Noble Gas Analyses Research Articles

Noble gas systematics in new popping rocks from the Mid-Atlantic Ridge (14°N): Evidence for small-scale upper mantle heterogeneities

The origin and evolution of terrestrial volatiles is a fundamental question in Earth and planetary sciences. To better address this, it is necessary to estimate the present-day isotopic and elemental composition of volatiles in the mantle, which is difficult because degassing during magma transport to the surface changes the initial elemental composition. However, some basalts are considered to be relatively un-degassed and therefore can be used to determine the concentrations of C, N, H2O and noble gases in the mantle. This is the case of the so-called popping rock sample 2πD43, found near 14°N on the Mid-Atlantic Ridge in 1985, which has been considered as a reference for upper mantle isotopic and elemental volatile compositions due to its high gas content. Such gas-rich samples have been recovered in only a few places on mid-ocean ridges but are crucial to characterizing the heterogeneities in mantle composition and therefore the cycling of volatiles on Earth. In order to better understand the occurrence of popping rocks, mantle heterogeneities and volatile cycling on Earth, cruise AT33-03 on R/V Atlantis was conducted in 2016 on the Mid-Atlantic Ridge near 14°N. Popping rocks were found in the same area as sample 2πD43, using the submersible Alvin, and their noble gas compositions were determined by step-crushing and laser ablation. A new anaerobic sampling protocol was tested in an attempt to minimize atmospheric contamination, which is ubiquitous in basaltic glass samples used for noble gas analyses, and to evaluate laser extraction techniques. Preliminary step-crushing results suggest that the new anaerobic sampling protocol reduces atmospheric influence on the noble gas measurements. Comparison with laser ablation data shows that there is still some atmospheric contamination, suggesting that contamination begins on the seafloor. Two groups of samples have been identified based on their noble gas compositions: one group with their highest measured ratios similar to those of sample 2πD43 (20Ne/22Ne of ∼12.5, 40Ar/36Ar of ∼27,000), and another group with lower maximum noble gas ratios (20Ne/22Ne of 12.2, 40Ar/36Ar of 16,100) and higher argon concentrations. Based on these results, along with Cl/K and H2O/Ce ratios and preliminary lead, strontium and neodymium isotope data, it is suggested that these two groups of samples reflect separate eruptive events that originated from distinct mantle sources. This provides evidence for upper mantle heterogeneity at small scales and could imply that the mantle source with lower neon and argon isotopic ratios has been more influenced by recycling of atmospheric noble gases.

Neon isotopes in individual presolar low-density graphite grains from the Orgueil meteorite.

We present He and Ne isotopes of individual presolar graphite grains from a low-density separate from Orgueil. Two grain mounts were analyzed with the same techniques but in a different sequence: The first one was measured with NanoSIMS followed by noble gas mass spectrometry, and the second one in reverse order. No grain contained 4He and only one grain on the second mount contained 3He. On the first mount, the grains had been extensively sputtered with NanoSIMS ion beams prior to noble gas analysis; we found only one grain out of 15 with presolar 22Ne above detection limit. In contrast, we found presolar 22Ne in six out of seven grains on the second mount that was not exposed to an ion beam prior to noble gas analysis. All 22 grains on the two mounts were imaged with scanning electron microscopy (SEM) and/or Auger microscopy. We present evidence that this contrasting observation is most likely due to e-beam-induced heating of the generally smaller grains on the first mount during SEM and Auger imaging, and not primarily due to the NanoSIMS analysis. If thermal contact of the grains to the substrate is absent, such that heat can only be dissipated via radiation, then the smaller, sputter-eroded grains are heated to higher temperatures such that noble gases can diffuse out. We discuss possible gas loss mechanisms and suggest solutions to reduce heating during e-beam analyses by minimizing voltages, beam currents, and dwell times. We also found small amounts of 21Ne in five grains. Using isotope data we determined that the dominant sources of most grains are core-collapse supernovae, congruent with earlier studies of low-density presolar graphite from Murchison. Only two of the grains are most likely from AGB stars, and two others have an ambiguous origin.

Origin of isotopically light nitrogen in meteorites

Bulk meteorite samples of various chemical classes and petrologic types (mainly carbonaceous chondrites) were systematically investigated by the stepped combustion method with the simultaneous isotopic analysis of carbon, nitrogen, and noble gases. A correlation was revealed between planetary noble gases associating with the Q phase and isotopically light nitrogen (δ15N up to –150‰). The analysis of this correlation showed that the isotopically light nitrogen (ILN) is carried by Q. In most meteorites, isotopically heavy nitrogen (IHN) of organic compounds (macromolecular material) is dominant. The ILN of presolar grains (diamond and SiC) and Q can be detected after separation from dominant IHN. Such a separation of nitrogen from Q and macromolecular material occurs under natural conditions and during laboratory stepped combustion owing to Q shielding from direct contact with oxygen, which results in Q oxidation at temperatures higher than the temperatures of the release of most IHN. There are arguments that ILN released at high temperature cannot be related to nanodiamond and SiC. The separation effect allowed us to constrain the contents of noble gases in Q, assuming that this phase is carbon-dominated. The directly measured 36Ar/C and 132Xe/C ratios in ILN-rich temperature fractions are up to 0.1 and 1 × 10–4 cm3/g, respectively. These are only lower constraints on the contents. The analysis of the obtained data on the three-isotope diagram δ15N–36Ar/14N showed that Q noble gases were lost to a large extent from most meteorites during the metamorphism of their parent bodies. Hence, the initial contents of noble gases in Q could be more than an order of magnitude higher than those directly measured. Compared with other carbon phases, Q was predominantly transformed to diamond in ureilites affected by shock metamorphism. The analysis of their Ar–N systematics showed that, similar to carbonaceous chondrites, noble gases were lost from Q probably before its transformation to diamond.

Further evidence for an impact origin of the Tsenkher structure in the Gobi-Altai, Mongolia: geology of a 3.7 km crater with a well-preserved ejecta blanket

AbstractThe Tsenkher structure in the Gobi-Altai, Mongolia is ac. 3.7 km diameter crater with a well-preserved ejecta blanket. It has been hypothesized to be either of impact or volcanic origin in our previous work. Observations during our 2007 expedition and related sample analyses give further support for an impact origin. The evidence includes the presence of a structurally uplifted near-circular rim surrounded by an ejecta blanket, and abundant breccias, some of which are melt- and millimetre-scale spherule-bearing. Planar deformation features (PDFs) were found in one quartz grain in a breccia sample. Fe-rich grains are found in a vesicular melt sample that is also characterized by elevated platinum group element (PGE) abundances with respect to the sedimentary bedrock of the area (approximately an order of magnitude). Noble gas analysis of one breccia sample yielded an elevated3He/4He value of (5.0±0.2) × 10−6. Although not conclusive alone, these geochemical results are consistent with a contribution of meteoritic components. A volcanic origin, in particular a maar formation, would require explanations for the unusual conditions associated with Tsenkher, including its large size occurring in isolation, the structurally uplifted rim and the lack of a bedded base surge deposit. A pronounced rampart structure observed at the eastern ejecta is also unusual for any volcanic origin.40Ar–39Ar dating of a vesicular melt sample gives an age of the Tsenkher structure of 4.9±0.9 Ma. The rampart structure could provide insights into the formation of similar ejecta morphologies associated with numerous impact craters on Mars.

The origin and degassing history of the Earth's atmosphere revealed by Archean xenon

Xenon (Xe) is an exceptional tracer for investigating the origin and fate of volatile elements on Earth. The initial isotopic composition of atmospheric Xe remains unknown, as do the mechanisms involved in its depletion and isotopic fractionation compared with other reservoirs in the solar system. Here we present high precision analyses of noble gases trapped in fluid inclusions of Archean quartz (Barberton, South Africa) that reveal the isotopic composition of the paleo-atmosphere at ≈3.3 Ga. The Archean atmospheric Xe is mass-dependently fractionated by 12.9±2.4 ‰ u−1 (± 2σ, s.d.) relative to the modern atmosphere. The lower than today 129Xe excess requires a degassing rate of radiogenic Xe from the mantle higher than at present. The primordial Xe component delivered to the Earth's atmosphere is distinct from Solar or Chondritic Xe but similar to a theoretical component called U-Xe. Comets may have brought this component to the Earth's atmosphere during the last stages of terrestrial accretion.

Atmospheric helium isotope composition as a tracer of volcanic emissions: A case study of Erta Ale volcano, Ethiopia

Abstract The composition of atmospheric helium is generally considered to be constant (3He/4He = 1.39 × 10− 6) on a large spatial scale. However, local variations may arise in tectonically active areas due to focussed degassing of one of its two isotopes, for example degassing of mantle-derived 3He or crustal-derived 4He. If detected, such variations have the potential to trace open conduit degassing of magmatic bodies and/or diffusive emissions from volcanic and/or crustal sources. Here, we test the possibility of detecting such variations in the Afar region of north-eastern Ethiopia, which is located over a well-developed rift system. Special attention was paid to the Erta Ale volcanic edifice, where both magma lake activity and strong degassing occur. We conducted high-precision 3He/4He ratio measurements of air samples from this region using an analytical facility at the Centre de Recherches Petrographiques et Geochimiques (CRPG), Nancy (France) that was specially designed for high-precision noble gas analyses. Within the precision of our measurements (0.2–0.4%, 95% confidence interval), the helium isotopic compositions of air from both the Afar rift zone and the crater zone of Erta Ale are similar to the composition of air collected at Brabois Park in Villers-les-Nancy, France (labelled here as RBB). An additional air sample collected in a large tank (500 cm3) in Afar in 2015 permitted replicate analysis (n = 8) and improved precision. The 3He/4He ratio of this additional sample was also identical to RBB within 0.19% (95% confidence interval, CI). However, a clear excess of 3He (1.32 ± 0.64%, 95% CI, relative to RBB based on a weighted mean of two samples) was detected in air collected above the active lava lake located in the central pit crater of Erta Ale volcano. Such excess requires a 3He flux of 0.15 ± 0.09 mol/yr from the crater lava lake to be sustained. A similar 3He flux of 0.12 ± 0.06 mol/yr is calculated from SO2 flux measurements and fumerolic gas data obtained during the same field trip. At several sites in the rim of the Northern crater, we conducted soil flux measurements using an accumulation chamber. Both the CO2 contents and the helium isotope ratios increased over time within the chamber, allowing us to evaluate the soil CO2 and 3He fluxes outside the lava lake area. These fluxes were found to be minor (~ 1%) compared to the lava lake flux. The CO2/3He ratio of (3.1 ± 0.7) × 109 of soil gases is comparable to that of the high temperature (1084 °C) fumaroles sited in the north pit crater. Using this ratio and our estimated 3He flux, we determined a CO2 flux of 4.6 ± 3.0 × 108 mol/yr for the lava lake, which is about 105 times lower than the global volcanic subaerial CO2 flux. Based on this pilot study, we suggest that 3He excesses in air could provide another means with which to evaluate the fluxes of CO2 and other volatile species in specific environments, such as highly active volcanic areas.

Effective radium-226 concentration in meteorites

The analysis of noble gases in meteorites provides constraints on the early solar system and the pre-solar nebula. This requires a better characterization and understanding of the capture, production, and release of noble gases in meteorites. The knowledge of transfer properties of noble gases for each individual meteorite could benefit from using radon-222, radioactive daughter of radium-226. The radon-222 emanating power is commonly quantified by the effective radium-226 concentration (ECRa), the product of the bulk radium-226 concentration and of the emanation coefficient E, which represents the probability of one decaying radium-226 to inject one radon-222 into the free porous network. Owing to a non-destructive, high-sensitivity accumulation method based on long photomultiplier counting sessions, we are now able to measure ECRa of meteorite samples, which usually have mass smaller than 15g and ECRa<0.5Bqkg−1. We report here the results obtained from 41 different meteorites, based on 129 measurements on 70 samples using two variants of our method, showing satisfactory repeatability and a detection limit below 10−2Bqkg−1 for a sample mass of 1g. While two meteorites remain below detection level, we obtain for 39 meteorites heterogeneous ECRa values with mean (min–max range) of ca. 0.1 (0.018–1.30)Bqkg−1. Carbonaceous chondrites exhibit the largest ECRa values and eucrites the smallest. Such values are smaller than typical values from most terrestrial rocks, but comparable with those from Archean rocks (mean of ca. 0.18Bqkg−1), an end-member of terrestrial rocks. Using uranium concentration from the literature, E is inferred from ECRa for all the meteorite samples. Values of E for meteorites (mean 40±4%) are higher than E values for Archean rocks and reported values for lunar and Martian soils. Exceptionally large E values likely suggest that the 238U-226Ra pair would not be at equilibrium in most meteorites and that uranium and/or radium are most likely not uniformly distributed. ECRa of meteorites is correlated with E and seems to mainly reflect the gas permeability of the meteorite, which could be one important property, preserved in the meteorite, of its parent body, characterizing its history in space, possibly modified by alteration, shock metamorphism, and eventually weathering on Earth. Larger radon emanation values are associated with larger concentrations of the heaviest noble gases (argon, krypton, xenon), and larger 20Ne/22Ne and 36Ar/38Ar ratios, suggesting Earth’s atmosphere contamination or solar wind implantation, and probably a similar carrier phase such as Q phase. An unclear correlation is observed with 40Ar, which may rule out a purely radiogenic effect on radon emanation. Thus, larger radon emanation suggests a larger capacity of collecting solar and terrestrial gases, which should imply higher loss of gases generated in the meteorite and larger dispersion of Pb/U ratios for age determination. This study provides the first quantification of natural radon-222 loss from meteorites and opens promising perspectives to quantify the relationship between pore space connectivity and the transfer properties for noble gases in meteorites and other extraterrestrial bodies.

Nucleosynthetic Signatures in Presolar SiC and Graphite Grains.

Presolar SiC and graphite grains are the grain types whose isotopic signatures have been extensively studied. Isotopic compositions of light and intermediate elements in single grains have been analyzed mostly using secondary ion mass spectrometry. Detailed information about nucleosynthetic conditions can be obtained from isotopic compositions of heavy elements. Isotopic compositions of heavy elements in SiC and graphite grains have been analyzed using resonant ionization mass spectrometry. Analyses of heavy elements and noble gases are likely to produce new insights into presolar grains using newly-developed instruments.

From supernova to Solar System: Few years only; first Solar System components apatite and spinel determined

We show data for the very first years of our Solar System development after an interaction between undisturbed, cold interstellar dust and supernova type II explosion gases. All manual work was done in 1976–1982 as part of 3 theses works but fundamentally new data interpretation was reached within the last three years.From the CI1 meteorite Orgueil, we are able to separate 1.4 per mill of material containing supernova related noble gases He, Ne and Ar as well as P.We separate minerals using essentially density gradient centrifugation followed by stepwise heating noble gas analysis. Our procedure loses nearly no material and is in sharp contrast to the otherwise used dissolution of >99% of material to obtain single presolar grains (Anders and Zinner, 1993). Our method safeguards minerals considerably more fragile than SiC or TiC presolar grains, such as apatite, Mg-Al-spinel, graphite clusters and even apatite coated graphite clusters. We find graphite, apatite and Mg-Al-spinel containing highly anomalous noble gases. For the first time, apatite, containing anomalous Ar with an isotope ratio for 38Ar/36Ar of 0.35, twice the normal ratio, is reported. Such a ratio is produced by a 20 solar mass type II supernova in the C-O-Ne-burning shell. Unmatched pure Ne-E from 22Na measured in the same samples sets the timeframe for this interaction to a maximum of only a few years.

Using noble-gas and stable-isotope data to determine groundwater origin and flow regimes: Application to the Ceneri Base Tunnel (Switzerland)

Tunnel drilling provides a unique opportunity to sample and study deep groundwaters that are otherwise difficult to access. Understanding deep groundwater flow is of primary importance in assessing the possible impacts of tunnelling on hydrogeological systems. During this study, water was sampled for noble-gas analysis from tunnel inflows in the AlpTransit Ceneri Base Tunnel (Canton Ticino, southern Switzerland), which passes through an area mainly characterized by metamorphic rocks (gneiss). Furthermore, water was sampled from springs located in the same geological environment.Based on the measurement of noble-gas concentrations and isotope ratios, tritium concentrations, the stable isotope composition of hydrogen (δ2H) and oxygen (δ18O), and the concentrations of major ions in the water, a conceptual hydrogeological model was established for this case study that allowed the most probable origin of the groundwaters sampled at different locations to be determined. The measured abundances of 3He, 4He, and 20Ne allow the geochemical characterization of old groundwaters strongly enriched in terrigenic helium of crustal origin and the identification of mixing with water that circulates preferentially through cataclastic structures. Noble-gas concentrations and isotope ratios as well as tritium are useful proxies for the characterization of faults that may be critical for tunnel drilling because of their active hydrogeological role and their influence on the mechanics of the rocks.

Methane Sources and Migration Mechanisms in Shallow Groundwaters in Parker and Hood Counties, Texas-A Heavy Noble Gas Analysis.

This study places constraints on the source and transport mechanisms of methane found in groundwater within the Barnett Shale footprint in Texas using dissolved noble gases, with particular emphasis on 84Kr and 132Xe. Dissolved methane concentrations are positively correlated with crustal 4He, 21Ne, and 40Ar and suggest that noble gases and methane originate from common sedimentary strata, likely the Strawn Group. In contrast to most samples, four water wells with the highest dissolved methane concentrations unequivocally show strong depletion of all atmospheric noble gases (20Ne, 36Ar, 84Kr, 132Xe) with respect to air-saturated water (ASW). This is consistent with predicted noble gas concentrations in a water phase in contact with a gas phase with initial ASW composition at 18 °C-25 °C and it suggests an in situ, highly localized gas source. All of these four water wells tap into the Strawn Group and it is likely that small gas accumulations known to be present in the shallow subsurface were reached. Additionally, lack of correlation of 84Kr/36Ar and 132Xe/36Ar fractionation levels along with 4He/20Ne with distance to the nearest gas production wells does not support the notion that methane present in these groundwaters migrated from nearby production wells either conventional or using hydraulic fracturing techniques.

Characterization of carbonaceous matter in xenolithic clasts from the Sharps (H3.4) meteorite: Constraints on the origin and thermal processing

Primitive xenolithic clasts, often referred to as “dark clasts”, are well known in many regolith breccias. The Sharps H3.4 ordinary chondrite contains unusually large dark clasts up to ∼1cm across. Poorly-graphitized carbon (PGC), with Fe, Ni metal and described as “carbon-rich aggregates”, has been reported in these clasts (Brearley, 1990). We report detailed analyses of carbonaceous matter in several identical Sharps clasts using FTIR, Raman, C-XANES, and TEM that provide insight on the extent of thermal processing and possible origin of such clasts. We also prepared acid residues of the clasts using the HCl/HF method and conducted mass spectrometric analysis of the entrained noble gases.Carbonaceous matter is often used to infer thermal history due to its sensitivity to thermal processes. The FTIR spectra of the acid residue from the Sharps clast suggest that carbonaceous matter in the clast contains less hydrogen and oxygen compared to acid residues from typical type 3.4 ordinary chondrites. The metamorphic temperatures obtained by Raman spectroscopy ranges between ∼380°C and ∼490°C. TEM observations indicate that the clasts experienced a peak temperature of 300°C to 400°C, based on the carbon d002 layer lattice spacing of C-rich aggregates. These estimates are consistent with an earlier estimate of 330±50°C, that is also estimated by the d002 layer lattice spacing (Brearley, 1990). It should be noted that the lattice spacing thermometer is based on terrestrial metamorphose rocks, and thus temperature was probably underestimated. Meanwhile, the C-XANES spectra of the C-rich aggregates show high exciton intensities, indicative of graphene structures that developed at around 700–800°C following an extensive period of time (millions of years), however, the surrounding matrix areas experienced lower temperatures of less than 300–500°C. Noble gas analysis of the acid residue from the Sharps clasts shows that the residue is almost identical with some material reported in carbonaceous chondrites, i.e., heavily enriched in the Q-gas component as well as HL-gas from presolar diamonds and Ne-E(H) from presolar SiC.These results indicate that the C-rich aggregates in the Sharps clasts formed under relatively high temperature conditions, up to 800°C, and were subsequently mixed with lower temperature matrix, probably in a different parent body, before they were incorporated into the final Sharps lithology by collision.

Noble gas composition and 40Ar/39Ar age in eclogites from the main hole of the Chinese Continental Scientific Drilling project

We present the first comprehensive noble gas study on eclogites. The four eclogite samples were recovered during the Chinese Continental Scientific Drilling and are from two distinct profile depth sections differing in their degree of interaction with meteoric water, based on their δ 18O-values (surface related and of mantle-type). Hence, noble gas analyses offer the potential to further discriminate between shallow (meteoric) and deep (mantle) fluid sources. Noble gas compositions reveal typical crustal fluid compositions, characterized by a variable mixture of atmospheric gases with significant contributions of nucleogenic neon, radiogenic 4He*, radiogenic 40Ar*, fissiogenic 131–136Xe, and presumably bariogenic 131Xe, but no significant addition of mantle gases. This signature can be also considered to represent one endmember component of eclogitic diamonds. Concentrations of non-radiogenic noble gases are rather low, with depletion of light relative to the heavier noble gases. Eclogites from lower depth which experienced a higher degree of interaction with meteoric water also showed higher contributions of atmospheric gas compared with eclogites recovered from greater depth. This is interpreted to result from interaction with high-salinity fluids during ultrahigh pressure (UHP). It demonstrates that the atmospheric noble gas abundance is a proxy for interaction with surface related fluids. 40Ar/39Ar (inverse) isochron ages of two phengite separates (241.2 ± 0.4 Ma and 275.0 ± 1.8 Ma, 1σ-errors) predate the main phase of UHP metamorphism (ca. 220 Ma). Biotite yields an integrated age of about 1100 Ma. These age values are interpreted to reflect the likely addition of excess 40Ar without any chronological meaning.

The Cryogenic Enrichment System in Chromatographic Analysis of Noble Gases in Groundwater

The concentration of helium in groundwater may be a good environmental tracer for groundwater dating in hydrogeology. In this work, we present a chromatographic method for simultaneous analysis of helium, neon, and argon in groundwater from a single groundwater sample. Proper use of environmental tracers for dating purposes requires the knowledge of the recharge temperature of the system and the excess air. Both parameters can be determined by measuring the concentration of argon and neon in groundwater. The lowest helium concentration in groundwater is 4.8 × 10−8 cmSTP/gH2O at sea level and temperature 10 ◦C. In view of the deficiency of a suitable detector with a limit of detection for helium at the level of 4.8× 10−8 cmSTP/gH2O, the application of the enrichment method is necessary. In this work, the cryogenic method of enrichment with activated charcoal at abated pressure conditions was applied. Helium, neon and argon are analyzed on two gas chromatographs equipped with capillary and packed columns (filled with molecular sieve 5A and activated charcoal) and three thermo-conductive detectors. The chromatographic method was applied to groundwater dating from Krakow and Żarnowiec aquifers. The levels of detection for measurement systems for the tested compounds are: 1.9 × 10−8 cmSTP/cm for neon, 3.1× 10−6 cmSTP/cm for argon, and 1.2× 10−8 cmSTP/cm for helium.

Improved Method for the Quantification of Methane Concentrations in Unconsolidated Lake Sediments.

There is conclusive evidence that the methods most commonly used to sample methane (CH4) dissolved in the pore water of lake sediments produce results that are likely to be affected by gas loss or gas exchange with the atmosphere. To determine the in situ amount of CH4 per unit mass of pore water in sediments, we developed and validated a new method that combines techniques developed for noble-gas analysis in pore waters with a standard headspace technique to quantify the CH4 present in the pore space in dissolved and gaseous form. The method was tested at two sites: Lake Lungern, where CH4 concentrations were close to saturation; and Lake Rotsee, where CH4 concentrations are known to exceed saturation and where CH4 bubble formation and gas ebullition are commonly observed. We demonstrate that the new method, in contrast to the available methods, more reliably captures the total amount of CH4 per unit mass of pore water consisting of both dissolved and free CH4 (i.e., gas bubbles) in the pore space of the sediment.

Determination of halogen abundances in terrestrial and extraterrestrial samples by the analysis of noble gases produced by neutron irradiation

The lack of a reliable database for heavy halogens (bromine and iodine) in terrestrial and extraterrestrial samples is mainly due to the analytical challenges of determining their very low abundances (<1ppm) in the materials of interest. The neutron irradiation noble gas mass spectrometric (NI-NGMS) technique initially developed in the 1960s is the only viable technique currently capable of determining concentrations below 1ppb of iodine for small (<10mg) sample sizes. We describe in detail the analytical protocols and provide a comprehensive and transparent overview of the data reduction procedures in order to fully explore the uncertainties of the technique. We demonstrate how the capabilities of modern mass spectrometers used for Ar-Ar dating, can be readily extended to incorporate halogen measurements. A new and critical assessment of the use of standards is presented based on results from multiple irradiations, including a meteorite (Shallowater aubrite), scapolite minerals introduced by Kendrick (2012) and a novel internal calibration method based on using barium.

Widespread distribution of ascending fluids transporting mantle helium in the fore-arc region and their upwelling processes: Noble gas and major element composition of deep groundwater in the Kii Peninsula, southwest Japan

Chemical and isotopic studies including analyses of noble gases were comprehensively conducted on the groundwater of the entire Kii Peninsula, which is located in the fore-arc region of southwest Japan. Groundwater of Na–Cl–HCO3, Na–HCO3–Cl, and Na–Cl types was shown to be distributed across the whole area. Groundwater in the inland central part of the peninsula shows relatively low salinity, whereas groundwater from the area along the ENE-trending Median Tectonic Line (MTL), on the north side of the peninsula, shows high salinity (up to 18,800mg/L of Cl−) and the presence of unusual heavy oxygen isotopes. This trend is similar to that documented in saline waters from the Arima region (the so-called “Arima-type thermal water”). High 3He/4He ratios relative to the atmospheric value (up to 6.7Ra) were recorded throughout the Kii Peninsula, covering a wider area than documented previously. The saline groundwater is also strongly depleted in 20Ne and heavy noble gases.From the wide distribution of high 3He/4He values and the associated 20Ne and Cl− concentrations, we infer that aqueous fluids derived from dehydration of the subducting slab are present at depth beneath almost the entire Kii Peninsula. These aqueous fluids may ascend along the major north-dipping boundary faults. The isotopic composition of groundwater from the southern part of the peninsula suggests that the contribution from these dehydration-derived fluids is relatively small in this region. However, volatile components (e.g., noble gases and CO2) in the groundwater of this area may originate from the dehydration-derived fluids. Upwelling of Arima-type thermal water of the Na–Cl–HCO3 type is expected to undergo a phase separation of volatile species due to decompression as the fluid ascends. The variety of water types documented may be due to this water–gas separation and the subsequent incorporation of gaseous species into shallow meteoric groundwater. The observed high 3He/4He ratios in the absence of a mantle wedge below the southern part of the Kii Peninsula may reflect the oblique ascent of these fluids along north-dipping boundary faults.

Irreversible adsorption of atmospheric helium on olivine: A lobster pot analogy

This study reports new experimental results that demonstrate that large amounts of atmospheric helium may be adsorbed onto the surfaces of olivine grains. This behavior is surface-area-related in that this contamination preferentially affects grains that are smaller than 125μm in size. One of the most striking results of our study is that in vacuo heating at 900°C for 15min is not sufficient to completely remove the atmospheric contamination. This suggests that the adsorption of helium may involve high-energy trapping of helium through irreversible anomalous adsorption. This trapping process of helium can thus be compared to a “lobster pot” adsorption: atmospheric helium easily gets in, but hardly gets out. While this type of behavior has previously been reported for heavy noble gases (Ar, Kr, Xe), this is the first time that it has been observed for helium. Adsorption of helium has, until now, generally been considered to be negligible on silicate surfaces. Our findings have significant implications for helium and noble gas analysis of natural silicate samples, such as for cosmic-ray exposure dating or noble gas characterization of extraterrestrial material. Analytical procedures in future studies should be adapted in order to avoid this contamination. The results of this study also allow us to propose an alternative explanation for previously described matrix loss of cosmogenic 3He.

Experimental Studies and Model Analysis of Noble Gas Fractionation in Porous Media

Core Ideas Ambient noble gases in porous media fractionate from through‐flow of other gases. CO2 fluxes and noble‐gas fractionation are measured in simple sandbox experiments. Modeled fractionation provides indirect proxy, e.g., of steady CO2 leakage fluxes. The noble gases, which are chemically inert under normal terrestrial conditions but vary systematically across a wide range of atomic mass and diffusivity, offer a multicomponent approach to investigating gas dynamics in unsaturated soil horizons, including transfer of gas between saturated zones, unsaturated zones, and the atmosphere. To evaluate the degree to which fractionation of noble gases in the presence of an advective–diffusive flux agrees with existing theory, a simple laboratory sand column experiment was conducted. Pure CO2 was injected at the base of the column, providing a series of constant CO2 fluxes through the column. At five fixed sampling depths within the system, samples were collected for CO2 and noble gas analyses, and ambient pressures were measured. Both the advection–diffusion and dusty gas models were used to simulate the behavior of CO2 and noble gases under the experimental conditions, and the simulations were compared with the measured depth‐dependent concentration profiles of the gases. Given the relatively high permeability of the sand column (5 × 10−11 m2), Knudsen diffusion terms were small, and both the dusty gas model and the advection–diffusion model accurately predicted the concentration profiles of the CO2 and atmospheric noble gases across a range of CO2 flux from ∼700 to 10,000 g m−2 d−1. The agreement between predicted and measured gas concentrations demonstrated that, when applied to natural systems, the multi‐component capability provided by the noble gases can be exploited to constrain component and total gas fluxes of non‐conserved (CO2) and conserved (noble gas) species or attributes of the soil column relevant to gas transport, such as porosity, tortuosity, and gas saturation.

Indigenous nitrogen in the Moon: Constraints from coupled nitrogen–noble gas analyses of mare basalts

Nitrogen and noble gas (Ne–Ar) abundances and isotope ratios, determined by step-wise CO2 laser-extraction, static-mass spectrometry analysis, are reported for bulk fragments and mineral separates of ten lunar mare basalts (10020, 10057, 12008, 14053, 15555, 70255, 71557, 71576, 74255, 74275), one highland breccia (14321), and one ferroan anorthosite (15414). The mare basalt sub-samples 10057,183 and 71576,12 contain a large amount of solar noble gases, whereas neon and argon in all other samples are purely cosmogenic, as shown by their 21Ne/22Ne ratios of ≈0.85 and 36Ar/38Ar ratios of ≈0.65. The solar-gas-free basalts contain a two-component mixture of cosmogenic 15N and indigenous nitrogen (<0.5 ppm). Mare basalt 74255 and the olivine fraction of 15555,876 record the smallest proportion of 15Ncosm; therefore, their δ15N values of −0.2 to +26.7‰ (observed at the low-temperature steps) are thought to well represent the isotopic composition of indigenous lunar nitrogen. However, δ15N values ≤−30‰ are found in several basalts, overlapping with the isotopic signature of Earth's primordial mantle or an enstatite chondrite-like impactor. While the lowest δ15N values allow for nitrogen trapped in the Moon's interior to be inherited from the proto-Earth and/or the impactor, the more 15N-enriched compositions require that carbonaceous chondrites provided nitrogen to the lunar magma ocean prior to the solidification of the crust. Since nitrogen can efficiently be incorporated into mafic minerals (olivine, pyroxene) under oxygen fugacities close to or below the iron-wustite buffer (Li et al., 2013), the mare basalt source region is likely characterized by a high nitrogen storage capacity. In contrast, anorthosite 15414 shows no traces of indigenous nitrogen, suggesting that nitrogen was not efficiently incorporated into the lunar crust during magma ocean differentiation.