Ar Isotopic Ratios Research Articles

A unified method for measuring noble gas isotope ratios in air, water, and volcanic gases via dynamic mass spectrometry

We describe a new method for the purification, transfer, and analysis of heavy noble gas isotope ratios in water, air, or volcanic gas samples using a dynamic dual-inlet isotope-ratio mass spectrometer (IRMS). IRMS noble gas analysis offers the potential for orders-of-magnitude higher precision than traditional (static) noble gas mass spectrometry. However, due to both inherent IRMS requirements and challenges associated with sample collection and purification, measuring noble gases via IRMS has typically been limited to samples with an air-like elemental composition. Our new method allows for highly accurate and precise measurements of Ar, Kr, and Xe isotope ratios via IRMS, independent of sample composition, thereby opening the door to air, water, or volcanic gas analyses all made with the same technique. This approach exploits the fact that purified heavy noble gas samples are dominated by 40Ar, such that matching 40Ar ion beams between sample and reference gas streams results in pressure balancing. We further introduce new techniques for precisely quantifying and correcting for (i) Kr and Xe isotope non-linearity and matrix effects, (ii) interference of the low energy 40Ar tail on 36Ar and 38Ar ion beams, and (iii) the impact of short-term background fluctuations on trace Xe isotope measurements. We also present results from adsorption-desorption experiments with multiple adsorbent materials across a range of temperatures. For adsorption on silica gel, we find small equilibrium isotopic fractionation of Kr (εeq = −0.37 ± 0.04‰ amu−1, ±2 SE) and Xe (εeq = −0.06 ± 0.02‰ amu−1, ±2 SE) that may be relevant to studies of terrestrial Xe isotope signatures. We demonstrate that adsorption and desorption of heavy noble gases on silica gel at 77 K and 303 K, respectively, is highly reproducible and thus constitutes a viable and efficient method for transferring gases without reliance on cryostats or liquid helium.

Data analysis and isotopic ratios measured onboard the Spacecraft Atmosphere Monitor

The Spacecraft Atmosphere Monitor (S.A.M.) is a scientific payload, which includes a Paul quadrupole ion trap (QIT) mass spectrometer and has been continuously operated onboard the International Space Station (ISS) between August 2019 and July 2021. S.A.M. measures the chemical composition of ISS cabin air and reports on major constituents (N2, O2, Ar, CO2, and CH4) once every 2 s. Each second, S.A.M. measures one hundred individual mass spectra stored in 4800 1μs-long channels. This report presents the autonomous onboard data analysis software that calibrates the mass spectrum, removes the residual gas background signal, identifies species of interest, and reports relevant abundances. In addition, we describe a ground-based analysis of downloaded S.A.M.'s data to extract the H, C, N, O, and Ar isotope ratios. We also describe experiments, requested by the ISS Vehicle office, to ascertain if benzene was present in the ISS cabin air (it was not). We demonstrate that S.A.M. instrument measures isotope ratios with precision and accuracy comparable to the state-of-the-art laboratory-size magnetic sector mass spectrometers.

Partial melting and subduction-related metasomatism recorded by geochemical and isotope (He-Ne-Ar-Sr-Nd) compositions of spinel lherzolite xenoliths from Coyhaique, Chilean Patagonia

Spinel lherzolite xenoliths from Coyhaique, Chilean Patagonia, are located ~320 km from the Chile Trench and, therefore, their geochemical and isotopic compositions may record slab-derived metasomatism from current and/or earlier subduction zones. We present new mineral and whole-rock major and trace elements that are discussed together with noble gas data (He, Ne, Ar) and Sr-Nd isotopic ratios. Partial melting is recorded by negative correlations between basaltic elements vs. MgO in whole-rock and minerals. In general, silicate minerals show depleted light-rare earth element (LREE) over heavy-REE (HREE) patterns. Whole-rock compositions and clinopyroxene record variable degrees of partial melting (1–15%), which is corroborated by the Sr-Nd depleted character of the lherzolites. Cryptic metasomatism is evidenced by: 1) enriched LREE patterns in whole-rock and type-2 clinopyroxenes; 2) negative anomalies of Nb-Ta-Ti in whole-rock samples coupled with enrichment of large ion lithophile elements and chalcophile elements over high field strength elements and HREE; 3) positive correlations of Pb and U vs. La and negative correlations of Nb/Nb* and Ti/Ti* vs. La in clinopyroxene, and 4) positive anomalies of Li in all silicate minerals. Geochemical differences between whole-rocks and their mineral constituents indicate selective enrichment of the whole-rock samples due to grain-boundary components. 3He/4He ratios reported here are the first strongly radiogenic values found in the Patagonian subcontinental lithospheric mantle (SCLM; 0.20–2.52 RA). Most Ne isotopic ratios are indistinguishable from air composition with few samples showing a nucleogenic component, whereas Ar isotopic ratios are extensively affected by an atmospheric component. The correlation between 3He/4He and 4He/40Ar* corroborates the metasomatism recorded by lithophile and chalcophile elements. Therefore, we identify a heterogeneous SCLM affected by low but variable degrees of partial melting and subsequent enrichment by melts or fluids from recycled subduction-related materials in the spinel stability field (1.06–1.90 GPa and 886–1150 °C).

Granite is an Effective Helium Source Rock: Insights from the Helium Generation and Release Characteristics in Granites from the North Qinling Orogen, China

AbstractGlobal helium (He) shortage is a challenging problem; however, the types of helium source rock and the mechanisms of He generation and release therein remain still poorly understood. In this study, in order to evaluate the potential of granite as an effective helium source rock, we collected granitic samples from the North Qinling Orogen, Central China, in the south of the helium‐rich Weihe Basin. The helium generation and release behaviors in granite were studied through analysis of U and Th concentrations, EMPA images, and He and Ar concentrations and isotopic ratios extracted by crushing and stepwise heating. The results indicate that Ar has a better retention and a lower mobility than He. 3He/4He ratios released by crushing and stepwise heating are 0.016–0.056 RA and 0.003–0.572 RA, respectively, where RA is the atmospheric 3He/4He of 1.4×10‐6, reflecting a crustal and radiogenic source. Helium concentrations extracted by the two ways are 0.13–0.95 ucm3 STP/g and 7.82–115.62 ucm3 STP/g, respectively, suggesting that matrix‐sited He accounts for more than 98% of total helium preserved in granite. In addition, the total generated He amounts in granites are calculated based on the measured U and Th concentrations in granitic samples. Dividing the preserved He quantities by the generated He amounts, it turned out that less than 10% of He produced since the formation of the granite is preserved in the rock over geological time, suggesting that more than 90% generated He can be transferred to the Weihe Basin. Temperature and fracture are the two critical factors controlling He release. Based on the relationship between He diffusivity of granites and temperature and the He closure temperatures of a variety of U‐ and Th‐rich minerals (27–250°C), we estimate that He can be partially released out of granite at the depths <400 m and totally released at the depths >7800 m. Fractures provide effective transfer of free He from deep source rocks to shallow reservoirs. Finally, a model on granite as an effective helium source rock is established. We suggest exploring He resources in hydrocarbon basins with granitic basement (or adjacent to granite bodies), high geothermal field, and young active fractures.

Constraining the early evolution of Venus and Earth through atmospheric Ar, Ne isotope and bulk K/U ratios

The atmospheric noble gas isotope and elemental bulk ratios on Venus and Earth provide important information on their origin and evolution. If the protoplanets grew to a certain mass (i.e. > 0.5 MEarth), they could have captured H2-dominated primordial atmospheres by accreting gas from the circumstellar disk during the formation of the Solar System, which were then quickly lost by hydrodynamic escape after the disk dissipated. In such a case, the EUV-driven hydrodynamic flow of H atoms dragged heavier elements with it at different rates, leading to changes in their initial isotope ratios. For reproducing Earth and Venus present atmospheric 36Ar/38Ar, 20Ne/22Ne, 36Ar/22Ne, isotope and bulk K/U ratios we applied hydrodynamic upper atmosphere escape and Smooth Particle Hydrodynamics (SPH) impact models for the loss calculations of captured H2-dominated primordial atmospheres for various protoplanetary masses. We investigated a wide range of possible EUV evolution tracks of the young Sun and initial atmospheric compositions based on mixtures of captured nebula gas, outgassed and delivered material from ureilite, enstatite and carbonaceaous chondrites. Depending on the disk lifetime of ≈ 3–5 Myr (Bollard et al., 2017; Wang et al., 2017) and the composition of accreted material after disk dispersal, we find from the reproduction of the present atmospheric Ar, Ne, and bulk K/U ratios, that early Earth's evolution can be explained if proto-Earth had accreted masses between ≈ 0.53 − 0.58 MEarth by the time the nebula gas dissipated. If proto-Earth would have accreted a higher mass during the disk lifetime the present atmospheric Ar and Ne isotope ratios can not be reproduced with our model approach. For masses > 0.75 MEarth, Earth would have had a problem to get get rid of its primordial atmosphere. If proto-Earth accreted ≈ 0.53 − 0.58MEarth of enstatite-dominated material as suggested by Dauphas (2017) during the disk lifetime, it would have captured a tiny primordial atmosphere that was lost ≈ 3 Myr after the disk dissipated. In such a case we find that the present-day atmospheric Ar and Ne isotope ratios can be best reproduced if the post-nebula impactors contained ≈ 5% weakly depleted carbonaceous chondritic material and ≈ 95% enstatite chondrites that are strongly depleted in Ar, Ne and moderately volatile elements like potassium. If higher amounts of carbonaceous chondrites were involved in early Earth's accretion as recently suggested by Schiller et al. (2018), then the Earth's present atmospheric Ar and Ne ratios can only be reproduced if the involved carbonaceous chondritic post-nebula material was also highly depleted in these noble gases and/or had to be partially be delivered as long as the primordial atmosphere was yet escaping. As long as primordial atmospheres surround the growing protoplanets the abundance of their volatile elements is overwritten by their respective captured solar-like atmospheric abundances. Therefore the initial composition of the protoplanets at the disk dispersal time can not be identified by our method. For masses < 0.5 MEarth atmospheric escape cannot explain the present-day ratios, i.e. if Earth grew slower then these ratios have to be explained differently (Marty, 2012). If proto-Venus captured a primordial atmosphere it should have grown to masses of ≈ 0.85− 1.0 MVenus during the time until the disk dissipated and if early Venus accreted its main mass during the disk lifetime than the present atmospheric Ar and Ne isotope ratios and the observed K/U ratios on Venus surface can also be reproduced by the escape of a captured primordial atmosphere that is lost within ≤ 100 Myr, if the Sun was born as a weakly to moderately active young G star. New precise re-measurements of atmospheric noble gases are necessary by future Venus missions to better constrain the material that was involved in the planet's accretion history and possibly also the EUV activity evolution of the young Sun. In addition, measurements of other moderately volatile element and isotope ratios on the surface such as Rb/U, 64Zn/66Zn, and 39K/41K can give an insight on whether Venus accreted slow or fast, i.e. almost to its final mass within the disk lifetime.

Heavy Noble Gas Isotopes as New Constraints on the Ventilation of the Deep Ocean

AbstractPast studies of noble gas concentrations in the deep ocean have revealed widespread, several percent undersaturation of Ar, Kr, and Xe. However, the physical explanation for these disequilibria remains unclear. To gain insight into undersaturation set by deep‐water formation, we measured heavy noble gas isotope and elemental ratios from the deep North Pacific using a new analytical technique. To our knowledge, these are the first high‐precision seawater profiles of 38Ar/36Ar and Kr and Xe isotope ratios. To interpret isotopic disequilibria, we carried out a suite of laboratory experiments to measure solubility fractionation factors in seawater. In the deep North Pacific, we find undersaturation of heavy‐to‐light Ar and Kr isotope ratios, suggesting an important role for rapid cooling‐driven, diffusive air‐to‐sea gas transport in setting the deep‐ocean undersaturation of heavy noble gases. These isotope ratios represent promising new constraints for quantifying physical air‐sea gas exchange processes, complementing noble gas concentration measurements.

Precise determination of Ar, Kr and Xe isotopic fractionation due to diffusion and dissolution in fresh water

Dissolved noble gases are ideal conservative tracers of physical processes in the Earth system due to their chemical and biological inertness. Although bulk concentrations of dissolved Ar, Kr, and Xe are commonly measured to constrain physical models of atmosphere, ocean, and terrestrial hydrosphere processes, stable isotope ratios of these gases (e.g. 136Xe/129Xe) are seldom used because of low signal-to-noise ratios. Here we present the first results from a new method of dissolved gas sampling, extraction and analysis that permits measurement of stable Ar, Kr, and Xe isotope ratios at or below ∼5 per meg amu−1 precision (1σ), two orders-of-magnitude below conventional Kr and Xe isotopic measurements. This gain in precision was achieved by quantitative extraction and subsequent purification of dissolved noble gases from 2-L water samples via helium sparging and viscous dual-inlet isotope ratio mass spectrometry. We have determined the solubility fractionation factors (αsol) for stable Ar, Kr, and Xe isotope ratios between ∼2 and 20°C via laboratory equilibration experiments. We have also conducted temperature-controlled air-water gas exchange experiments to estimate the kinetic fractionation factors (αkin) of these isotope ratios. We find that both αsol and αkin, normalized by isotopic mass difference (Δm), decrease in magnitude with atomic number but are proportional to Δm for isotope ratios of the same element. With the new ability for high precision isotopic measurements, we suggest that dissolved Kr and Xe isotope ratios in groundwater represent a promising, novel geochemical tool with important applications for groundwater modeling, water resource management, and paleoclimate.

Genetic implications for the Damajianshan W‐Cu‐As polymetallic deposit in Lvchun, Southwest China: Constraints from H–O, He–Ar, S, and Pb isotopes

The Sanjiang Tethyan domain in SE Asia is one of the most important mineral belts in China. The Damajianshan (DMJS) W‐Cu‐As polymetallic deposit is located in the southern part of Sanjiang Tethyan domain, related to Triassic quartz porphyry. Detailed exploration thereafter shows that the reserves of W, Cu, and As are 0.09, 0.42, and 0.12 Mt, respectively. The W is a typical oxyphile element and always coexisted with Sn–Li–Be–Nb–Ta. The Cu–As are typical sulphophile elements and always coexisted with Au–Ag–Pb–Zn–Sb–Hg. It is rarely reported that a deposit preserves tungsten and copper–arsenic simultaneously in the southern Sanjiang region. In this study, the S–Pb isotopic ratios of sulphides, the H–O isotopic compositions of fluid inclusions in hydrothermal quartz, and the He–Ar isotopic ratios of fluid inclusions in sulphides were analysed to constrain the origin of the DMJS deposit. Studies have shown that the δ34S values of sulphides from ores are mainly between −6.17‰ and +0.02‰; the sulphur isotope compositions indicate that the ore‐forming materials might originate from deep sources. The Pb isotope characteristics indicate that the ore metals might originate from deep sources and be closely related to the quartz porphyry. The δ18Ofluid values of ore‐forming fluids calculated from hydrothermal quartz range from −5.5‰ to +7.5‰, and the δD values of the fluid inclusions in quartz are from −81.7‰ to −50.2‰. The H–O isotope systematics indicate that the ore fluids in the DMJS deposit were probably initially sourced from magmatic water and later gradually mixed with Mesozoic meteoric water. Fluid inclusions in pyrite, chalcopyrite, arsenopyrite, and pyrrhotite possess 3He/4He ratios of 0.02–0.05 Ra, and their 40Ar/36Ar ratios range from 562.47 to 4159.15, indicating a complete crustal fluids and with higher radiogenic 40Ar. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore‐forming fluids have a deep source. The unique geologic structure of the Sanjiang Tethys tectonic metallogenic region and the evolution of ore‐forming fluids both promoted the formation of this rare DMJS deposit.

Argon isotopes as tracers for martian atmospheric loss

Recent measurements of the present-day Ar abundance and isotopic ratios in the martian atmosphere by the SAM instrument suite onboard the Curiosity rover can be used to constrain the atmospheric and volatile evolution. We have examined the role of volcanic outgassing, escape to space via sputtering, crustal erosion, impact delivery, and impact erosion in reproducing the Ar isotope ratios from an initial state 4.4 billion years ago. To investigate the effects of each of these processes, their timing, and their intensity we have modeled exchanges of Ar isotopes between various reservoirs (mantle, crust, atmosphere, etc.) throughout Mars’ history. Furthermore, we use present-day atmospheric measurements to determine the parameter space consistent with observations. We find that significant loss to space (at least 48% of atmospheric 36Ar) is required to match the observed 36Ar/38Ar ratio. Our estimates of volcanic outgassing do not supply sufficient 40Ar to the atmosphere to match observations, so in our model at least 31% of 40Ar produced in the crust must have also been released to the atmosphere. Of the total 40Ar introduced into the atmosphere about 25% must have been lost to space. By adding the present-day isotopic abundances with our results of total integrated Ar loss we find a “restored” value of atmospheric 40Ar/36Ar, which represents what that ratio would be if the total integrated Ar loss had remained in the atmosphere. We determine the restored value to be ∼900–1500. This is below the present martian atmospheric value (1900 ± 300), but 3–5 times greater than the terrestrial value.

Postcollisional mafic igneous rocks record recycling of noble gases by deep subduction of the continental crust

Recycling of noble gases from crustal rocks into the mantle is indicated not only by oceanic basalts and mantle xenoliths, but also by ultrahigh-pressure metamorphic rocks in collisional orogens. It is intriguing whether noble gases in continental crust were recycled into the mantle by deep subduction of the continental crust to mantle depths. Here we firstly report the He, Ne and Ar isotopic compositions of pyroxene from postcollisional mafic igneous rocks in the Dabie orogen, China. The results show that the pyroxene separates from the mafic rocks have low 3He/4He ratios of 0.002 to 1.8Ra and air-like Ne isotope compositions. Furthermore, the pyroxene exhibits low 40Ar/36Ar ratios of 393.6 to 1599.8, close to those of the air. In combination with whole-rock geochemistry it is found that pyroxene 3He/4He ratios are correlated with whole-rock (La/Yb)N and Sr/Y ratios, εNd(t) values and MgO contents. These observations demonstrate the mass transfer from the deeply subducted continental crust to the overlying mantle wedge, recording the source mixing between the crust-derived melt and the mantle peridotite in the continental subduction zone. A direct addition of the crustal He via crust-derived melt to the mantle leads to the extremely low 3He/4He ratios in the orogenic lithospheric mantle, and the dissolved atmospheric Ar and Ne in the subducted supracrustal rocks results in the air-like Ar and Ne isotope ratios. Therefore, the noble gas isotopic signatures of supracrustal rocks were carried into the mantle by the continental deep subduction to subarc depths and then transferred to the postcollisional mafic igneous rocks via the melt–peridotite reaction at the slab–mantle interface in a continental subduction channel. Our finding firstly establishes the slab–mantle interaction model for recycling of supracrustal noble gases in the continental subduction zone.

Tracing injected CO2 in the Cranfield enhanced oil recovery field (MS, USA) using He, Ne and Ar isotopes

The He, Ne and Ar isotopic composition of gases collected in 2009 and 2012 from 13 production wells, injection wells and the CO2 supply pipeline at the Cranfield CO2-enhanced oil recovery field (MS, USA) have been measured in order to determine the extent to which they trace the fate of injected CO2 in the reservoir. In the absence of samples of CO2 pre-injection reservoir gas we use the Ne isotope composition of the production and injection well gases to determine the isotopic composition of the natural gas. The noble gas isotopes display binary mixing trends between the injected CO2 and a CH4-rich natural gas that is characterised by radiogenic He, Ne and Ar isotope ratios. 3He/4He and 40Ar*/4He ratios (where 40Ar* represents the non-atmospheric 40Ar) display coherent relationships with CO2 concentrations that can be used to trace and quantify the injected CO2 in an engineered site over a sustained period of injection. The presence of a small amount of air-derived Ar, from a non-atmospheric source, in many gas samples rules out using 40Ar/36Ar to track the injected CO2. The noble gases identify the loss of a significant proportion of the CO2 from the gas phase sampled by five production wells in 2009. Using 3He/4He and 40Ar*/4He ratios to reconstruct the major gas composition, it appears that between 22% and 96% of the CO2 has been lost in individual wells. This study demonstrates that the naturally occurring noble gases have the potential to trace the fate and quantify the sequestration of CO2 at injection sites.

Reconstructing the evolution of Lake Bonney, Antarctica using dissolved noble gases

Lake Bonney (LB), located in Taylor valley, Antarctica, is a perennially ice-covered lake with two lobes, West Lake Bonney (WLB) and East Lake Bonney (ELB), which are separated by a narrow ridge. Numerous studies have attempted to reconstruct the evolution of LB because of its sensitivity to climatic variations and the lack of reliable millennial-scale continental records of climate in this region of Antarctica. However, these studies are limited by the availability of accurate lacustrine chronologies. Here, we attempt to better constrain the chronology of LB and thus, the evolution of past regional climate by estimating water residence times based on He, Ne and Ar concentrations and isotopic ratios in both WLB and ELB.3He and 4He excesses up to two and three orders of magnitude and 35–150 times the atmospheric values are observed for WLB and ELB samples, respectively. In comparison, while measured 40Ar/36Ar ratios are atmospheric (∼295.5) in ELB, WLB samples display 40Ar/36Ar ratios of up to ∼315 reflecting addition of radiogenic 40Ar. Both 4He and 40Ar excesses clearly identify the addition of subglacial discharge (SGD) from underneath Taylor Glacier into WLB at depths of 25m and 35m. He isotopic ratios suggest that He excesses are predominantly crustal (>93%) in origin with small mantle contributions (<7%). These crustal 4He and 40Ar excesses are used together with basement rock production rates of these isotopes to derive first-order approximations of water residence times for both lobes. Numerous factors capable of affecting water residence times are evaluated and corrected 4He and 40Ar water ages are used to place further constrains into the reconstruction of both WLB and ELB history. Combined 4He and 40Ar ages in WLB suggest maximum water residence times of ∼250kyrs BP. These results support the presence of remnant water from proglacial lakes that existed during Marine Isotope Stage 7 (160–240kyrs) in WLB, in agreement with previous studies. In comparison, 4He ages in ELB are much younger (<27kyrs BP) and display a complex evolutionary history that is very different from WLB. 4He ages also suggest that the ELB ice cover formed significantly earlier (∼1.5kyrs BP) than previously reported. The timing of these hydrologic changes in ELB appears to correspond to regional and global climatic events that are recorded in both the Taylor Dome ice-core record as well as in other Dry Valley Lakes.

C, N, He, and Ar isotope and element ratios in fluid inclusions from MORB chilled glasses: Stepwise crushing data

C, N, He, and Ar isotope and element ratios in fluid inclusions from MORB chilled glasses: Stepwise crushing data

He, Ar, C and N isotopes in thermal springs of the Chukotka Peninsula: Geochemical evidence of the recent rifting in the north-eastern Asia

Thermal springs with water temperatures up to 97°C were sampled in the Chukotka Peninsula of the north-eastern Asia on the area of ~60,000km2. He and Ar isotope ratios in bubbling gases were measured in 30 samples of from 23 spring groups. 3Не/4Не=R values vary in the range of (20.2÷172)×10−8=(0.14÷1.23)Ratm. The over-atmospheric 40Ar/36Ar ratio values were found out in 18 groups. The compositions of gas phase were determined in 26 groups. In 16 of them N2 content exceeds 90vol.%. at δ15N values=(0.0÷+4.5) ‰. CO2 contents vary from <1 to 95vol.% and δ13CCO2 values determined in 19 samples vary from −20.4 to −4.4‰. In several samples δ13C values were also measured in methane and ethane.Geochemical specificity of gases was revealed in Kolyuchin–Mechigmen Zone (KMZ) of the Peninsula. The zone coincides with the trough of NW–SE strike that dissected the Precambrian basement of this part of the Chukotka–Seward microcontinent during the Triassic phase of tectonic activation. The latest re-activation of KMZ resulted in its subsidence during Quaternary, seismic activity and basic volcanism. The KMZ gases contain maximal contribution of light (mantle-derived) 3He and carbon dioxide whereas this CO2, and associated non-atmospheric N2 and Ar are all enriched by heavy isotopes (13C, 40Ar, and 15N, respectively). The waters of these springs are characterized by the highest reservoir temperatures estimated by solute geothermometers. These features are related to upwelling of mantle-derived melt supplying into the crust 3He-enriched helium and thermal energy stimulating rock metamorphism.

Helium and Argon Isotopic Studies of Fossil Material and the Theoretical Evolution of He and Ar in Earth’s Atmosphere through Time

We analyzed the elemental concentrations and the isotopic compositions of helium and argon in Cambrian to Jurrassic aged Gastropod, Ammonite and Trilobite fossils in order to examine variation in these gases through time. Fossil samples yielded He and Ar isotopic ratios close to the present day atmospheric values, but also indicated some addition of a radiogenic component. We compared the results to theoretical values calculated from a mathematical model of Earth’s atmosphere assuming mantle degassing. Results from our mathmatical models showed that the 40Ar/36Ar ratio of Earth’s atmosphere increased rapidly after the formation of the Earth, but has been almost identical to the present day value for the last 1 Ga. For atmospheric helium, model results were consistent with present day atmospheric values, assuming complete helium degassing from the continental crust into the atmosphere. The model suggests that the atmospheric 3He/4He ratio has remained relatively constant for the last 0.1 Ga. Given the similarity between present day and ancient He and Ar isotopic ratios, we conclude that the corresponding ratios measured in ancient fossil material may partially reflect composition of the ancient atmosphere and are not necessarily due to contamination by the present day atmosphere.

New high-precision measurements of the isotopic composition of atmospheric argon

Earth’s atmosphere is used as a standard reference gas for mass spectrometric determinations of argon (Ar) isotopes used principally in geochronology. There are three published independent determinations of the Ar isotope composition of modern atmosphere that differ subtly. We have made new high-precision measurements of Ar isotope ratios of five different sources of air using a high-sensitivity multi-collector noble gas mass spectrometer in order to distinguish between them. The isotope ratios, corrected only for backgrounds, reside on a inverse square-root mass law fractionation line that passes through the air value proposed by Lee et al. [Lee J. Y., Marti K., Severinghaus J. P., Kawamura K., Yoo H. S., Lee J. B. and Kim J. S. (2006) A redetermination of the isotopic abundances of atmospheric Ar. Geochim. Cosmochim. Acta 70, 4507–4512]. They are distinct from both the other proposed compositions and provide the first independent confirmation of the atmospheric Ar isotope composition. We suggest that the revised values should now be in routine employment.

Noble gases in Lakes Nyos and Monoun, Cameroon

He, Ne and Ar isotopic ratios and concentrations of noble gases in different depths of Lakes Nyos and Monoun in Cameroon, West Africa, were investigated. Samples were collected in November 1999, January 2001 and December 2001. Improved sampling method applied in 2001 enabled us to detect detailed isotopic distribution in the lakes. Concentrations of Ne and Xe in the lake waters were low by factors of ~10 and ~3, respectively, compared with those of air saturated water (ASW), whereas in CO 2 gas they were depleted by 2 orders of magnitude compared with those in air. In contrast, He was strongly enriched in lake water by more than three orders of magnitude compared to ASW. 3 He/ 4 He ratios of 8 × 10 ‐6 (Lake Nyos) and 5 × 10 ‐6 (Lake Monoun) indicated recharge of magmatic gases of mantle origin to the lakes, though the ratios were lowered by the addition of radiogenic 4 He from crustal rocks. A small contribution (<2%) of MORB-like Ne was detected in both lake waters. A simple mixing between two end members, i.e., atmosphere and gases from a deepseated reservoir, can explain the noble gas signatures. Depth profile of He concentration in Lake Nyos showed a clear layered structure with a maximum at 190 m, 20 m above the lake bottom. Below this depth, 3 He/ 4 He ratio decreased slightly, suggesting that the fluid from the deep-seated reservoir is supplied to the lake mainly at ca. 190 m. C/ 3 He ratios were 1.5 × 10 10 and 0.6 × 10 10 in deep and shallow layers of Lake Nyos, respectively. The C/ 3 He ratio was 1.7 × 10 10 at

Atmospheric 38Ar/ 36Ar in the mantle: Implications for the nature of the terrestrial parent bodies

Analyses of argon in basaltic glasses from mid-oceanic ridges and oceanic islands show that the 38Ar/ 36Ar isotopic ratio in the mantle does not differ from the atmospheric value and is different from the solar value. This contrasts with neon isotope systematics that shows solar-like, non-atmospheric, values in mantle-derived rocks. The air-like argon isotopic ratio for the Earth's mantle is not due to air contamination since they are associated with high 20Ne/ 22Ne ratios (> 12). We present new He, Ne, and Ar data for a suite of glasses from ocean ridges and ocean islands, with the goal of improving the accuracy of the mantle 38Ar/ 36Ar determination. The 20Ne/ 22Ne– 40Ar/ 36Ar correlation observed in these samples allows us to give a well-constrained mantle 38Ar/ 36Ar ratio, which is clearly air-like. The Ne–Ar systematics in the mantle suggest that the Ne and Ar isotopic compositions may have been probably acquired by implantation of solar wind in the Earth's parent bodies rather than by solubilization in a magma ocean from the solar nebula or a primordial atmosphere. Indeed, a scenario of solar wind implantation with mass fractionation (as shown for Genesis targets) coupled with sputtering, is able to produce the Ne and Ar isotopic ratios observed in the Earth's mantle and in gas-rich meteorites. However, this scenario fails to explain the Kr and Xe isotopic signatures and a different origin for the Kr and Xe is required.

A consistent picture of early hydrodynamic escape of Venus atmosphere explaining present Ne and Ar isotopic ratios and low oxygen atmospheric content

A time dependent model of hydrogen hydrodynamic escape powered by solar EUV flux and solar wind, and accounting for oxygen frictional escape, has been implemented in primitive Venus conditions. The model is constrained by the present 20Ne/ 22Ne and 36Ar/ 38Ar ratios in Venus atmosphere. It suggests that the net total amount of water delivered to the planet during accretion (≈ 10–100 Myr) is not in excess of the content of ≈ 5 Terrestrial Oceans (5 TO). In our preferred scenario, 60% of the oxygen (3 TO) is left behind the hydrogen during the first 100 Myr. From a comparison with Earth's case, we suggest that hydrodynamic escape has dried up Venus atmosphere early in its history (≈ 70 Myr), triggering the crystallization of the magma ocean, and leaving no available water in the atmosphere to condense out and form an Earth-size water ocean. On the contrary, Earth, possibly endowed with more water, and subject to a weaker hydrodynamic escape, would have remained wet after the crystallization of its magma ocean. We suggest that the oxygen left behind the escaping hydrogen during the main hydrodynamic phase on Venus has been dissolved in the magma ocean, and lost through oxidation. In the proposed scenario, the dense Venus CO 2 atmosphere doesn't result from an initial episode of runaway (or moist) greenhouse, but has been formed during the crystallization of the magma ocean, by progressive exsolution of carbon dioxide, at a time when the atmospheric partial pressure of water was of a few hundred bar. In the subsequent period, from ≈ 100 to ≈ 500 Myr, the hydrogen of the water delivered by comets may have been removed by continuing thermal escape, resulting at 500 Myr in a water global equivalent layer (GEL) of a few meters depth (or less), probably under the form of water vapor in the atmosphere, and a molecular oxygen atmosphere of ≈ 10 bar or so. At later times, pick-up ion escape may have removed most of the remaining water, and led to the present D/ H atmospheric enhancement factor of 150. The ≈ 10 bar of oxygen may have been absorbed by crustal oxidation.

Solar and Solar-Wind Composition Results from the Genesis Mission

The Genesis mission returned samples of solar wind to Earth in September 2004 for ground-based analyses of solar-wind composition, particularly for isotope ratios. Substrates, consisting mostly of high-purity semiconductor materials, were exposed to the solar wind at L1 from December 2001 to April 2004. In addition to a bulk sample of the solar wind, separate samples of coronal hole (CH), interstream (IS), and coronal mass ejection material were obtained. Although many substrates were broken upon landing due to the failure to deploy the parachute, a number of results have been obtained, and most of the primary science objectives will likely be met. These objectives include He, Ne, Ar, Kr, and Xe isotope ratios in the bulk solar wind and in different solar-wind regimes, and 15N/14N and 18O/17O/16O to high precision. The greatest successes to date have been with the noble gases. Light noble gases from bulk solar wind and separate solar-wind regime samples have now been analyzed. Helium results show clear evidence of isotopic fractionation between CH and IS samples, consistent with simplistic Coulomb drag theory predictions of fractionation between the photosphere and different solar-wind regimes, though fractionation by wave heating is also a possible explanation. Neon results from closed system stepped etching of bulk metallic glass have revealed the nature of isotopic fractionation as a function of depth, which in lunar samples have for years deceptively suggested the presence of an additional, energetic component in solar wind trapped in lunar grains and meteorites. Isotope ratios of the heavy noble gases, nitrogen, and oxygen are in the process of being measured.