Palma show no distinct correlations with whole-rock 87Sr/ 86Sr, 143Nd/ 144Nd, 187Os/ 188Os, or Pb isotopes, but 3He/ 4He ratios for La Palma lavas correlate with 18O/ 16O measured for the same phenocryst populations. Despite limited 3He/ 4He variations for El Hierro and La Palma, their He–O isotope systematics are consistent with derivation from mantle sources containing distinct recycled oceanic basaltic crust (El Hierro) and gabbroic lithosphere (La Palma) components that have mixed with depleted mantle, and a high- 3He/ 4He component (> 9.7R A) in the case of La Palma. The new data are consistent with models involving generation of compositionally and lithologically (e.g., pyroxenite, eclogite, peridotite) heterogeneous mantle sources containing recycled oceanic crust and lithosphere entrained within upwelling high- 3He/ 4He mantle that has been severely diluted by interaction with depleted mantle. We propose that the noble gas systematics of HIMU-type lavas and ocean island basalts (OIB) in general, are most simply interpreted as being controlled by the most gas-rich reservoir involved in mixing to generate their mantle sources. In this scenario, HIMU and enriched mantle (EM) sources are dominated by depleted mantle, or high- 3He/ 4He mantle, because recycled crust and lithosphere have low He concentrations. Consequently, high- 3He/ 4He OIB would predominantly reflect derivation from a less depleted mantle source with sub-equal to higher He contents than depleted mantle. The available coupled He–O isotope systematics measured for OIB lavas are consistent with this hypothesis.

Detection of tritium and alpha decaying radionuclides in L/ILW by measurements of helium isotopes

As decay products, helium isotopes can clearly indicate the presence of tritium and alpha decaying isotopes in a closed system. This study presents the helium and neon measurements and their interpretation of long-term headspace gas investigations in L/ILW waste drums from Paks Nuclear Power Plant and closed vaults of the Radioactive Waste Treatment and Disposal Facility, Puspokszilagy, Hungary. Development of special sampling methods and preparation lines as well as isotope-analytical measurements of the headspace gas samples were done in the Hertelendi Laboratory of Environmental Studies in the ATOMKI. In the gas samples helium isotopes as well as neon isotopes have been determined mass spectrometrically. While neon content can be of atmospheric origin only, helium can be produced either by alpha decay (4He) or decay of tritium (3He). 3H/4He and He/Ne ratios have been used to determine the different origin of the helium isotopes. Helium isotope ratios always represented 3He enrichment in the headspace gases produced by the decay of the tritium in the waste. Using the recent 3He concentration in headspace gas the total amount of 3H restored in L/ILW vaults was estimated. The investigated seven different vaults were closed between 1979 and 1995 when they had been full with L/ILW. The calculated tritium activities based on the He measurements showed good agreement with the documented isotope inventory of the vaults. Typical tritium activity concentrations were between 0.1 and 10 Bq/L gas in the drums and between 10 and 1000 Bq/L gas in the vaults. Additionally, one drum showed a higher He/Ne ratio compared to air, which clearly indicates 4He excess, thus the presence an alpha source in the waste.

Spatial variability of erosion rates inferred from the frequency distribution of cosmogenic 3He in olivines from Hawaiian river sediments

To constrain the spatial distribution of erosion rates in the Waimea river watershed, on the western side of the island of Kauai, Hawaii, we calculate the frequency distribution of cosmogenic 3He concentrations ([ 3He] c ) from helium isotopic measurements in olivine grains from a single sample of river sediment. Helium measurements were made in 26 aliquots of ∼ 30 olivine grains each. The average [ 3He] c from the 26 aliquots was used to estimate a basin-wide average erosion rate of 0.056 mm/yr, a value that is similar to erosion rates obtained from geochemical analyses of river sediments from tectonically stable landforms. However, forward models of cosmogenic nuclide production and sediment generation rates are inconsistent with the hypothesis that the observed [ 3He] c frequency distribution is the result of a homogeneous, basin wide, erosion rate. Instead, a distribution of erosion rates, from ∼ 0 to 4 mm/yr, may account for the observed frequency distribution. The distribution of erosion rates can be modeled by both non-linear slope- and curvature-dependent erosion rates with power law exponents ranging from 2.0 to 2.5. However, the spatial distribution of cosmogenic nuclides for slope- and curvature-dependent erosion rates are distinct, and we propose strategies to test further the extent to which erosion rates are controlled by the macroscale topographic features. These results demonstrate that the observed frequency distribution of cosmogenic nuclide concentrations in river sediments, combined with numerical modeling of erosion rates, can provide constraints on both the spatial variability of erosion rates in a drainage basin and the form of parameterized erosion laws.

How many mantle plumes in Africa? The geochemical point of view

The association of anomalous topographic swells and widespread Cenozoic volcanism within the African plate (Hoggar, Tibesti, Darfur, Ethiopian highlands, Kenyan dome) may reflect either the involvement of one, or several, deep mantle plumes, or, alternatively, be attributed to tectonic processes involving only the lithosphere or the shallow asthenosphere. We present here new helium isotopic measurements that, added to existing data, allow us to restrict the spatial extent of a high- 3He component (up to 20 Ra) to the Ethiopia–Afar volcanic province, for places where large volumes of Oligocene pre-rift flood basalts and ignimbrites erupted within a short (1–2 Ma) time interval. All other investigated African volcanic provinces display MORB-type, and/or continental lithosphere-like, 3He / 4He signatures (7 ± 2 Ra) often modified by a contribution of crustal He. The distribution of He isotopic signatures in Africa, together with other isotopic (Sr–Nd–Pb) tracers measured in Miocene to Plio–Quaternary alkaline lavas in East-Africa, is fully consistent with the occurrence of two types of mantle plumes: (i) a large, deep-sited mantle plume characterized by a high- 3He signature, possibly originating from the core–mantle boundary according to seismic mantle tomography, which triggered the flood basalt eruptions 30 Ma ago and which subsequently interacted with shallower mantle sources to produce the syn-rift volcanism of the Ethiopia–Afar province; and (ii) a second-order type of shallow mantle upwelling, presumably originating from depths shallower than 400 km as suggested by seismic wave imaging, distinct from the main Afar plume and disseminated within the African plate under the uplifted and rifted swells. The above conclusions do not support the view of a unique large mantle plume feeding all Cenozoic African volcanic provinces. The fact that high- 3He signals are associated with the largest lava volumes erupted in Africa since the beginning of the Cenozoic argue against models advocating a shallow origin for high 3He / 4He signatures. Instead, they confirm that such signatures characterize hot material coming from the deep mantle.

Pitch angle distribution of trapped energetic protons and helium isotope nuclei measured along the Resurs-01 No. 4 LEO satellite

Abstract. The NINA detector on board the Resurs-01 No. 4 satellite (835 km, 98° inclination) is equipped with particle trackers based on silicon strip detectors. From the energy deposited in each of its silicon layers the mass, the momentum direction and energy of incident particles have been determined. The resolutions in mass and energy allow identification of H and He isotopes over the 10-50 MeV/n energy range. The angular resolution is about 2.5°. We present the direct measurements of proton and helium isotopes pitch angle distributions derived from Resurs-01 No.4/NINA observations and their variations as functions of (B, L) coordinates and energy. The measurements of trapped helium isotopes spectrum are also presented.

Methane and methane carbon isotope ratios in the Northeast Atlantic including the Mid-Atlantic Ridge (50°N)

Previous work has shown that methane anomalies frequently occur within the rift valley of the Mid-Atlantic Ridge (MAR). The plumes appear confined within the high, steep walls of the valley, and it is not known whether methane may escape to the open ocean outside. In order to investigate this question, the concentration and 13C/ 12C ratio of methane together with CCl 3F concentration were measured in the northeastern Atlantic including the rift valley near 50°N. This segment contained methane plumes centered several 100 m above the valley floor with δ 13C values mostly between –15‰ and –10‰. A limited number of helium isotope measurements showed that δ 3He increased to 17% at the bottom of the valley, which suggests the helium and methane sources may be spatially separated. In the eastern Atlantic away from the ridge (48°N, 20°W), the methane concentration decreased monotonically from the surface to the bottom, but the methane δ 13C exhibited a mid-water maximum of about –25‰. The bottom water methane contained a significantly lower δ 13C of about –36‰. Thus, it appears that isotopically heavy methane escapes from the MAR into North Atlantic Deep Water (NADW) that contacts the ridge crest while circulating to the east. The formation of NADW supplies isotopically light methane that dilutes the input of heavy carbon from the ridge. We employed a time-dependent box model to calculate the extent of isotope dilution and thereby the flux of MAR methane into the NADW circulation. The degree of methane oxidation, which affects the 13C/ 12C of methane through kinetic isotope fractionation, was estimated by comparing methane and CFC-11 model results with observations. The model calculations indicate a MAR methane source of about 0.06×10 −9 mol L −1 yr −1 to waters at the depth of the ridge crest. Assuming this extends to a 500 m thick layer over half of the entire Atlantic, the amount of methane escaping from the MAR to the open ocean is estimated to be about 1×10 9 mol yr −1. The total production of methane within the rift valley is likely much greater than the flux from the valley to the outside because of local oxidation. This implies that serpentinization of ultramafic rocks supports much of methane production in the rift valley because the amount expected from basalt degassing in association with mantle helium (<0.6×10 9 mol CH 4 yr −1) is less than even the net amount escaping from the valley. The model results also indicate the methane specific oxidation rate is about 0.05 yr −1 in open waters of the northern Atlantic.

Accretion of interplanetary dust in polar ice

Measurements of helium isotopes in particles separated from polar ice demonstrate that extraterrestrial ³He dominates the ³He flux at the GISP2 (Greenland) and Vostok (Antarctica) ice core sites. Replicate measurements of late Holocene ice samples yield ³He fluxes of 0.62±0.27×10−12 cm³ STP cm−2 ka−1 (GISP2) and 0.77 ± 0.25 × 10−12 cm³ STP cm−2 ka−1 (Vostok), similar to results from marine sediments. These are the first detailed measurements of ³He in particles from ice core samples, and they demonstrate the utility of the ice core record for evaluating the temporal history of the extraterrestrial dust flux. Results from Vostok samples from 1096–1403 m depth (75–97 ka B.P.) are similar to the late Holocene data, with the exception of two highly anomalous results from 1307 m. The latter probably indicate the presence of rare, large or gas‐rich extraterrestrial particles.

³He in Indonesian Seas: Inferences on deep pathways

Helium isotope measurements from the Indonesian Seas, and from the Java‐Australia Dynamic Experiment cruises define mantle ³He sources and deep water spreading patterns. ³He sources in the Sulawesi Sea and in the Makassar Strait are found, but these are prevented from spreading into the Banda Sea and the Indian Ocean by blocking sills. A major mantle source is identified below 2000m in the Flores Sea. The ³He distribution suggests that the Banda Sea water migrates westward along the floor of the Flores Sea, upwells and then advects the high ³He Flores signal. A tongue of higher‐than‐ambient ³He on the Indian Ocean side of the Flores chain indicates a deep outflow of the Flores and the Banda Sea water into the Indian Ocean near 1300m.

Tritium-3He ages of deep waters in the NEW Polynya

Abstract The measurements of tritium and helium isotopes in the Northeast Water (NEW) Polynya region in the summer of 1992 and 1993 show that the deep waters in the north (Westwind Trough) are 3–5 years older than those in the south (Belgica Trough). The distribution of these tracers in the region indicates that a deep ‘flushing’ event took place between the 1992 and 1993 field studies. This result corroborates information obtained by others from hydrographic data and from year-long bottom-moored current meters.

Helium isotope fluxes and groundwater ages in the Dogger Aquifer, Paris Basin

Helium isotopic and rare gas measurements in the Dogger aquifer of the Paris Basin have shown the helium concentration and helium isotopic ratio to be first‐order homogeneous with only slight differences in relation to specific chemical water types. Helium in the Dogger aquifer contains quantities of 3He 3–4 times in excess of radiogenic production/accumulation from natural decay of U, Th series elements. Calculations of cosmogenic production/accumulation of 3He cannot account for the observed excesses, and 3He excess is attributed to the addition of a mantle‐derived He magmatic component. Calculations of the isotopic fluxes of helium out of the Dogger (4He flux, 2×10−7 mol 4He m−2 yr−1; 3He flux, 1.8×10−14 mol 3He m−2 yr−1 (±50%)) suggest that the helium residence time and the groundwater residence time in the Dogger aquifer are 4 Myr (±50%). This age estimate is at least 10 times as great as estimates derived from hydrologic models but is more consistent with the geochemistry of the water.

Geochemical constraints on formation fluid ages, hydrothermal heat flux, and crustal mass transport mechanisms at Cajon Pass

He, Ne, and helium isotopic measurements have been made on borehole fluids collected during drill stem tests at the Cajon Pass scientific drill hole. These fluids represent a dominant component (greater than 90%) of the formation fluids as determined from fluoroscein tracer measurements. The results indicate a crustally derived radiogenic helium source with excess He concentrations up to 100x air‐saturated water conditions. The gas content and the chemical composition of the borehole waters reflect a variation in the formation waters entering the hole from several discrete, identified, and poorly interconnected fracture systems. These conditions indicate a local source for the excess 4He which allows 4He model ages of 33,000 to 5×106 years to be placed on the formation fluids at this depth. This constraint suggests an upper limit of &lt;7% of the total heat flow can be removed by hydrothermal processes. These data thus provide geochemical support for a low stress fault. These model ages further indicate an effective vertical component of the fluid transport velocity of the order of 0.04–6 cm yr−1. Pressure head and in situ permeability measurements indicate flow velocities of 10−5 cm yr−1. To satisfy both of these extant conditions, fluid exchange must have occurred (as least once) in the last ∼33,000 to 5×106 years under flow conditions considerably faster than presently operable. Fluid flow in this region is therefore discontinuous with long periods of slow flow interrupted by periods of very rapid transport. Such a conclusion is consistent with both local and general observations of metamorphic geology and suggests a deep crustal fluid transport mechanism that is mechanically and/or tectonically controlled.