Martian xenon components in Shergotty mineral separates: Locations,sources, and trapping mechanisms

Abstract— Isotopic signatures and concentrations of xenon have been measured in Shergotty mineral separates by laser step heating. Martian atmosphere and ‘martian interior’ xenon are present, as is a spallation component. Martian atmospheric xenon is 5–10 times more concentrated in opaque minerals (magnetite, ilmenite, and pyrrhotite) and maskelynite than in pyroxenes, perhaps reflecting grain size variation. This is shown to be consistent with shock incorporation. A component consisting of solar xenon with a fission contribution, similar to components previously identified in martian meteorites and associated with the martian interior, is best defined in the pyroxene‐dominated separates. This component exhibits a consistent ¹²⁹Xe (¹²⁹Xe/¹³²Xe ?1.2) excess over solar/planetary (¹²⁹Xe/¹³²Xe ?1.04). We suggest that gas present in the melt, perhaps a mixture of interior xenon and martian atmosphere, was incorporated into the pyroxenes in Shergotty as the minerals crystallized.     

Noble gases and chemical composition of Shergotty mineral fractions,Chassigny, and Yamato 793605: The trapped argon-40/argon-36 ratio and ejection times of Martian meteorites

Abstract— We report the elemental and isotopic composition of the noble gases as well as the chemical abundances in pyroxene, maskelynite/mesostasis glass, and bulk material of Shergotty and of bulk samples from Chassigny and Yamato 793605. The ⁴⁰K-⁴⁰Ar isochron for the Shergotty minerals yields a gas retention age of 196 Ma, which is, within errors, in agreement with previously determined Rb-Sr internal isochron ages. Argon that was trapped at this time has a ⁴⁰Ar/³⁶Ar ratio of 1100. For Chassigny and Y-793605, we obtain trapped ⁴⁰Ar/³⁶Ar ratios of 1380 and 950, respectively. Using these results and literature data, we show that the three shergottites, Shergotty, Zagami, and QUE 94001; the lherzolites ALH 77005, LEW 88516, and Y-793605; as well as Chassigny and ALH 84001 contain a mixture of Martian mantle and atmospheric Ar; whereas, the trapped ⁴⁰Ar/³⁶Ar ratio of the nakhlites, Nakhla, Lafayette, and Governador Valadares cannot be determined with the present data. We show that Martian atmospheric trapped Ar in Martian meteorites is correlated with the shock pressure that they experienced. Hence, we conclude that the Martian atmospheric gases were introduced by shock into the meteoritic material. For the Shergotty minerals, we obtain ³He-, ²¹Ne-, and ³⁸Ar-based cosmic-ray exposure ages of 3.0 Ma, and for the lherzolite Y-793605, 4.0 Ma, which confirms our earlier conclusion that the lherzolites were ejected from Mars ～1 Ma before the shergottites. Chassigny yields the previously known ejection age of 11.6 Ma.     

Shergottites Dhofar 019, SaU 005, Shergotty,and Zagami: 40Ar‐39Ar chronology and trapped Martian atmospheric and interior argon

Abstract— We report a high‐resolution ⁴⁰Ar‐³⁹Ar study of mineral separates and whole‐rock samples of olivine‐phyric (Dhofar 019, Sayh al Uhaymir [SaU] 005) and basaltic (Shergotty, Zagami) shergottites. Excess argon is present in all samples. The highest (⁴⁰Ar/³⁶Ar)_trapped ratios are found for argon in pyroxene melt inclusions (?1500), maskelynite (?1200), impact glass (?1800) of Shergotty and impact glass of SaU 005 (?1200). A high (⁴⁰Ar/³⁶Ar)_trapped component‐usually uniquely ascribed to Martian atmosphere‐can also originate from the Martian interior, indicating a heterogeneous Martian mantle composition. As additional explanation of variable high (⁴⁰Ar/³⁶Ar)_trapped ratios in shocked shergottites, we suggest argon implantation from a “transient atmosphere” during impact induced degassing. The best ⁴⁰Ar‐³⁹Ar age estimate for Dhofar 019 is 642 ± 72 Ma (maskelynite). SaU 005 samples are between 700–900 Ma old. Relatively high ⁴⁰Ar‐³⁹Ar ages of melt inclusions within Dhofar 019 (1086 ± 252 Ma) and SaU 005 olivine (885 ± 66 Ma) could date entrapment of a magmatic liquid during early olivine crystallization, or reflect unrecognized excess ⁴⁰Ar components. The youngest ⁴⁰Ar‐³⁹Ar age of Shergotty separates (maskelynite) is ?370 Ma, that of Zagami is ?200 Ma. The ⁴⁰Ar‐³⁹Ar chronology of Dhofar 019 and SaU 005 indicate >1 Ga ages. Apparent ages uncorrected for trapped (e.g., Martian atmosphere, mantle) argon components approach 4.5 Ga, but are not caused by inherited ⁴⁰Ar, because excess ⁴⁰Ar is supported by ³⁶Ar_trapped. Young ages obtained by ⁴⁰Ar‐³⁹Ar and other chronometers argue for primary rather than secondary events. The cosmic ray exposure ages calculated from cosmogenic argon are 15.7 ± 0.7 Ma (Dhofar 019), 1.0–1.6 Ma (SaU 005), 2.1–2.5 Ma (Shergotty) and 2.2–3.0 Ma (Zagami).     

Noble gases in enstatite chondrites I: Exposure ages,pairing, and weathering effects

Abstract— Cosmic‐ray exposure ages calculated from cosmogenic noble gas nuclides are reported for 57 enstatite (E) chondrites, 43 of them were measured for the first time. With a total of 62 individual E chondrites (literature and this data, corrected for pairing) the observed spectrum of ages ranges between 0.07 and 66 Ma. Three clusters seem to develop at about 3.5, 8, and 25 Ma, respectively. Since the uncertainty of ages is estimated to be ～20% (in contrast to 10 to 15% for ordinary chondrites) and the number of examined samples is still comparatively small, these peaks have to be confirmed by more measurements. Regarding the two subgroups, EH and EL chondrites, no systematic trend is apparent in the distribution of cosmic‐ray exposure ages. Several E chondrites yield significantly lower ³⁸Ar ages compared to those calculated from cosmogenic ³He and ²¹Ne. For these E chondrites, we suggest a reduction of cosmogenic ³⁸Ar as a result of weathering. In order to prove the possible influence of terrestrial alteration on the cosmogenic noble gas record of E‐chondritic material, we simulated terrestrial weathering in an experiment of 12 weeks duration. The treatment showed that a significant amount of cosmogenic ³⁸Ar is lost on Earth by the influence of water.     

Liebermannite,KAlSi3O8, a new shock-metamorphic,high-pressure mineral from the Zagami Martian meteorite

In this paper, we discuss the occurrence of liebermannite (IMA 2013-128), KAlSi₃O₈, a new, shock-generated, high-pressure tetragonal hollandite-type structure silicate mineral, in the Zagami basaltic shergottite meteorite. Liebermannite crystallizes in space group I4/m with Z = 2, cell dimensions of a = 9.15 ± 0.14 (1σ) Å, c = 2.74 ± 0.13 Å, and a cell volume of 229 ± 19 Å³ (for the type material), as revealed by synchrotron diffraction. In Zagami, liebermannite likely formed via solid-state transformation of primary igneous K-feldspar during an impact event that achieved pressures of ~20 GPa or more. The mineral name is in honor of Robert C. Liebermann, a high-pressure mineral physicist at Stony Brook University, New York, USA.     

Noble gases and nitrogen in Muong Nong tektites

Abstract— Three samples of Muong Nong tektites have been studied for N and noble gases. The isotopic composition of noble gases is airlike. The noble gas amounts are much higher in Muong Nong tektites than in splash-form tektites. As compared to air, He and Ne have been enriched, most likely due to inward diffuion from ambient air, subsequent to glass formation. Nitrogen contents range from 0.3 to 1.34 ppm, with a non-atmospheric δ¹⁵N ranging from 8 to 17%. The release pattern of δ¹⁵N clearly shows the presence of two N components. Higher N/³⁶Ar values than those of air, together with positive δ¹⁵N, show that a major portion of N in Muong Nong tektites is a remnant from the sedimentary source material.     

Rumuruti chondrites: Noble gases,exposure ages,pairing, and parent body history

Abstract— In this paper, we present concentration and isotopic composition of the light noble gases He, Ne, and Ar as well as of ⁸⁴Kr, ¹³²Xe, and ¹²⁹Xe in bulk samples of 33 Rumuruti (R) chondrites. Together with previously published data of six R chondrites, exposure ages are calculated and compared with those of ordinary chondrites. A number of pairings, especially between those from Northwest Africa (NWA), are suggested, so that only 23 individual falls are represented by the 39 R chondrites discussed here. Eleven of these meteorites, or almost 50%, contain solar gases and are thus regolithic breccias. This percentage is higher than that of ordinary chondrites, howardites, or aubrites. This may imply that the parent body of R chondrites has a relatively thick regolith. Concentrations of heavy noble gases, especially of Kr, are affected by the terrestrial atmospheric component, which resides in weathering products. Compared to ordinary chondrites, ¹²⁹Xe/¹³²Xe ratios of R chondrites are high.     

Noble gases in enstatite chondrites II: The trapped component

Abstract— The trapped noble gas record of 57 enstatite chondrites (E chondrites) has been investigated. Basically, two different gas patterns have been identified dependent on the petrologic type. All E chondrites of type 4 to 6 show a mixture of trapped common chondritic rare gases (Q) and a subsolar component (range of elemental ratios for E4–6 chondrites: ³⁶Ar/¹³²Xe = 582 ± 270 and ³⁶Ar/⁸⁴Kr = 242 ± 88). E3 chondrites usually contain Q gases, but also a composition with lower ³⁶Ar/¹³²Xe and ³⁶Ar/⁸⁴Kr ratios, which we call sub‐Q (³⁶Ar/¹³²Xe = 37.0 ± 18.0 and ³⁶Ar/⁸⁴Kr = 41.7 ± 18.1). The presence of either the subsolar or the sub‐Q signature in particular petrologic types cannot be readily explained by parent body metamorphism as postulated for ordinary chondrites. We therefore present a different model that can explain the bimodal distribution and composition of trapped heavy noble gases in E chondrites. Trapped solar noble gases have been observed only in some E3 chondrites. About 30% of each group, EH3 and EL3 chondrites, amounting to 9% of all analyzed E chondrites show the solar signature. Notably, only one of those meteorites has been explicitly described as a regolith breccia.     

Noble gases in ten Nullarbor chondrites: Exposure ages,terrestrial ages,and weathering effects

Abstract— We present concentration and isotopic composition of He, Ne, and Ar in ten chondrites from the Nullarbor region in Western Australia as well as the concentrations of ⁸⁴Ke, ¹²⁹Xe, and ¹³²Xe. From the measured cosmogenic ¹⁴C concentrations (Jull et al. 1995), shielding‐corrected production rates of ¹⁴C are deduced using cosmogenic ²²Ne/²¹Ne ratios. For shielding conditions characterized by ²²Ne/²¹Ne >1.10, this correction becomes significant and results in shorter terrestrial ages. The exposure ages of the ten Nullarbor chondrites are in the range of values usually observed in ordinary chondrites. Some of the meteorites have lost radiogenic gases as well as cosmogenic ³He. Most of the analyzed specimens show additional trapped Ar, Kr, and Xe of terrestrial origin. The incorporation of these gases into weathering products is common in chondrites from hot deserts.     

Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages,trapped gases,and a re‐evaluation of the evidence for solar cosmic ray‐produced neon in shergottites and other achondrites

We present noble gas data for 16 shergottites, 2 nakhlites (NWA 5790, NWA 10153), and 1 angrite (NWA 7812). Noble gas exposure ages of the shergottites fall in the 1–6 Ma range found in previous studies. Three depleted olivine‐phyric shergottites (Tissint, NWA 6162, NWA 7635) have exposure ages of ~1 Ma, in agreement with published data for similar specimens. The exposure age of NWA 10153 (~12.2 Ma) falls in the range of 9–13 Ma reported for other nakhlites. Our preferred age of ~7.3 Ma for NWA 5790 is lower than this range, and it is possible that NWA 5790 represents a distinct ejection event. A Tissint glass sample contains Xe from the Martian atmosphere. Several samples show a remarkably low (²¹Ne/²²Ne)_cos ratio < 0.80, as previously observed in a many shergottites and in various other rare achondrites. This was explained by solar cosmic ray‐produced Ne (SCR Ne) in addition to the commonly found galactic cosmic ray‐produced Ne, implying very low preatmospheric shielding and ablation loss. We revisit this by comparing measured (²¹Ne/²²Ne)_cos ratios with predictions by cosmogenic nuclide production models. Indeed, several shergottites, acalpulcoites/lodranites, angrites (including NWA 7812), and the Brachina‐like meteorite LEW 88763 likely contain SCR Ne, as previously postulated for many of them. The SCR contribution may influence the calculation of exposure ages. One likely reason that SCR nuclides are predominantly detected in meteorites from rare classes is because they usually are analyzed for cosmogenic nuclides even if they had a very small (preatmospheric) mass and hence low ablation loss.     

Noble gases in twenty-one Saharan LL-chondrites: Exposure ages and possible pairings

Abstract— Nineteen LL-chondrites and two L/LL-chondrites (Adrar 003 and Tanezrouft 010) from the Saharan desert of Algeria and Libya have been analysed for their He, Ne and Ar composition as well as for their abundance of ⁸⁴Kr and ¹³²Xe. Calculated ²¹Ne cosmic-ray exposure ages vary between 2 and 35 Ma. The age distribution is consistent with that of modern LL-chondrite falls except that no dominant peaks can be observed, especially not the one related to a 15 Ma collisional event. However, the lack of young exposure ages of <8 Ma is obvious. This is a characteristic feature of LL-chondrites. Only one of the 21 LL-chondrites, namely Acfer 066, contains solar gases and is thus considered a regolith breccia. Three specimens reveal considerable loss of ³He, probably due to periods of elevated temperatures in orbits with small perihelion distances. Furthermore, severe loss of ⁴He and ⁴⁰Ar is found in two samples. Considering possible pairings, we suggest 14 individual falls basically on the basis of the noble gas data, the petrographic sub-classification and by taking the find location into consideration. However, there are constraints on confirming pairings solely on the basis of our studies. Thus, we can only exclude individual samples with a unique noble gas fingerprint from paired specimens.     

Noble gases and strontium isotopes in the unique meteorite Divnoe

Abstract— We present an isotope study of noble gases in Divnoe, an anomalous meteorite, and also Rb-Sr and K-Ar dating of this meteorite. The relatively young Rb-Sr age obtained (3.39 Ga) seems doubtful and, most probably, results from weathering or contamination. The ancient K-Ar age (4.67^+0.20_–0.40), together with clear excess of ¹²⁹Xe, allows the suggestion of very early formation of the Divnoe meteorite. Concentrations and isotope ratios of noble gases in Divnoe are: 17.9 ≤ ³He ≤ 29.0 × 10^?8; ²⁰Ne = 6.22 × 10^?8; 2.44 ≤ ³⁶Ar ≤ 5.10 × 10^?8; ¹³⁰Xe = 41.3 × 10^?12 cm³/g; 0.079 ≤ ³He/⁴He ≤ 0.193; ²⁰Ne/²²Ne = 0.860; ²¹Ne/²²Ne = 0.927; 3.47 ≤ ⁴⁰Ar/³⁶Ar ≤ 9.47; 2.22 ≤ 36Ar/³⁸Ar ≤ 3.27; ¹²⁹Xe/¹³²Xe = 1.09. The exposure age calculated from cosmogenic ³He, ²¹Ne, and ³⁸Ar is 17.9 ± 0.9 Ma. On the basis of the isotope data for the noble gases and O, and abundances of K, Rb, and Sr, an attempt was made to estimate the relationship of Divnoe to other meteorite types. The O-isotope characteristics of Divnoe are clearly distinct from those of ordinary chondrites, acapulcoites/lodranites, and SNC meteorites (Petaev et al., 1994, Clayton, 1993). In plots of ¹³⁶Xe vs. ¹²⁹Xe/¹³⁰Xe, the Divnoe data fall outside of the data fields for carbonaceous and enstatite chondrites. The light noble gas data, especially the ⁴⁰Ar/³⁸Ar ratio, and the ⁴⁰Ar, ³⁸Ar, ³He, and ⁴He contents of Divnoe differ significantly from those of all meteorite types except diogenites. The K, Rb, and Sr abundances in Divnoe are substantially lower than in most other meteorites. In the concentrations of these elements, as well as in the REE pattern, the Divnoe meteorite is similar only to diogenites. Divnoe probably should be treated as a restite remaining after partial melting of the chondritic mantle of a parent asteroid body.     

Noble gas studies in CAIs from CV3 chondrites: No evidence for primordial noble gases

Abstract— Calcium‐aluminum‐rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka. The ²⁰Ne/²²Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne‐HL, Ne‐Q, or solar wind Ne. The ²¹Ne/²²Ne ratios range from 0.86 to 0.72, with fine‐grained, more altered CAIs usually showing lower values than coarse‐grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na‐rich alteration phases rather than to the presence of Ne‐G or Ne‐R (essentially pure ²²Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The ³⁶Ar/³⁸Ar ratios are between 0.7 and 4.8, with fine‐grained CAIs within one meteorite showing higher ratios than the coarse‐grained ones. This agrees with higher concentrations of cosmogenic ³⁶Ar produced by neutron capture on ³⁵Cl with subsequent β^?‐decay in finer‐grained, more altered, and thus, more Cl‐rich CAIs than in coarser‐grained, less altered ones. Although our data do not strictly contradict the presence of small amounts of Ne‐G, Ne‐R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na‐, Ca‐, and Cl‐rich minerals.     

Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures

Noble gases in the five angrites Northwest Africa (NWA) 1296, 2999, 4590, 4801, and 4931 were analyzed with total melting and stepwise heating methods. The noble gases consist of in situ components: spallogenic, radiogenic, nucleogenic, and fission. Cosmic-ray exposure ages of the angrites (including literature data) spread uniformly from <0.2 to 56 Ma, and coarse-grained angrites have longer exposure ages than fine-grained angrites. It is implied that the parent bodies from which the two subgroups of angrites were ejected are different and have distinct orbital elements. The ²⁴⁴Pu-¹³⁶Xe relative ages of the angrites obtained by using ²⁴⁴Pu/¹⁵⁰Nd ratios are as old as that of Angra dos Reis, reflecting their early formation. On the other hand, another method to obtain ²⁴⁴Pu-¹³⁶Xe relative ages, using fission ¹³⁶Xe, spallogenic ¹²⁶Xe, and Ba/REE ratios, yields systematically older ²⁴⁴Pu-¹³⁶Xe ages than those obtained by using ²⁴⁴Pu/¹⁵⁰Nd ratios, which is explained by apparently high Ba/REE ratios caused by Ba contamination during terrestrial weathering. The ²⁴⁴Pu/²³⁸U ratio at 4.56 Ga of angrites is estimated as 0.0061 ± 0.0028, which is consistent with those for chondrules, chondrites, achondrites, and a terrestrial zircon. It is suggested that initial ²⁴⁴Pu/²³⁸U ratio has been spatially homogeneous at least in the inner part of the early solar system.     

Noble gases in presolar diamonds II: Component abundances reflect thermal processing

Abstract— Using the isotopic compositions derived in Huss and Lewis, 1994a (Paper I), abundances of the P3, HL, and P6 noble-gas components were determined for 15 diamond separates from primitive chondrites of 8 chondrite classes. Within a meteorite class, the relative abundances of these components correlate with the petrologic subtype of the host meteorite, indicating that metamorphism is primarily responsible for the variations. Relative abundances of P3, HL, and P6 among diamond samples can be understood in terms of thermal processing of a single mixture of diamonds like those now found in CI and CM2 chondrites. With relatively gentle heating, primitive diamonds first lose their low-temperature P3 gases and a “labile” fraction of the HL component. Mass loss associated with release of these components produces an increase in the HL and P6 content of the remaining diamond relative to unprocessed diamond. Higher temperatures initiate destruction of the main HL carrier, while the HL content of the surviving diamonds remains essentially constant. At the same time, the P6 carrier begins to preferentially lose light noble gases. Meteorites that have experienced metamorphic temperatures ?650 °C have lost essentially all of their presolar diamond through chemical reactions with surrounding minerals. The P3 abundance seems to be a function only of the maximum temperature experienced by the diamonds and thus is independent of the nature of the surrounding environment. If all classes inherited the same mixture of primitive diamonds, then P3 abundances would tie together the metamorphic scales in different meteorite classes. However, if the P3 abundance indicates a higher temperature than do other thermometers applicable to the host meteorite, then the P3 abundance may contain information about heating prior to accretion. Diamonds in the least metamorphosed EH, CV, and CO chondrites seem to carry a record of pre-accretionary thermal processing.     

Terrestrial and Martian weathering signatures of xenon components in shergottite mineral separates

Abstract– Xenon‐isotopic ratios, step‐heating release patterns, and gas concentrations of mineral separates from Martian shergottites Roberts Massif (RBT) 04262, Dar al Gani (DaG) 489, Shergotty, and Elephant Moraine (EET) 79001 lithology B are reported. Concentrations of Martian atmospheric xenon are similar in mineral separates from all meteorites, but more weathered samples contain more terrestrial atmospheric xenon. The distributions of xenon from the Martian and terrestrial atmospheres among minerals in any one sample are similar, suggesting similarities in the processes by which they were acquired. However, in opaque and maskelynite fractions, Martian atmospheric xenon is released at higher temperatures than terrestrial atmospheric xenon. It is suggested that both Martian and terrestrial atmospheric xenon were initially introduced by weathering (low temperature alteration processes). However, the Martian component was redistributed by shock, accounting for its current residence in more retentive sites. The presence or absence of detectable ¹²⁹Xe from the Martian atmosphere in mafic minerals may correspond to the extent of crustal contamination of the rock’s parent melt. Variable contents of excess ¹²⁹Xe contrast with previously reported consistent concentrations of excess ⁴⁰Ar, suggesting distinct sources contributed these gases to the parent magma.     

Noble gases in planetary atmospheres: Implications for the origin and evolution of atmospheres

The radiogenic and primordial noble gas content of the atmospheres of Venus, Earth, and Mars are compared with one another and with the noble gas content of other extraterrestial samples, especially meteorites. The fourfold depletion of ⁴⁰Ar for Venus relative to the Earth is attributed to the outgassing rates and associated tectonics and volcanic styles for the two planets diverging significantly within the first billion or so years of their history, with the outgassing rate for Venus becoming much less than that for the Earth at subsequent times. This early divergence in the tectonic style of the two planets may be due to a corresponding early onset of the runaway greenhouse on Venus. The 16-fold depletion of ⁴⁰Ar for Mars relative to the Earth may be due to a combination of a mild K depletion for Mars, a smaller fraction of its interior being outgassed, and to an early reduction in its outgassing rate. Venus has lost virtually all of its primordial He and some of its radiogenic He. The escape flux of He may have been quite substantial in Venus' early history, but much diminished at later times, with this time variation being perhaps strongly influenced by massive losses of H₂ resulting from efficient H₂O loss processes.Key trends in the primordial noble gas content of terrestial planetary atmospheres include (1) a several orders of magnitude decrease in ²⁰Ne and ³⁶Ar from Venus to Earth to Mars; (2) a nearly constant ²⁰Ne/³⁶Ar ratio which is comparable to that found in the more primitive carbonaceous chondrites and which is two orders of magnitude smaller than the solar ratio; (3) a sizable fractionation of Ar, Kr, and Xe from their solar ratios, although the degree of fractionation, especially for ³⁶Ar/¹³²Xe, seems to decrease systematically from carbonaceous chondrites to Mars to Earth to Venus; and (4) large differences in Ne and Xe isotopic ratios among Earth, meteorites, and the Sun. Explaining trends (2), (2) and (4), and (1) pose the biggest problems for the solar-wind implantation, primitive atmosphere, and late veneer hypotheses, respectively. It is suggested that the grain-accretion hypothesis can explain all four trends, although the assumptions needed to achieve this agreement are far from proven. In particular, trends (1), (2), (3), and (4) are attributed to large pressure but small temperature differences in various regions of the inner solar system at the times of noble gas incorporation by host phases; similar proportions of the host phases that incorporated most of the He and Ne on the one hand (X) and Ar, Kr, and Xe on the other hand (Q); a decrease in the degree of fractionation with increasing noble-gas partial pressure; and the presence of interstellar carriers containing isotopically anomalous noble gases.Our analysis also suggests that primordial noble gases were incorporated throughout the interior of the outer terrestial planets, i.e., homogeneous accretion is favored over inhomogeneous accretion. In accord with meteorite data, we propose that carbonaceous materials were key hosts for the primordial noble gases incorporated into planets and that they provided a major source of the planets' CO₂ and N₂.     

Noble gases in enstatite chondrites released by stepped crushing and heating

Abstract– Enstatite chondrites (ECs) were subjected to noble gas analyses using stepped crushing and pyrolysis extraction methods. ECs can be classified into subsolar gas‐carrying and subsolar gas‐free ECs based on the ³⁶Ar/⁸⁴Kr/¹³²Xe ratios. For subsolar gas‐free ECs, elemental ratios, and Xe isotopic compositions indicate that Q gas is the dominant trapped component, the Q gas concentration can be correlated with the petrologic type, reasonably explained by gas release from a common EC parental material during subsequent heating. Atmospheric Xe with sub‐Q elemental ratios is found in Antarctic E3s at 600–800 °C and through crushing. The ¹³²Xe released in these fractions accounts for 30–60% of the bulk concentrations. Hence, the sub‐Q signature is generally due to contamination of elementally fractionated atmosphere. Subsolar gas is mainly released (up to 78% of the bulk ³⁶Ar) at 1300–1600 °C and through crushing, suggesting that enstatite and friable phases are the host phases. Subsolar gas is isotopically identical to solar gas, but elementally fractionated. These observations are consistent with a previous study, which suggested that subsolar gas could be fractionated solar wind having been implanted into chondrule precursors ( Okazaki et al. 2001 ). Unlike subsolar gas‐free ECs, the primordial gas concentrations of subsolar gas‐carrying ECs are not simply correlated with the petrologic type. It is inferred that subsolar gas‐rich chondrules were heterogeneously distributed in the solar nebula and accreted to form subsolar gas‐carrying ECs. Subsequent metamorphic and impact‐shock heating events have affected noble gas compositions to various degrees.     

Noble gases in Muong Nong‐type tektites and their implications

Abstract— We have the elemental abundances and isotopic compositions of noble gases in Muong Nong‐type tektites from the Australasian strewn field by crushing and by total fusion of the samples. We found that the abundances of the heavy noble gases are significantly enriched in Muong Nong‐type tektites compared to those in normal splash‐form tektites from the same strewn field. Neon enrichments were also observed in the Muong Nong‐type tektites, but the Ne/Ar ratios were lower than those in splash‐form tektites because of the higher Ar contents in the former. The absolute concentrations of the heavy noble gases in Muong Nong‐type tektites are similar to those in impact glasses. The isotopic ratios of the noble gases in Muong Nong‐type tektites are mostly identical to those in air, except for the presence of radiogenic ⁴⁰Ar. The obtained K‐Ar ages for Muong Nong‐type tektites were about 0.7 Myr, similar to ages of other Australasian tektites. The crushing experiments suggest that the noble gases in the Muong Nong‐type tektites reside mostly in vesicles, although Xe was largely affected by adsorbed atmosphere after crushing. We used the partial pressure of the heavy noble gases in vesicles to estimate the barometric pressure in the vesicles of the Muong Nong‐type tektites. Likely, Muong Nong‐type tektites solidified at the altitude (between the surface and a maximum height of 8–30 km) lower than that for splash‐form tektites.