Atmospheric Noble Gases Research Articles

A whole ocean thermometer from atmospheric noble gas ratios

A whole ocean thermometer from atmospheric noble gas ratios

Ancient atmospheric noble gases preserved in post-impact hydrothermal minerals of the 200 Ma-old Rochechouart impact structure, France

The evolution of Earth's atmosphere over billions of years provides important constraints on the geological history of our planet. However, major challenges in studying this have been finding samples in which ancient atmospheric signals are reliably preserved and precisely determining the age of the trapped atmosphere. Hydrothermal minerals can preserve fluid inclusions that formed from air-saturated waters. However, the use of hydrothermal minerals as paleo-atmospheric proxies is often limited by contamination by deeply-sourced crustal gases (e.g., radiogenic noble gases) and the difficulty of providing precise age constraints on the crystallization of the hydrothermal minerals.In this study, we tested the hypothesis that meteorite impact craters could provide an untapped archive of atmospheric evolution by investigating noble gases (Ne, Ar, Kr and Xe) contained in fluid inclusions within post-impact hydrothermal minerals from the Rochechouart impact structure (France). Impact craters can be dated using a variety of methods and because meteorite impacts cause localized fracturing of country rocks and the development of minor thermal anomalies, they are often associated with short-lived (up to a few Ma, depending on the size of the crater) and shallow hydrothermal systems dominated by surface-derived hydrothermal fluids.We found that the elemental ratios of noble gases in quartz from Rochechouart are close to the elemental ratios of modern Air. Furthermore, most noble gas isotope ratios are close to modern atmosphere, except for rare deviations caused by radiogenic ingrowth, nucleogenic effects and/or minor mass fractionation. After a minor correction for post-entrapment decay of 40K, an upper limit for the 40Ar/36Ar ratio of the ∼200 Ma atmospheric component trapped in Rochechouart quartz is determined as 292.7 ± 3.6 (2σ s.d.), which is significantly lower than the modern value of 298.6 ± 0.3. A Mesozoic 40Ar/36Ar ratio lower than 292.7 ± 3.6 is in line with the existing paleo-atmospheric dataset and existing models of the temporal evolution of the atmospheric 40Ar/36Ar ratio and conclusively proves the preservation of an ancient atmospheric component in rocks from Rochechouart. Given that meteorite impacts have occurred throughout the entire Earth's history they might collectively provide a unique, yet unexplored, repository of the atmospheric evolution. The same approach might also be applicable to investigating the atmospheric evolution of other terrestrial planets.

A journey in Noble Gas Cosmochemistry and Geochemistry

I started my journey in science by studying noble gases implanted by the solar wind in dust grains on the surface of the Moon, and with many colleagues I have studied solar wind implanted noble gases in natural and artificial samples throughout my career, the latter exposed primarily by the Genesis space mission. Major questions are what noble gases in the solar wind can tell us about the present and the past Sun, and how they can contribute to understanding the formation and history of the planets and their building blocks, represented, for example, by meteorites. Since my early years as a postdoc, I have also been interested in noble gases (and radioactive nuclides) produced in meteorites and other extraterrestrial samples by interactions with energetic elementary particles from galactic cosmic radiation (and the Sun). These so called “cosmogenic” nuclides allow us to study the transport of meteorites to Earth, and the dynamics of the top surface layers (“regoliths”) on the Moon, asteroids, and comets. Cosmogenic noble gases are also crucial for studying even more exotic topics such as the history of tiny presolar grains that formed in the cooling envelopes of earlier generations of stars towards the end of their lives and were eventually incorporated into the meteoritic matter where they are found today. Cosmogenic noble gases in some tiny phases in meteorites are also likely tracers of our highly active Sun at a very early stage in its history. A few years later, I started my third major research topic in cosmochemistry, the study of primordial noble gases in meteorites and other extraterrestrial samples. These noble gases were incorporated into meteorites or their precursors in the early solar system or even in a presolar environment. I also participated in studies by colleagues of isotopic anomalies of other elements important in cosmochemistry, my expertise being mainly in aspects of the influence of cosmic rays on these elements. Although working in an Earth Science institution, it took quite a while before I started to also study noble gases (and radionuclides) in terrestrial samples. This is described in the second part of this contribution. A major focus was on cosmogenic noble gases and radionuclides produced in samples near the Earth’s surface. Although production rates of cosmogenic nuclides on Earth are several orders of magnitude lower than in space, making their analysis more challenging, they have become an important tool in geomorphology. Because stable noble gas nuclides are particularly well suited to the study of ancient landscapes, much of our work focused on areas with arid climates, such as Antarctica and the Andes in Chile, in collaboration with geoscience colleagues. We also participated in the large multinational CRONUS collaboration, funded by the European Union, a community effort to improve our knowledge of nuclide production rates at the Earth’s surface. In another major collaboration with external colleagues we are involved in noble gas analyses of water samples, ranging from lakes to aquifers to tiny inclusions in stalagmites. This research focuses on studying lake and groundwater dynamics, including contributions of mantle-derived noble gases such as in volcanic lakes. Atmospheric noble gases dissolved in suitable samples are also palaeotemperature indicators, supplementing information from other proxies such as oxygen isotopes.

The Effect of Past Saturation Changes on Noble Gas Reconstructions of Mean Ocean Temperature

AbstractThe ocean's immense ability to store and release heat on centennial to millennial time scales modulates the impacts of climate perturbations. To gain a better understanding of past variations in mean ocean temperature (MOT), a noble gas‐based proxy measured from ancient air in ice cores has been developed. Here we assess non‐temperature effects that may influence the atmospheric noble gas ratios reconstructed from polar ice and how they impact the temperature signal with an intermediate complexity Earth system model. We find that changes in wind speed, sea‐ice extent, and ocean circulation have partially compensating effects on mean‐ocean noble gas saturation, leading to a slight reduction of noble gas undersaturation at the Last Glacial Maximum (LGM). Taking these effects and ice core measurements into account, our model suggests a revised MOT difference between the LGM and pre‐industrial of −2.1 ± 0.7°C that is also in improved agreement with other independent temperature reconstructions.

Noble gases in aquitard provide insight into underlying subsurface stratigraphy and free gas formation

AbstractBiogeochemical gas production resulting in free gas phase formation can severely affect groundwater and solute transport in aquifers. Such gas–water interactions are important in aquifers affected by geogenic As, which are commonly associated with biogeochemical CH4 production. Additionally, the influence of aquitards on As concentrations in contaminated aquifers has recently been challenged. These observations prompted the analysis through a heterogeneous aquitard overlying a high CH4−gas‐producing zone of an As‐contaminated aquifer. A sediment core taken through the aquitard was analyzed for noble gases to assess how the aquitard physically contributes to the underlying gas production. Results reveal that the aquitard pore space is unsaturated in two separate layers resulting in hanging pore water constrained by an air‐like gas phase. This interlayering of unsaturated and saturated zones identifies the aquitard's stratigraphy as key in determining hydrostatic pressure—a main control of free gas formation (i.e., CH4) in the underlying aquifer. The partly unsaturated conditions reduce the hydrostatic pressure by 30% compared with fully saturated conditions. To our knowledge, this is the first study applying noble gases to examine the influence of an aquitards physical state on gas production in an underlying aquifer. Further, such partly unsaturated sediment layers of low conductivity might provide preferential pathways for periodic water flow, fostering aquitard–aquifer solute transport. Groundwater samples additionally collected throughout the study site confirm more widespread degassing than previously reported. Up to 90% of the expected atmospheric noble gas concentrations is lost from groundwater immediately below the investigated sediment core.

Mantle helium in pore fluids of deep-sea sediments around the Azores archipelago

We present noble gas concentrations and helium isotope ratios measured in the pore fluids of two deep-sea sediment cores taken at the Azores Plateau. NeXe concentrations were used to estimate the atmospheric noble gas component within the pore fluids and to then determine the terrigenic helium component. The concentrations of terrigenic 3He indicate the presence of hydrothermal fluids and a mantle He component within the sediment pore space. At both sampling locations, the concentration of terrigenic helium increases with sediment depth. The observed concentration gradients suggest a diffusive transport of He through the sediments. The helium isotope ratio of the source of the terrigenic helium is characterized by a typical mid-ocean ridge basalt (MORB). These results support previous studies suggesting that the pelagic sediments surrounding the Azores host active hydrothermal systems.Our data indicate that the transport of terrigenic helium takes place diffusively over larger areas of the ocean floor, and that the concentration of terrigenic helium in the pelagic sediments increases towards the Mid-Atlantic Ridge and Terceira Rift.The data further suggests that in the Azores region the terrigenic noble gas component is only detectable in the sediment pore fluids and cannot be observed in the overlying bottom water. This highlights the potential of measuring noble gas concentrations and isotope ratios in the sediment column to identify hydrothermal activity in the ocean.

Pre-subduction mantle noble gas elemental pattern reveals larger missing xenon in the deep interior compared to the atmosphere

Understanding volatile evolution on Earth provides critical information on the processes that shape the Earth, and hence the solar system. The noble gas elemental and isotopic composition of the Earth's mantle traces the sources of Earth's volatiles as well as evolution processes, such as mantle degassing and regassing via subduction. However, ubiquitous shallow-level atmospheric contamination of mantle-derived samples has hampered determining the deep mantle heavy noble gas (Kr, Xe) isotopic and elemental composition. Moreover, the present-day elemental composition of the mantle reflects mixing of initial volatiles with atmospheric noble gases recycled through subduction that makes understanding the elemental signatures acquired during accretion difficult. Using a recently-developed protocol, we previously measured the krypton and xenon isotopic and elemental compositions of the Galápagos and Iceland plume sources, which have among the most primitive helium and neon isotopic signatures, sampling one of the least degassed, most primordial mantle reservoirs. Based on these measurements, here we introduce a new approach to correct for recycling and hence determine the initial noble gas elemental ratios of the deep mantle. Our study leaves room for a substantial proportion of 36Ar in the mantle to be primordial in origin. Most importantly, we show that the deep mantle, prior to the injection of any atmospheric volatiles, was depleted in Xe relative to Kr by about two orders of magnitude when compared to chondritic compositions, and the Xe depletion was larger in the mantle than in the modern-day atmosphere. Unlike the atmosphere that has protracted history of Xe depletion through the Archean, the deep mantle xenon depletion was acquired very early in Earth's history, most likely during accretion. Hence, missing xenon in the Earth's interior and in the atmosphere appears to be two distinct problems. Three possible scenarios, potentially acting in sync, may have led to the mantle xenon depletion: Xe partitioning into the core, magma ocean outgassing and a xenon deficit in the Earth's parent bodies, such as comets.

87Sr/86Sr Ratios and Atmospheric Noble Gases in Theistareykir Geothermal Fluids: A Record of Glacial Water

The determination of the current and past recharge sources, as well as the reconstruction of the timing of the recharge in geothermal reservoirs, is required in order to correctly assess the resource potential of these systems. Theistareykir is a newly developed geothermal field close to the well-known exploited fields of Krafla and Námafjall in NE Iceland. In this study, the 87Sr/86Sr ratios measured in deep geothermal fluids are presented and, together with the Cl and noble gas signatures, are used to place constraints on the fluid sources. The Cl/Sr and 87Sr/86Sr ratios show a peculiar and unique composition among Icelandic geothermal fluids. The 87Sr/86Sr ratios range from 0.70355 to 0.70671, suggesting the presence of a significant seawater component—possibly marine aerosols added to rain or snowfall—as well as an additional source of Sr leached from local basalts. Moreover, a correlation between the atmospheric noble gas (ANGs) elemental ratios Ne/Ar, Kr/Ar and Xe/Ar, and the 87Sr/86Sr ratios is observed. The latter results from the mixing of meteoric water with Sr leached from local basalts, meteoric water containing unrelated Sr from seawater, and recharge water with ANGs derived from trapped air bubbles in snow. We suggest that the combined ANGs and Sr seawater signatures are representative of a glacial water source derived from the melting of compacting snow.

Deep-mantle krypton reveals Earth's early accretion of carbonaceous matter.

Establishing when, and from where, carbon, nitrogen and water were delivered to Earth is a fundamental objective in understanding the origin of habitable planets such as Earth. Yet, volatile delivery to Earth remains controversial1-5. Krypton isotopes provide insights on volatile delivery owing to their substantial isotopic variations among sources6-10, although pervasive atmospheric contamination has hampered analytical efforts. Here we present the full suite of krypton isotopes from the deep mantle of the Galápagos and Iceland plumes, which have the most primitive helium, neon and tungsten isotopic compositions11-16. Except for 86Kr, the krypton isotopic compositions are similar to a mixture of chondritic and atmospheric krypton. These results suggest early accretion of carbonaceous material by proto-Earth and rule out any combination of hydrodynamic loss with outgassing of the deep or shallow mantle to explain atmospheric noble gases. Unexpectedly, the deep-mantle sources have a deficit in the neutron-rich 86Kr relative to the average composition of carbonaceous meteorites, which suggests a nucleosynthetic anomaly. Although the relative depletion of neutron-rich isotopes on Earth compared with carbonaceous meteorites has been documented for a range of refractory elements1,17,18, our observations suggest such a depletion for a volatile element. This finding indicates that accretion of volatile and refractory elements occurred simultaneously, with krypton recording concomitant accretion of non-solar volatiles from more than one type of material, possibly including outer Solar System planetesimals.

Noble gas signatures constrain oil-field water as the carrier phase of hydrocarbons occurring in shallow aquifers in the San Joaquin Basin, USA

Noble gases record fluid interactions in multiphase subsurface environments through fractionation processes during fluid equilibration. Water in the presence of hydrocarbons at the subsurface acquires a distinct elemental signature due to the difference in solubility between these two fluids. We find the atmospheric noble gas signature in produced water is partially preserved after hydrocarbons production and water disposal to unlined ponds at the surface. This signature is distinct from meteoric water and can be used to trace oil-field water seepage into groundwater aquifers. We analyse groundwater (n = 30) and fluid disposal pond (n = 2) samples from areas overlying or adjacent to the Fruitvale, Lost Hills, and South Belridge Oil Fields in the San Joaquin Basin, California, USA. Methane (2.8 × 10−7 to 3 × 10−2 cm3 STP/cm3) was detected in 27 of 30 groundwater samples. Using atmospheric noble gas signatures, the presence of oil-field water was identified in 3 samples, which had equilibrated with thermogenic hydrocarbons in the reservoir. Two (of the three) samples also had a shallow microbial methane component, acquired when produced water was deposited in a disposal pond at the surface. An additional 6 samples contained benzene and toluene, indicative of interaction with oil-field water; however, the noble gas signatures of these samples are not anomalous. Based on low tritium and 14C contents (≤ 0.3 TU and 0.87–6.9 pcm, respectively), the source of oil-field water is likely deep, which could include both anthropogenic and natural processes. Incorporating noble gas analytical techniques into the groundwater monitoring programme allows us to 1) differentiate between thermogenic and microbial hydrocarbon gas sources in instances when methane isotope data are unavailable, 2) identify the carrier phase of oil-field constituents in the aquifer (gas, oil-field water, or a combination), and 3) differentiate between leakage from a surface source (disposal ponds) and from the hydrocarbon reservoir (either along natural or anthropogenic pathways such as faulty wells).

The dichotomy in noble gas signatures linked to tectonic deformation in Wufeng-Longmaxi Shale, Sichuan Basin

Geochemical homogeneity in shale is often assumed when tracing subsurface fluids and characterizing sedimentary basins. This study presents measurements of the bulk gas composition, stable isotopes, and noble gas volume fraction and isotopes for shale gas samples collected from gas wells in the Wufeng-Longmaxi Shale, the southern Sichuan Basin, China. The dryness [C1/(C2 + C3)] ranging from 166.3 to 251.2, combined with δ13C1 and δDC1 that vary from −28.8 to −27.3‰ and − 153 to −145‰, respectively, point to a late mature thermogenic origin of hydrocarbon gas. 3He/4He ratios of gas samples are around 0.01 times the air value suggesting dominantly crust-derived He. 21Ne/22Ne and 40Ar/36Ar ratios of many gas samples are higher than the corresponding air values indicating the mixing of crustal and atmospheric noble gases. Multiple dichotomous patterns are observed in noble gas signatures of forelimb and backlimb samples, and depression and crest samples. 20Ne/22Ne ratios of some crest samples are higher than that of depression samples in the backlimb, pointing to the presence of diffusion-driven fractionation that is likely caused by the long-distance migration from depression to crest. Elemental ratios of air-derived noble gas isotopes – 22Ne/36Ar, 84Kr/36Ar, and 132Xe/36Ar are compared to the recharge water values, suggesting the interactions of oil, gas, and water phases in the shale over geologic time. Forelimb samples generally display older ages than backlimb samples, indicating a larger flux of external radiogenic 4He due to the higher density of deep faults in the forelimb area caused by the basement-involved deformation. The basement-involved deformation also causes pore collapse especially in the forelimb leading to a lower porosity that results in a more pristine noble gas signature in the forelimb due to the reduced impact of younger recharge water.

Formation of Venus, Earth and Mars: Constrained by Isotopes

We discuss the current state of knowledge of terrestrial planet formation from the aspects of different planet formation models and isotopic data from 182Hf-182W, U-Pb, lithophile-siderophile elements, 48Ca/44Ca isotope samples from planetary building blocks, 36Ar/38Ar, 20Ne/22Ne, 36Ar/22Ne isotope ratios in Venus' and Earth's atmospheres, the expected solar 3He abundance in Earth's deep mantle and Earth's D/H sea water ratios that shed light on the accretion time of the early protoplanets. Accretion scenarios that can explain the different isotope ratios, including a Moon-forming event after ca. 50 Myr, support the theory that the bulk of Earth's mass (>80%) most likely accreted within 10-30 Myr. From a combined analysis of the before mentioned isotopes, one finds that proto-Earth accreted 0.5-0.6 MEarth within the first ~4-5 Myr, the approximate lifetime of the protoplanetary disk. For Venus, the available atmospheric noble gas data are too uncertain for constraining the planet's accretion scenario accurately. However, from the available Ar and Ne isotope measurements, one finds that proto-Venus could have grown to 0.85-1.0 MVenus before the disk dissipated. Classical terrestrial planet formation models have struggled to grow large planetary embryos quickly from the tiniest materials within the typical lifetime of protoplanetary disks. Pebble accretion could solve this long-standing time scale controversy. Pebble accretion and streaming instabilities produce large planetesimals that grow into Mars-sized and larger planetary embryos during this early accretion phase. The later stage of accretion can be explained well with the Grand-Tack, annulus or depleted disk models. The relative roles of pebble accretion and planetesimal accretion/giant impacts are poorly understood and should be investigated with N-body simulations that include pebbles and multiple protoplanets.

Reference Model Payload for Ice Giant Entry Probe Missions

Descent probes afford the opportunity to make essential atmospheric measurements that are beyond the reach of remote sensing, including the atmospheric abundances of noble gases and key isotopes, and the structure of the atmosphere beneath the cloud tops. Measurements are defined as Tier 1, representing threshold science required to justify the probe mission, and Tier 2 representing valuable science that significantly complement and enhance the threshold measurements, but of themselves are not sufficient to justify the mission. Tier 1 measurements comprise atmospheric noble gas abundances including helium, key noble gas isotope ratios, and the thermal structure of the atmosphere. Instrumentation required to achieve the Tier 1 measurements include a mass spectrometer, a helium abundance detector, and an atmospheric structure instrument comprising both sensors for pressure, temperature, and atmospheric acoustic properties (speed of sound). Tier 1 science can be achieved with a probe making measurements near one to several bars. Tier 2 science includes measurements of key isotopic ratios, the abundances of atmospheric condensables and disequilibrium species, atmospheric dynamics, the net radiative flux transfer profile of the atmosphere, and the location, composition, properties, and structure of the clouds. To achieve all the Tier 2 science objectives requires a probe descending through at least ten bars carrying the full Tier 1 suite of instruments as well as a nephelometer, net flux radiometer, and an ultrastable oscillator to enable Doppler wind tracking of the probe throughout descent.

Original composition and formation process of slab-derived deep brine from Kashio mineral spring in central Japan

Brine samples from the wells in the Kashio mineral spring (an “Arima-type” hot spring at Ooshika-Mura, central Japan) were analyzed to determine the original chemical and isotopic compositions of the deep brine end-member before its dilution by meteoric water and to elucidate the origin of the end-member. The trends of variation between Cl, δD, and δ18O indicated the existence of a two-component mixing system and a systematic variation in the mixing ratio, which were mentioned in previous studies. By carefully tracking the variation in tritium (3H) and atmospheric noble gas in the brine, the Cl concentration in the end-member was determined to be 24,000 mg/L. This value is consistent with the result of previous studies. Based on the estimated composition and other related data, we inferred that the end-member originated from slab-derived fluid, which in turn may have undergone oxygen isotope exchange reactions with minerals. Although both the Arima and Kashio brines are considered to be derived from fluid dehydrated from the Philippine Sea slab, the chemical and isotopic compositions of the Kashio end-member are different from those of the Arima end-member. In particular, the Kashio end-member is characterized by low Cl concentration (~ 40% lower than that in the Arima end-member), low hydrogen isotope ratio, and low 3He/4He ratio (1.4 Ra). These results indicate that the chemical and isotopic compositions of the slab-derived fluid are different for each location. The significant difference in δD could reflect the difference in the dehydration depth. Finally, the low temperature and relatively low 3He/4He ratio of the brine end-member could be explained by its long residence time within the crust.

Diffusion of noble gases in subduction zone hydrous minerals

Subduction of atmospheric noble gases has been considered to play an important role in altering the primordial isotopes of Earth’s mantle over geological time. Analysis of natural samples and experiments indicate that large quantities of noble gases can be dissolved in volatile-bearing hydrous minerals in the subduction slabs. To quantitatively investigate the recycling efficiency of noble gases and relevant consequences on the mantle noble gas isotopic evolution, the diffusivities of noble gases in these minerals are needed. In this study, diffusion of He, Ne, Ar, Kr and Xe in lizardite, antigorite and tremolite have been calculated by first-principles methods based on density functional theory. Our results disclose that diffusion is slower with increasing radius of the noble gas atom (DHe > DNe > DAr > DKr > DXe) as expected. The common ring-structures in hydrous silicate minerals provide incorporation sites for the noble gas atoms and control their mobility. The diffusion activation energies are 84.9, 157.3, 287.5, 347.4, 414.9 kJ/mol from He to Xe in lizardite, and despite the very similar lattice structure between lizardite and antigorite, the activation energies are found to be significantly higher in antigorite, which are 120.6, 267.3, 449.6, 497.9 and 550.0 kJ/mol, respectively. In tremolite, the energy barriers are 93.6, 158.2, 266.3, 322.2 and 385.0 kJ/mol, which are also found to be in very good agreement with available experimental values and similar to those in lizardite. We also calculated diffusion activation energies at higher pressures (1 GPa for liazardite, 3 GPa for antigorite and tremolite) to better understand how much noble gases can be preserved against diffusive loss during subduction. Our result show that the oceanic crust and the lithospheric mantle of the subduction slab play different roles in delivering noble gases into the mantle. We find that all Ar, Kr, Xe and possibly part of the Ne can be entrained by the serpentine-dominated lithospheric mantle into the deep mantle due to the high diffusive energy barriers in antigorite. In contrast, noble gases in the amphibole-enriched oceanic crust would be characterized by fractionated noble gas signature, with the concentrations of retained noble gases in the crust following their respective ionic radius (Ne < Ar < Kr < Xe).

(Invited) Electrochemical Characterization of the Interface between Atmospheric Noble Gas Plasma Jet and Aqueous Solution

Elucidation of general features of the interface between atmospheric pressure plasma jets and aqueous solutions would be beneficial to environmental, agricultural and biomedical applications. Such fundamental descriptions are critically important to design process chemistries and scale-up production. In this presentation, our recent efforts towards that end will be discussed. The focus of the talk will be on free jets of low temperature radiofrequency (RF) plasma generated using noble gas in contact with water, in contrast to the often used two-electrode configuration with e.g. micro-plasmas. Experiments will be presented in support of a perspective that views the plasma-liquid interface as a boundary condition that locally constrains the reduction potential, which can be controlled using parameters such as the plasma gas composition, applied RF power, and jet stand-off distance. A unique spatial distribution of the reduction potential that develops to maintain charge neutrality in the aqueous solution will be presented. The consequences for process design and scale-up will be expanded upon.

Krypton storage capacity of the Earth's lower mantle

Noble gases are important geochemical tracers allowing reconstructing global volatile cycles in Earth's reservoirs. To constrain these fundamental processes, precise data on their partitioning behavior at deep Earth conditions are needed. Such data are only available at moderate pressures up to 25 GPa due to experimental challenges. We have investigated the possibility of noble gas storage in the Earth's lower mantle up to 115 GPa. We studied the incorporation of krypton in the second most abundant lower mantle mineral (Mg1-x,Fex)O (ferropericlase) as well as in liquid metal-alloys by performing experiments up to 115 GPa and 3700 K using the laser-heated diamond anvil cell coupled to post-mortem EMPA analysis and X-ray absorption spectroscopy. The results reveal that, at these extreme conditions, up to 3 wt.% of krypton can be stored in (Mg1-x,Fex)O and 3000 ppm in the Fe-rich liquid metal. For both phases the storage capacities increase with pressure (between 40 GPa and 60 GPa) at a constant high temperature of 2300 K. Fpc has never been considered as a NG host, despite being the second most abundant mineral in the Earth's LM. Using recent accurate compressibility data, we demonstrate that a substitution of krypton into the anion site of (Mg1-x,Fex)O in form of neutral oxygen Schottky defects at diluted lower mantle conditions is possible. This noble gas incorporation mechanism is in agreement with a previous study on bridgmanite. We show that (Mg1-x,Fex)O exhibits higher noble gas storage capacities than bridgmanite through the lower mantle using lattice strain modeling and including experimental solubility and thermoelastic data for neon, argon, krypton and xenon. We also demonstrate that both phases exhibit the highest solubilities for argon and krypton. We used the solubility data from lattice strain modeling to predict noble gas abundances stored in the solid lower mantle after magma ocean crystallization. The modeled abundances show apparent similarities with estimates for the deep noble gas reservoir that are based on either 3He abundances in ocean island basalts or radiogenic 40Ar abundances in the bulk Earth. This strongly indicates that the crystalline lower mantle may play an important role as deep noble gas storage reservoir. We propose, based on considerations on noble gas replenishment from the lower mantle to the atmosphere, that the lower mantle can only contribute to a small fraction of the present-day atmospheric noble gases. This suggests that the lower mantle is an un-degassed reservoir.

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.

Missing atmospheric noble gases in a large, tropical lake: The case of Lake Kivu, East-Africa

Lake Kivu is a 485 m deep tropical rift lake in East-Africa and well-known for its very high concentrations of dissolved carbon dioxide and methane in the stratified deep waters. In view of future large-scale methane extraction for power production, there is a need for predicting the evolution of gas concentrations and lake stability using numerical modelling. However, knowledge about the geochemical origin and transport processes affecting dissolved gases in the lake is still partially missing. Due to their inert nature, the analysis of dissolved noble gases can help to shed light on such questions. To learn more about transport processes in Lake Kivu, we extended a well-established sampling method for dissolved noble gases to work in the lake's high gas pressure waters. The results of our analysis show a distinct non-atmospheric isotopic signal in the deep waters (below ~250 m) with 3He/4He and 40Ar/36Ar ratios ~250% and ~20% higher than air saturated water (ASW). Moreover, the gas concentration profiles reveal a striking lack of atmospheric noble gases in the deep waters with respect to ASW. While Ne is depleted by ~45%, the more soluble 36Ar and Kr even decrease by ~70%. In contrast, 4He concentrations increase with depth by a factor of up to ~600. We attribute this excess He and the increases in 3He/4He and 40Ar/36Ar to the inflow of magmatic gases into Lake Kivu, along with a significant contribution of radiogenic 4He. To explain the depletion of atmospheric noble gases, we present and discuss three different scenarios, namely continuous outgassing, the inflow of depleted groundwater and a large past outgassing event. Due to the best agreement with our observations, we conclude that the inflow of depleted groundwater is likely responsible for the observed atmospheric noble gas depletions.

Halogens in serpentinites from the Isua supracrustal belt, Greenland: An Eoarchean seawater signature and biomass proxy?

Abstract Serpentine in modern seafloor and ophiolitic environments incorporates and often retains high concentrations of atmospheric noble gases and seawater-derived halogens. Ancient serpentinites therefore provide the potential to trace the composition of early surface environments. Antigorite-serpentinites locally carbonated to talc-magnesite schist outcropping in a low strain zone within the Eoarchean Isua supracrustal belt (Greenland) are investigated here, to test the retention of paleo-atmospheric noble gases and Eoarchean seawater halogens, and to further determine the genetic setting and metamorphic history of some of Earth’s oldest serpentinites. Based on field relationships, whole rock major and trace element geochemistry, and mineral chemistry, the investigated serpentinites are shown to represent hydrated and variously carbonated magmatic olivine ± orthopyroxene + Cr-spinel cumulates emplaced at the base of a lava flow of boninitic affinity pillowed in its upper portion. In addition, rare zircons extracted from one of the serpentinised cumulates have distinct magmatic trace element signatures and a U-Pb age of 3721 ± 27 Ma indicating the pillowed lava flow erupted on the Eoarchean seafloor. The serpentinites have high concentrations of noble gases, but the presence of parentless radiogenic ‘excess’ 40Ar, introduced by crustal-derived metamorphic fluids, obscures the 40Ar/36Ar ratio of Eoarchean seawater. Local carbonation of the serpentinites also caused halogen loss and fractionation. However, the least carbonated antigorite serpentinites preserve Br/Cl and I/Cl ratios within the range of modern seafloor serpentinites, which is interpreted as indicating Archaean serpentinising fluids were similar in composition to modern seawater-derived fluids. Importantly, the lowest measured I/Cl ratio of 29 (±2) × 10−6, taken as a maximum value for the Eoarchean ocean, is an order of magnitude lower than estimates for the primitive mantle I/Cl value. Iodine has a low concentration relative to Cl in modern seawater because it is sequestered by organic matter. If the inferred low I/Cl of Eoarchean seawater is correct, then similar I-sequestration was likely occurring in the Eoarchean, a process requiring the presence of significant biomass in Earth’s early oceans. Further constraining the Precambrian evolution of seawater I/Cl via serpentinites or other proxies may provide a novel method to explore the emergence and evolution of terrestrial biomass.