Xe Fraction Research Articles

Marangoni effect on hydrodynamics and mass transfer behavior in an internal loop airlift reactor under elevated pressure

AbstractThe Marangoni effect on the hydrodynamics and mass transfer was studied in an internal loop airlift reactor with the addition of ethanol under elevated pressure. The overall gas holdup αg increased by ~10% and the gas–liquid volumetric mass‐transfer coefficient kla increased by ~20% on average for each one bar increase in pressure. When the mole fraction of ethanol xE increased from 0 to 0.012, αg increased by ~110% and kla increased by ~50%. However, when xE increased from 0.012 to 0.11, αg decreased by ~40% and kla decreased by ~25%. This volcano‐shaped relationship between both hydrodynamics and mass transfer behavior and ethanol concentration was determined by the Marangoni effect. Semi‐empirical correlations of αg and kla were proposed by including the Marangoni effect in a wide range of ethanol concentration and operating conditions. These correlations gave good predictions of αg and kla compared with extensive experimental data in the literature.

Hadean isotopic fractionation of xenon retained in deep silicates.

Our understanding of atmosphere formation essentially relies on noble gases and their isotopes, with xenon (Xe) being a key tracer of the early planetary stages. A long-standing issue, however, is the origin of atmospheric depletion in Xe1 and its light isotopes for the Earth2 and Mars3. Here we report that feldspar and olivine samples confined at high pressures and high temperature with diluted Xe and krypton (Kr) in air or nitrogen are enriched in heavy Xe isotopes by +0.8 to +2.3‰ per AMU, and strongly enriched in Xe over Kr. The upper +2.3‰ per AMU value is a minimum because quantitative trapping of unreacted Xe, either in bubbles or adsorbed on the samples, is likely. In light of these results, we propose a scenario solving the missing Xe problem that involves multiple magma ocean stage events at the proto-planetary stage, combined with atmospheric loss. Each of these events results in trapping of Xe at depth and preferential retention of its heavy isotopes. In the case of the Earth, the heavy Xe fraction was later added to the secondary CI chondritic atmosphere through continental erosion and/or recycling of a Hadean felsic crust.

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.

Xenon isotope constraints on ancient Martian atmospheric escape

Trapped, paleoatmospheric xenon (Xe) in the Martian regolith breccia NWA 11220 is mass-dependently fractionated relative to solar Xe by 16.2 ± 2.7‰/amu. These data indicate that fractionation of atmospheric Xe persisted for hundreds of millions of years after planetary formation. Such a protracted duration of atmospheric Xe mass fractionation, which is particularly striking when compared to the non-fractionated state of Martian atmospheric krypton (Kr), cannot be easily reconciled with Xe escape as a neutral atom in a neutral hydrodynamic hydrogen wind. However, Xe escape as an ion coupled to a partially ionized hydrogen or oxygen wind provides a simple solution to problems associated with the neutral escape hypothesis. Ionic Xe escape requires a sufficiently high escape flux of a carrier ion (H+ or O+) and probably requires a structured planetary magnetic field to channel the flow. The end of Xe escape from Mars could be attributed to waning hydrogen sources from volcanic outgassing or from interactions of reduced impactors with surface water and ice. Alternatively, if Xe ions were driven off by O+, the end of Xe escape could be attributed to the decay of solar extreme ultra-violet radiation.

Simulations of Recent Argon Gas-Puff Implosions on Z With Xe and Kr Dopants

An intriguing result is that a small fraction of Xe (0.8% by number in the center jet) can have a significant impact on the emitted K-shell radiation in an Ar gas-puff implosion. This result was recently obtained from a pair of experiments on Sandia National Laboratories Z machine. These shots were Z2603 (with Xe) and Z2605 (without Xe). The pair had similar initial density profiles (outer shell, inner shell, and center jet) but one had the small aforementioned percentage of Xe dopant in the center jet. The resultant Ar K-shell yield was considerably reduced from 373 ± 9% to 129 ± 9% kJ without an analogous change in the total emitted power or total radiation yield. Furthermore, the K-shell power pulse for Z2603 reduced to a broad bimodal, 7 terra watts (TW) pulse, much smaller than the 32 TW conventional single peak for Z2605. Radiation-magnetohydrodynamic simulations presented herein qualitatively reproduce major trends from experiments. Detailed analyses of the implosion dynamics, temperature and density profiles, and emitted radiation agree qualitatively with the previous study. The effect of varying the xenon fraction on K-shell radiation suggests that enhanced Xe concentration reduces K-shell emission by diminishing K-Shell radiation from the central jet.

Strange messenger: A new history of hydrogen on Earth, as told by Xenon

Atmospheric xenon is strongly mass fractionated, the result of a process that apparently continued through the Archean and perhaps beyond. Previous models that explain Xe fractionation by hydrodynamic hydrogen escape cannot gracefully explain how Xe escaped when Ar and Kr did not, nor allow Xe to escape in the Archean. Here we show that Xe is the only noble gas that can escape as an ion in a photo-ionized hydrogen wind, possible in the absence of a geomagnetic field or along polar magnetic field lines that open into interplanetary space. To quantify the hypothesis we construct new 1-D models of hydrodynamic diffusion-limited hydrogen escape from highly-irradiated CO2-H2-H atmospheres. The models reveal three minimum requirements for Xe escape: solar EUV irradiation needs to exceed 10× that of the modern Sun; the total hydrogen mixing ratio in the atmosphere needs to exceed 1% (equiv. to 0.5% CH4); and transport amongst the ions in the lower ionosphere needs to lift the Xe ions to the base of the outflowing hydrogen corona. The long duration of Xe escape implies that, if a constant process, Earth lost the hydrogen from at least one ocean of water, roughly evenly split between the Hadean and the Archean. However, to account for both Xe’s fractionation and also its depletion with respect to Kr and primordial 244Pu, Xe escape must have been limited to small apertures or short episodes, which suggests that Xe escape was restricted to polar windows by a geomagnetic field, or dominated by outbursts of high solar activity, or limited to transient episodes of abundant hydrogen, or a combination of these. Xenon escape stopped when the hydrogen (or methane) mixing ratio became too small, or EUV radiation from the aging Sun became too weak, or charge exchange between Xe+ and O2 rendered Xe neutral. In our model, Xe fractionation attests to an extended history of hydrogen escape and Earth oxidation preceding and ending with the Great Oxidation Event (GOE).

Evolution of atmospheric xenon and other noble gases inferred from Archean to Paleoproterozoic rocks

We have analyzed ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3 Ga) to Paleozoic (404 Ma) rocks in an attempt to document the evolution of the elemental composition and isotopic signature of the atmosphere with time. Doing so, we aimed at understanding how physical and chemical processes acted over geological time to shape the modern atmosphere.Modern atmospheric xenon is enriched in heavy isotopes by 30–40‰ u−1 relative to Solar or Chondritic xenon. Previous studies demonstrated that, 3.3 Ga ago, atmospheric xenon was isotopically fractionated (enriched in the light isotopes) relative to the modern atmosphere, by 12.9 ± 1.2 (1σ) ‰ u−1, whereas krypton was isotopically identical to modern atmospheric Kr. Details about the specific and progressive isotopic fractionation of Xe during the Archean, originally proposed by Pujol et al. (2011), are now well established by this work. Xe isotope fractionation has evolved from 21‰ u−1 at 3.5 Ga to 12.9‰ u−1 at 3.3 Ga. The current dataset provides some evidence for stabilization of the Xe fractionation between 3.3 and 2.7 Ga. However, further studies will be needed to confirm this observation. After 2.7 Ga, the composition kept evolving and reach the modern-like atmospheric Xe composition at around 2.1 Ga ago.Xenon may be the second atmospheric element, after sulfur, to show a secular isotope evolution during the Archean that ended shortly after the Archean-Proterozoic transition. Fractionation of xenon indicates that xenon escaped from Earth, probably as an ion, and that Xe escape stopped when the atmosphere became oxygen-rich. We speculate that the Xe escape was enabled by a vigorous hydrogen escape on the early anoxic Earth. Organic hazes, scavenging isotopically heavy Xe, could also have played a role in the evolution of atmospheric Xe.For 3.3 Ga-old samples, Ar-N2 correlations are consistent with a partial pressure of nitrogen (pN2) in the Archean atmosphere similar to, or lower than, the modern one, thus requiring other processes than a high pN2 to keep the Earth's surface warm despite a fainter Sun. The nitrogen isotope composition of the atmosphere at 3.3 Ga was already modern-like, attesting to inefficient nitrogen escape to space since that time.

Negligible fractionation of Kr and Xe isotopes by molecular diffusion in water

Molecular diffusion is a key transport process for noble gases in water. Such diffusive transport is often thought to cause a mass-dependent fractionation of noble gas isotopes that is inversely proportional to the square root of the ratio of their atomic mass, referred to as the square root relation. Previous studies, challenged the commonly held assumption that the square root relation adequately describes the behaviour of noble gas isotopes diffusing through water. However, the effect of diffusion on noble gas isotopes has only been determined experimentally for He, Ne and Ar to date, whereas the extent of fractionation of Kr and Xe has not been measured. In the present study the fractionation of Kr and Xe isotopes diffusing through water immobilised by adding agar was quantified through measuring the respective isotope ratio after diffusing through the immobilised water. No fractionation of Kr and Xe isotopes was observed, even using high-precision noble gas analytics. These results complement our current understanding on isotopic fractionation of noble gases diffusing through water. Therefore this complete data set builds a robust basis to describe molecular diffusion of noble gases in water in a physical sound manner which is fundamental to assess the physical aspects of gas dynamics in aquatic systems.

Kinetic analysis of rare gas metastable production and optically pumped Xe lasers

Optically pumped all-rare-gas lasers use metastable rare gas atoms as the lasing species in mixtures with He or Ar buffer gas. The metastables are generated in a glow discharge, and we report model calculations for the optimal production of Ne*, Ar*, Kr* and Xe*. Discharge efficiency was estimated by solving the Boltzmann equation.Laser efficiency, gain and output power of the CW optically pumped Xe laser were assessed as functions of heavier rare gas content, pressure, optical pump intensity and the optical path length. It was found that, for efficient operation the heavier rare gas content has to be of the order of one percent or less, and the total pressure—in the range 0.3–1.5 atm. Output power and specific discharge power increase approximately linearly with pump intensity over the output range from 300–500 W cm−2. Ternary mixtures Xe:Ar:He were found to be the most promising. Total laser efficiency was found to be nearly the same for pumping the 2p8 or 2p9 state, reaching 61%–70% for a pump intensity of ~720 W cm−2 when the Xe fraction was in the range 0.001 ÷ 0.01 and Ar fraction—0.1 ÷ 0.5. However, when the 2p8 state was pumped, the maximum total efficiency occurred at larger pressures than for pumping of the 2p9 state. The discharge power density required to sustain a sufficient Xe* number density was in the range of tens of watts per cubic centimeter for 50% Ar in the mixture.

Meteorite constraints on Martian atmospheric loss and paleoclimate

The evolution of Mars' atmosphere to its currently thin state incapable of supporting liquid water remains poorly understood and has important implications for Martian climate history. Martian meteorites contain trapped atmospheric gases that can be used to constrain both the timing and effectiveness of atmospheric escape processes. In this paper, measurements of xenon isotopes in two ancient Martian meteorites, ALH 84001 and NWA 7034, are reported. The data indicate an early episode of atmospheric escape that mass fractionated xenon isotopes culminated within a few hundred million years of planetary formation, and little change to the atmospheric xenon isotopic composition has occurred since this time. In contrast, on Earth atmospheric xenon fractionation continued for at least two billion years (Pujol et al., 2011). Such differences in atmospheric Xe fractionation between the two planets suggest that climate conditions on Mars may have differed significantly from those on Archean Earth. For example, the hydrogen escape flux may not have exceeded the threshold required for xenon escape on Mars after 4.2–4.3 Ga, which indicates that Mars may have been significantly drier than Earth after this time.

Effect of gas mixing on physical properties of warm collisional helicon plasmas

The effect of inert gas mixing on the physical properties of a helicon plasma source with a Nagoya type III antenna is analytically investigated by taking into account the thermal and collisional effects. The dielectric permittivity tensor of this mixed gas plasma is obtained by using the Bhatnagar–Gross– Krook kinetic theory. Considering the dielectric tensor of mixed gas plasma and solving the electromagnetic field equations, the profiles of electromagnetic fields and plasma resistance are plotted and discussed. The results show that the plasma resistance peaks decrease with increasing Xe fraction in Ar–Xe plasma, and increase with the He fraction in Ar–He plasma. It is also shown that by increasing the xenon filling fraction, the electromagnetic field amplitudes are lowered, and by increasing the helium filling fraction, they are increased.

Effects of a Xe dopant on an Ar gas-puff implosion on Z

Two Ar gas-puff experiments, each using loads consisting of two annuli and a center jet, were recently performed on Sandia National Laboratories' Z machine. These shots had the same load except that one of them (Z2603) employed a Xe dopant of 0.8% by number in the jet, which was not present in the otherwise identical Z2605. The extensive diagnostics deployed in these experiments reveal that the presence of this small fraction of Xe had a significant effect on the emitted K-shell radiation. Use of the Xe dramatically reduced the Ar K-shell yield from 373 ± 9% to 129 ± 9% kJ. However, the total yield increased, from 1.02 ± 17% to 1.14 ± 17% MJ. Also, the K-shell power pulse for Z2603 exhibited two nearly equal peaks separated by ∼5 ns, but that of Z2605 consisted of a more conventional single peak. Analysis of time- and space resolved K-shell spectra using a detailed atomic model for Ar ions indicates that a much greater fraction of the load mass was heated to the K-shell in the non-Xe shot Z2605. Previous ...

The evolution of MORB and plume mantle volatile budgets: Constraints from fission Xe isotopes in Southwest Indian Ridge basalts

We present high-precision measurements of the fission isotopes of xenon (Xe) in basalts from the Southwest Indian Ridge (SWIR) between 16°E and 25°E. Corrections for syn- to post-eruptive atmospheric contamination yield the Xe isotopic compositions of SWIR mantle sources. We solve for the proportions of mantle Xe derived from the primordial mantle Xe budget, recycling of atmospheric Xe, decay of short-lived 129I, fission of extinct 244Pu, and fission of extant 238U. Xe isotope systematics evident in SWIR basalts and other mantle-derived samples provide new insights into the integrated history of mantle source degassing and regassing. We find that recycled atmospheric Xe dominates the Xe inventories of the SWIR Western and Eastern Orthogonal Supersegment mantle sources (∼80–90% of 132Xe is recycled in origin), consistent with results from studies of plume-influenced basalts from Iceland and the Rochambeau Rift. While significant regassing of the mantle is evident, we also find differences in the extent of degassing of the MORB and plume sources. MORB sources are consistently characterized by a lower fraction of fission Xe derived from Pu-fission, indicating a greater extent of degassing relative to the plume source. The prevalence of recycled atmospheric Xe in mantle sources indicates incorporation of depleted recycled material even into mantle sources with primitive He and Ne isotopic compositions. Consequently, depleted lithophile isotopic compositions in mantle sources with primitive He and Ne cannot be interpreted as evidence for a nonchondritic bulk silicate Earth.

Atmospheric noble gases as tracers of biogenic gas dynamics in a shallow unconfined aquifer

Atmospheric noble gases (NGs) were used to investigate biogenic gas dynamics in a shallow unconfined aquifer impacted by a crude oil spill, near Bemidji, MN. Concentrations of 3,4He, 20,22Ne, 36,40Ar, Kr, and Xe were determined for gas- and aqueous-phase samples collected from the vadose and saturated zones, respectively. Systematic elemental fractionation of Ne, Ar, Kr, and Xe with respect to air was observed in both of these hydrogeologic zones. Within the vadose zone, relative ratios of Ne and Ar to Kr and Xe revealed distinct process-related trends when compared to corresponding ratios for air. The degree of NG deviation from atmospheric concentrations generally increased with greater atomic mass (i.e., ΔXe>ΔKr>ΔAr>ΔNe), indicating that Kr and Xe are the most sensitive NG tracers in the vadose zone. Reactive transport modeling of the gas data confirms that elemental fractionation can be explained by mass-dependent variations in diffusive fluxes of NGs opposite to a total pressure gradient established between different biogeochemical process zones. Depletion of atmospheric NGs was also observed within a methanogenic zone of petroleum hydrocarbon degradation located below the water table. Solubility normalized NG abundances followed the order Xe>Kr>Ar>Ne, which is indicative of dissolved NG partitioning into the gas phase in response to bubble formation and possibly ebullition. Observed elemental NG ratios of Ne/Kr, Ne/Xe, Ar/Xe, and Kr/Xe and a modeling analysis provide strong evidence that CH4 generation below the water table caused gas exsolution and possibly ebullition and carbon transfer from groundwater to the vadose zone. These results suggest that noble gases provide sensitive tracers in biologically active unconfined aquifers and can assist in identifying carbon cycling and transfer within the vadose zone, the capillary fringe, and below the water table.

Thermal modeling of shock melts in Martian meteorites: Implications for preserving Martian atmospheric signatures and crystallization of high‐pressure minerals from shock melts

AbstractThe distribution of shock melts in four shergottites, having both vein and pocket geometry, has been defined and the conductive cooling time over the range 2500 °C to 900 °C calculated. Isolated 1 mm2 pockets cool in 1.17 s and cooling times increase with pocket area. An isolated vein 1 × 7 mm in Northwest Africa (NWA) 4797 cools to 900 °C in 4.5 s. Interference between thermal haloes of closely spaced shock melts decreases the thermal gradient, extending cooling times by a factor of 1.4 to 100. This is long enough to allow differential diffusion of Ar and Xe from the melt. Small pockets (1 mm2) lose 2.2% Ar and 5.2% Xe during cooling, resulting in a small change in the Ar/Xe ratio of the dissolved gas over that originally trapped. With longer cooling times there is significant fractionation of Xe from Ar and the Ar/Xe ratio increases rapidly. The largest pockets show less variation of Ar/Xe and likely preserve the original trapped gas composition. Considering all of the model calculations, even the smallest isolated pockets have cooling times greater than the duration of the pressure pulse, i.e., >0.01 s. The crystallization products of these shock melts will be unrelated to the peak shock pressure experienced by the meteorite.

Production of anomalous Xe in nanodiamond in chondrites during the last supernova explosion predating the origin of the Solar System

Anomalous Xe enriched in both heavy and light isotopes (Xe-HL) was identified in the high-temperature Xe fraction in relict nanodiamond grains from chondrites, whereas the low-temperature Xe fraction (Xe-P3) typically has the normal isotopic composition. The paper presents a concise review of current models put forth to account for the genesis of nanodiamond with anomalous noble gas components and specifies a real process and major regularities during the generation of the isotopic relations of the anomalous Xe-HL component in relict nanodiamond grains. This component is demonstrated to be formed and captured simultaneously with the synthesis of nanodiamond, when shock waves induced by supernova explosions propagated. It is important that diamond synthesis during the passage of shock waves and the enrichment of this diamond in Xe-HL are also possible in the wave forefront region under extremal P-T conditions, in the pressure drop region behind the wave front (by means of nucleation), and by means of irradiation of carbonic grains with high-energy particles. The isotopic composition of Xe-HL results from an increase in the hardness of the spectrum of nuclear-active particles and its enrichment in heavy ions at acceleration in shock waves. Arguments are presented in support of the hypothesis that the nanodiamond population found in chondrites was produced during the latest supernova explosion before the development of the Solar System, with the supernova likely being a SnIa carbon detonation supernova. This furnishes evidence in support of recently advanced hypotheses that the nanodiamond population of chondrites is not presolar.

Ion acceleration in Ar–Xe and Ar–He plasmas. II. Ion velocity distribution functions

Ion velocity distribution functions (ivdfs) are investigated by laser induced fluorescence in Ar–Xe and Ar–He expanding helicon plasmas as a function of gas composition. In the case of Ar–Xe plasma, it was found that in the helicon source, both the Ar+ and Xe+ vdfs are unimodal. Their parallel speeds are subsonic and unaffected by changes in gas composition. At the end of the source, the argon ivdf shows a bimodal structure indicative of an electric double layer upstream of the measurement location. The fast argon ion component parallel velocity increases with Xe fraction from 6.7 to 8 km/s as the Xe fraction increases from 0% to 4%. In the expansion region, the bimodal character of Ar ivdf is maintained with a supersonic fast component reaching parallel speeds of 10.5 km/s. For all the studied plasma conditions and different spatial locations, the Xe+ vdf exhibits a unimodal structure with a maximum parallel flow velocity of 2.2 km/s at the end of the source. For Ar–He plasma, the Ar ivdf is bimodal with the fast ion component parallel velocity increasing from 5.2 to 7.8 km/s as the He fraction increases from 0% to 30%. For the same He fraction range, the slow argon ion population distribution changes from a single Gaussian to a wide distribution extending all the way from the speed of the fast population to 0 m/s.

Ion acceleration in Ar–Xe and Ar–He plasmas. I. Electron energy distribution functions and ion composition

Electron energy distribution functions (eedf), ion production, and ion composition are studied in Ar–Xe and Ar–He expanding helicon plasmas. It was found that under the conditions of constant total flow rate, Xe, in addition to Ar, changes the eedf from Maxwellian-like to Druyvesteyn-like with a shortening of the high energy tail at ∼15 eV. The electron temperature exponentially decreases from ∼7 eV in pure Ar plasma to ∼4 eV in pure Xe plasma. Xenon ions dominate the ion population for Xe filling fractions greater than 10%. The plasma density increases by ∼15% with increasing Xe fraction. For an Ar–He plasma, increasing the helium fraction increases the electron temperature from ∼7 eV in pure Ar plasma to ∼14 eV for a He filling fraction of 80%. The plasma density drops by more than three orders of magnitude from 1.14×1011 cm−3 to 6.5×107 cm−3. However, the inferred ion densities indicate that even at a helium fraction of 80%, argon ions significantly outnumber helium ions.

Noble gases in two shergottites and one nakhlite from Antarctica: Y000027, Y000097, and Y000593

We have investigated secondary influences on the noble gas budget in rim and interior pairs of three Martian meteorites from Antarctica: the lherzolitic shergottites Y000027 and Y000097, and the nakhlite Y000593. Three factors have been found to influence the original Martian noble gas budget: shock metamorphic overprint, cosmic irradiation, and terrestrial weathering. The 3He/ 4He ratio of the shergottites is between 0.189 and 0.217, which indicates almost complete loss of radiogenic 4He. This is expected from the high shock pressure observed in the shergottite samples. The concentration of 4He in these shergottite samples ranges from 33.8 to 39.4 × 10 −8 ccSTP/g. 22Ne in the shergottites is on the order of 14 × 10 −9 ccSTP/g. The nakhlite has ∼800 × 10 −8 ccSTP/g 4He and ∼26 × 10 −9 ccSTP/g 22Ne. An indication for solar cosmic ray contribution to the neon budget can be found in the shergottites. As Y000027 and Y000097 are reported to be paired we conclude the cosmic ray exposure (CRE) age T (3+21) of this shergottite to be 4.41 ± 0.54 Ma. For the nakhlite Y000593 T (3+21) is 11.8 ± 0.3 Ma. Heavy noble gas concentrations show large differences between rim and interior samples with the rim samples having 1.3–2.9, 1.7–38, and 1.4–20 times as much 36Ar, 84Kr, and 132Xe, respectively. The enrichment of heavy noble gases in the rim samples indicates severe terrestrial contamination. The relation between 129Xe/ 132Xe and 84Kr/ 132Xe in the rim samples shows that the incorporation mechanism caused elemental fractionation of Kr and Xe to the extent that in the Y000027 shergottite samples any Martian signature is completely masked by terrestrial contamination, if the total is taken. Only the 1400 °C steps show clear evidence for Martian atmosphere. The Y000593 nakhlite interior sample, on the other hand, shows low 84Kr/ 132Xe in relation to 129Xe/ 132Xe, which is characteristic for fractionated Martian atmosphere observed in nakhlites.

DETERMINATION OF THE MINIMUM MASSES OF HEAVY ELEMENTS IN THE ENVELOPES OF JUPITER AND SATURN

We calculate the minimum mass of heavy elements required in the envelopes of Jupiter and Saturn to match the observed oversolar abundances of volatiles. Because the clathration efficiency remains unknown in the solar nebula, we have considered a set of sequences of ice formation in which the fraction of water available for clathration is varied between 0 and 100 %. In all the cases considered, we assume that the water abundance remains homogeneous whatever the heliocentric distance in the nebula and directly derives from a gas phase of solar composition. Planetesimals then form in the feeding zones of Jupiter and Saturn from the agglomeration of clathrates and pure condensates in proportions fixed by the clathration efficiency. A fraction of Kr and Xe may have been sequestrated by the H3+ ion in the form of stable XeH3+ and KrH3+ complexes in the solar nebula gas phase, thus implying the formation of at least partly Xe- and Kr-impoverished planetesimals in the feeding zones of Jupiter and Saturn. These planetesimals were subsequently accreted and vaporized into the hydrogen envelopes of Jupiter and Saturn, thus engendering volatiles enrichments in their atmospheres, with respect to hydrogen. Taking into account both refractory and volatile components, and assuming plausible molecular mixing ratios in the gas phase of the outer solar nebula, we show that it is possible to match the observed enrichments in Jupiter and Saturn, whatever the clathration efficiency. Our calculations predict that the O/H enrichment decreases from 6.7 to 5.6 times solar (O/H) in the envelope of Jupiter and from 18.1 to 15.4 times solar (O/H) in the envelope of Saturn with the growing clathration efficiency in the solar nebula.