Noble gases in ureilites released by crushing

Abstract— Noble gases in two ureilites, Kenna and Allan Hills (ALH) 78019, were measured with two extraction methods: mechanical crushing in a vacuum and heating. Large amounts of noble gases were released by crushing, up to 26.5% of ¹³²Xe from ALH 78019 relative to the bulk concentration. Isotopic ratios of the crush‐released Ne of ALH 78019 resemble those of the trapped Ne components determined for some ureilites or terrestrial atmosphere, while the crush‐released He and Ne from Kenna are mostly cosmogenic. The crush‐released Xe of ALH 78019 and Kenna is similar in isotopic composition to Q gas, which indicates that the crush‐released noble gases are indigenous and not caused by contamination from terrestrial atmosphere. In contrast to the similarities in isotopic composition with the bulk samples, light elements in the crush‐released noble gases are depleted relative to Xe and distinct from those of each bulk sample. This depletion is prominent especially in the ²⁰Ne/¹³²Xe ratio of ALH 78019 and the ³⁶Ar/¹³²Xe ratio of Kenna. The values of measured ³He/²¹Ne for the gases released by crushing are significantly higher than those for heating‐released gases. This suggests that host phases of the crush‐released gases might be carbonaceous because cosmogenic Ne is produced mainly from elements with a mass number larger than Ne. Based on our optical microscopic observation, tabular‐foliated graphite is the major carbon mineral in ALH 78019, while Kenna contains abundant polycrystalline graphite aggregates and diamonds along with minor foliated graphite. There are many inclusions at the edge and within the interior of olivine grains that are reduced by carbonaceous material. Gaps can be seen at the boundary between carbonaceous material and silicates. Considering these petrologic and noble gas features, we infer that possible host phases of crush‐released noble gases are graphite, inclusions in reduction rims, and gaps between carbonaceous materials and silicates. The elemental ratios of noble gases released by crushing can be explained by fractionation, assuming that the starting noble gas composition is the same as that of amorphous carbon in ALH 78019. The crush‐released noble gases are the minor part of trapped noble gases in ureilites but could be an important clue to the thermal history of the ureilite parent body. Further investigation is needed to identify the host phases of the crush‐released noble gases.     

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 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.     

Occurrence and implications of silicon nitride in enstatite chondrites

Abstract— Silicon nitride, Si₃N₄, has previously been observed to be a common constituent of acid residues of Qingzhen (EH3) and Indarch (EH4). Ion probe analysis of the Si, N and C isotopic compositions of individual Si₃N₄ grains from Qingzhen and Indarch acid residues suggest most, if not all, grains are Solar System in origin. A few grains have isotopically anomalous C but this is probably due to small presolar SiC grains adhering to them. In situ observations of the Si₃N₄ in Qingzhen show that it is only present within, and probably exsolved from, host phases which contain elemental Si in solid solution. Thermodynamic calculations suggest that the Si₃N₄ probably formed during metamorphism and not in the nebula. Thermodynamic calculations also show that sinoite (Si₂N₂O) and not Si₃N₄ should be the stable phase during metamorphism. It appears that kinetic factors must have inhibited the formation of sinoite in Qingzhen and Indarch.     

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.     

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.     

Manganese-chromium systematics in sulfides of unequilibrated enstatite chondrites

Abstract— We measured with a secondary ion mass spectrometer Mn/Cr ratios and Cr isotopes in individual grains of Mn-bearing sulfides (i.e., sphalerites, ZnS; alabandites, MnS; and niningerites, MgS) in nine unequilibrated enstatite chondrites (UECs). The goals were to determine whether live ⁵³Mn (half-life ～3.7 Ma) was incorporated in these objects at the time of their isotopic closure and to establish whether Mn-Cr systematics in sulfides in UECs can be used as a high-resolution chronometer to constrain formation time differences between these meteorites. Sulfide grains analysed in four of these UECs, MAC 88136 (EL3), MAC 88184 (EL3), MAC 88180 (EL3), and Indarch (EH4), have clear ⁵³Cr excesses. These ⁵³Cr excesses can be very large (δ⁵³Cr/⁵²Cr ranges up to ～18,400%, the largest ⁵³Cr excess measured so far) and, in some grains, are well correlated with the Mn/Cr ratios. Thus, they were most likely produced by the in situ decay of ⁵³Mn in the meteorite samples. In the remaining five meteorites, no detectable excesses of ⁵³Cr were found, and only upper limits on the initial ⁵³Mn/⁵⁵Mn ratios could be established. The four meteorites with ⁵³Cr excesses show variations in the inferred ⁵³Mn/⁵⁵Mn ratios in various sulfide grains of the same meteorite. The Mn-Cr systematics in these sulfides were disturbed (during and/or after the decay of ⁵³Mn) by varying degrees of reequilibration. Provided ⁵³Mn was homogeneously distributed in the region of the early solar system where these objects formed, the data suggest that the time of the last isotopic equilibration of sulfides in EL chondrites occurred at least 3 Ma after a similar episode in EH chondrites.     

The evolution of enstatite and chondrules in unequilibrated enstatite chondrites: Evidence from iron-rich pyroxene

Abstract— FeO-rich (Fs^6–34) pyroxene lacking cathodoluminescence (CL), hereafter black pyroxene, is a major constituent of some of the chondrules and fragments in unequilibrated (type 3) enstatite chondrites (UECs). It contains structurally oriented zones of Cr-, Mn-, V-rich, FeO-poor enstatite with red CL, associated with mm-sized blebs of low-Ni, Fe-metal and, in some cases, silica. These occurrences represent clear evidence of pyroxene reduction. The black pyroxene is nearly always rimmed by minor element (Cr, Mn, V)-poor enstatite having a blue CL. More commonly, red and blue enstatites, unassociated with black pyroxene, occur as larger grains in chondrules and fragments, and these constitute the major silicate phases in UECs. The REE abundance patterns of the black pyroxene are LREE-depleted. The blue enstatite rims, however, have a near-flat to LREE-enriched pattern, ～0.5–4x chondritic. The petrologic and trace element data indicate that the black pyroxene is from an earlier generation of chondrules that formed in a nebular region that was more oxidizing than that of the enstatite chondrites. Following solidification, these chondrules experienced a more reducing nebular environment and underwent reduction. Some, perhaps most, of the red enstatite that is common throughout the UECs may be the product of solid-state reduction of black pyroxene. The blue enstatite rims grew onto the surfaces of the black pyroxene and red enstatite as a result of condensation from a nebular gas. The evolutionary history of some of the enstatite and chondrules in enstatite chondrites can be expressed in a four-stage model that includes: Stage 1. Formation of chondrules in an oxidizing nebular environment Stage 2. Solid-state reduction of the more oxidized chondrules and fragments to red enstatite in a more reducing nebular environment Stage 3. Formation of blue enstatite rims on the black pyroxene as well as on the red enstatite. Stage 4. Reprocessing, by various degrees of melting, of many of the earlier-formed materials.     

Lewis Cliff 87223, an anomalous enstatite chondrite and the diversity of enstatite chondrites

We studied a thin section of Lewis Cliff (LEW) 87223, an unusual EL3-related, enstatite chondrite (EC) that has primary and secondary features not observed in other ECs. We studied its metal-rich nodules, possible shock features, and chondrules, eight of which are Al-rich chondrules (ARCs). LEW 87223 has petrologic and compositional features similar to EL3s. Enstatite is the dominant mineral; chondrule boundaries are well defined; Si content of metal (0.5–0.6 wt%) is consistent with typical EL3; it has Cr-bearing troilite, oldhamite, and alabandite; and its O-isotopic composition is similar to other ECs. However, metal abundance in LEW 87223 (~13 vol%) is slightly higher than in other EL3s and its metal nodules are texturally and mineralogically different from other ECs. Both high and low Ni metals are present, and its alabandite has higher Fe (27.8 wt% Fe) than in other EL3s. Silicates appear darkened in plane polarized light, largely due to reduction of Fe from silicate. A remarkable feature of LEW 87223 is the high abundance of ARCs, which contain Ca-rich plagioclase and varying amounts of Na-rich plagioclase along chondrule edges and as veins. This suggests Na metasomatism and the possibility of hydrothermal fluids, potentially related to an impact event. LEW 87223 expands the range of known EC material. It shows that ECs are more diverse and record a wider range of parent body processes than previously known. LEW 87223 is an anomalous EL3, potentially the first member of a new EC group should similar samples be discovered.     

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 and meteorites

Abstract— Noble gases repeatedly have served to widen the scope of meteorite research. During the first half century of such measurements, the emphasis was on the determination of U, Th/He-gas retention ages of iron meteorites, which is the most unsuitable class of meteorites for such studies. With the realization that the He in these meteorites results from the interaction of cosmic rays with meteoritic matter, meteorites became to be used as “the poor man's space probe” that yielded information on the constancy in time and space of the cosmic radiation. Another widening of scope came with the discovery of extremely high noble gas contents in the outermost layers of the individual grains that make up stony meteorites. These gases are of solar origin; they have been implanted as low-energy solar wind (SW) or as solar energetic particles (SEP) into the grains before their compaction. Presently they offer the only opportunity to precisely measure the isotopic composition of solar matter and to learn about potential changes of the Sun in time. Stony meteorites of the “carbonaceous” variety contain “stardust” that carries the undiluted nucleosynthesis products of individual stars that yield incredibly detailed information concerning the parameters that prevailed during the synthesis.     

Petrogenesis of anomalous Queen Alexandra Range enstatite meteorites and their relation to enstatite chondrites,primitive enstatite achondrites,and aubrites

Queen Alexandra Range (QUE) meteorite 94204 is an anomalous enstatite meteorite whose petrogenesis has been ascribed to either partial melting or impact melting. We studied the meteorite pairs QUE 94204, 97289/97348, 99059/99122/99157/99158/99387, and Yamato (Y)‐793225; these were previously suggested to represent a new grouplet. We present new data for mineral abundances, mineral chemistries, and siderophile trace element compositions (of Fe,Ni metal) in these meteorites. We find that the texture and composition of Y‐793225 are related to EL6, and that this meteorite is unrelated to the QUEs. The mineralogy and siderophile element compositions of the QUEs are consistent with petrogenesis from an enstatite chondrite precursor. We caution that potential re‐equilibration during melting and recrystallization of enstatite chondrite melt‐rocks make it unreliable to use mineral chemistries to assign a specific parent body affinity (i.e., EH or EL). The QUEs have similar mineral chemistries among themselves, while slight variations in texture and modal abundances exist between them. They are dominated by inclusion‐bearing millimeter‐sized enstatite (average En_99.1–99.5) with interstitial spaces filled predominantly by oligoclase feldspar (sometimes zoned), kamacite (Si approximately 2.4 wt%), troilite (≤2.4 wt% Ti), and cristobalite. Siderophile elements that partition compatibly between solid metal and liquid metal are not enriched like in partial melt residues Itqiy and Northwest Africa (NWA) 2526. We find that the modal compositions of the QUEs are broadly unfractionated with respect to enstatite chondrites. We conclude that a petrogenesis by impact melting, not partial melting, is most consistent with our observations.     

On the significance of diopside and oldhamite in enstatite chondrites and aubrites

Abstract— Enstatite is the primary silicate phase of equilibrated enstatite chondrites (EECs). The CaO contents of these enstatites lie close to or on the enstatite-diopside phase boundary, yet, curiously, diopside has always been absent from EEC assemblages. In contrast, aubrites contain abundant diopside even though they are thought to be derived from an E chondrite-like protolith. A phase equilibrium analysis of the Ca-Mg-Fe-Mn-Si-O-S system under reducing conditions solves this enigma and shows that diopside-bearing EECs should commonly be found. When S fugacity is sufficiently high (e.g., Fe-FeS buffer), low O fugacity limits the stability of diopside in favor of oldhamite. Under such conditions, the relative stability of diopside and oldhamite is described by the reaction: CaMgSi₂O₆ + MgS = CaS + Mg₂Si₂O₆ A large bulk compositional field exists where diopside and oldhamite are simultaneously stable. The existence of oldhamite does not preclude the stability of diopside. Phase diagram topology demonstrates that bulk compositions lying in the enstatite-oldhamite field and enstatite-oldhamite-alabandite field have enstatite CaO contents nearly identical to that of enstatite in equilibrium with diopside alone. This explains the high enstatite CaO contents of all EECs that do not contain diopside. This study also reports the discovery of the first EEC to contain metamorphic diopside, the Antarctic meteorite EET 90102. Elephant Moraine 90102 has a typical EL6 texture and contains the assemblage: enstatite, diopside, albite, kamacite, troilite, sinoite, and graphite. Trace quantities of alabandite, oldhamite and daubreelite are also present. Diopside is stable in EET 90102 because its bulk composition lies within either the enstatite-diopside-oldhamite-alabandite or diopside-alabandite-enstatite stability fields. In contrast, all other EECs analyzed to date have bulk compositions lying in the enstatite-oldhamite-alabandite stability field. The discovery of diopside in EET 90102 helps confirm the predictions of the phase equilibrium analysis. Elephant Moraine 90102 experienced a high-temperature metamorphic equilibration from which it was quenched. The enstatite-diopside, CaS in alabandite and Fe in alabandite, geothermometers yield temperatures of last equilibration of ～900 °C. The absence of daubreelite and schreibersite along with high troilite Cr contents and high kamacite P contents confirm a high-temperature metamorphic quench. The EET 90102 chondrite experienced a somewhat different cooling history and has a slightly different bulk composition than all other EECs studied to date; however, the close mineralogic, petrologic and textural similarities between EET 90102 and nominal EL6 chondrites signify that it should be classified as a diopside- and sinoite-bearing EL6 chondrite. Assuming that the aubrites formed from an E chondrite-like protolith, a source rock similar to that of a diopside-bearing EEC offers a clear advantage for aubrite formation. Melting of a diopside-saturated EEC protolith would not require conversion of CaS to achieve diopside-saturation upon cooling.     

A condensation model for the formation of chondrules in enstatite chondrites

Abstract— It is proposed that the chondrules in enstatite chondrites formed near the Sun from rain‐like supercooled liquid silicate droplets and condensed Fe‐Ni alloys in thermodynamic equilibrium with a slowly cooling nebula. FeO formed and dissolved in the droplets in an initial stage when the nucleation of iron was blocked, and was later mostly reduced to unalloyed Fe. At high temperatures, the silicate droplets contained high concentrations of the less volatile components CaO and Al₂O₃. At somewhat lower temperatures the equilibrium MgO content of the droplets was relatively high. As cooling progressed, some droplets gravitated toward the Sun, and moved in other directions, depleting the region in CaO, Al₂O₃, and MgO and accounting for the relatively low observed CaO/SiO₂, Al₂O₃/SiO₂, and MgO/SiO₂ ratios in enstatite chondrites. At approximately 1400 K, the remaining supercooled silicate droplets crystallized to form MgSiO₃ (enstatite) with small amounts of olivine and a high‐SiO₂ liquid phase which became the mesostases. The high enstatite content is the result of the supercooled chondrules crystallizing at a relatively low temperature and relatively high total pressure. Finally, FeS formed at temperatures below 680 K by reaction of the condensed Fe with H₂S. All calculations were performed with the evaluated optimized thermodynamic databases of the FactSage thermodynamic computer system. The thermodynamic properties of compounds and solutions in these databases were optimized completely independently of any meteoritic data. Agreement of the model with observed bulk and phase compositions of enstatite chondrules is very good and is generally within experimental error limits for all components and phases.     

The thermometry of enstatite chondrites: A brief review and update

Abstract— Due to the discoveries in Antarctica, the number of known enstatite chondrites has doubled in the last few years, and many rare or previously unknown types have been collected, most notably many EL3 and EH3 chondrites. We have applied the five major enstatite chondrite thermometers to the new and previously known enstatite chondrites, the thermometers being: (1) kamacite-quartz-enstatite-oldhamite-troilite (KQEOT), (2) oldhamite, (3) alabandite-niningerite, (4) sphalerite, and (5) phosphide-metal. Measured temperatures based on the KQEOT and oldhamite systems are 800 °C-1000 °C with the type 3 enstatite chondrites having values similar to those of type 4–6. It seems likely that these temperatures relate to events prior to parent body metamorphism, such as nebula condensation or chondrule formation, and were not significantly reset by later events. Measured temperatures for alabandite-niningerite, metal-phosphide and sphalerite in EH chondrites increase from 300 °C-400 °C to 600 °C-800 °C with petrographic indications of increasing metamorphism. In contrast, measured temperatures for all EL chondrites, including the most heavily metamorphosed, are generally <400 °C. Apparently EL chondrites cooled more slowly than the EH chondrites regardless of metamorphism experienced. Measured temperatures for the alabandite-niningerite, metal-phosphide and sphalerite are actually closure temperatures for the last thermal event suffered by the meteorite, and the fast cooling rates indicated are most consistent with processes occurring in thick regoliths.     

Properties of chondrules in EL3 chondrites,comparison with EH3 chondrites,and the implications for the formation of enstatite chondrites

Abstract— The study of chondrules provides information about processes occurring in the early solar system. In order to ascertain to what extent these processes played a role in determining the properties of the enstatite chondrites, the physical and chemical properties of chondrules from three EL3 chondrites and three EH3 chondrites have been examined by optical, cathodoluminescence (CL), and electron microprobe techniques. Properties examined include size, texture, CL, and composition of both individual phases and bulk chondrules. The textures, distribution of textures, and composition of silicates of the EL3 chondrules resemble those of EH3 chondrules. However, the chondrules from the two classes differ in that (1) the size distribution of the EL chondrules is skewed to larger values than EH chondrules, (2) the enstatite in EL chondrules displays varying shades of red CL due to the presence of fine‐grained sulfides and metal in the silicates, and (3) the mesostasis of EH chondrules is enriched in Na relative to that of EL chondrules. The similarities between the chondrules of the two classes suggest similar precursor materials, while the differences suggest that there was not a single reservoir of meteoritic chondrules, but that their origin was fairly local. The differences in the size distribution of chondrules in EH and EL chondrites may be explained by aerodynamic and gravitational sorting during accumulation of the meteoric material, while differences in CL and mesostasis properties may reflect differences in formation conditions and cooling rate following chondrule formation. We argue that our observations are consistent with the formation of enstatite chondrites in a thick dynamic regolith on their parent body.     

Primordial noble gases in “phase Q” in carbonaceous and ordinary chondrites studied by closed‐system stepped etching

Abstract— The HF/HCI‐resistant residues of the chondrites CM2 Cold Bokkeveld, CV3 (ox.) Grosnaja, CO3.4 Lancé, CO3.7 Isna, LL3.4 Chainpur, and H3.7 Dimmitt have been measured by closed‐system stepped etching (CSSE) in order to better characterise the noble gases in “phase Q”, a major carrier of primordial noble gases. All isotopic ratios in phase Q of the different meteorites are quite uniform, except for (²⁰Ne/²²Ne)_Q. As already suggested by precise earlier measurements (Schelhaas et al., 1990; Wieler et al., 1991, 1992), (²⁰Ne/²²Ne)_Q is the least uniform isotopic ratio of the Q noble gases. The data cluster ～10.1 for Cold Bokkeveld and Lancé and 10.7 for Chainpur, Grosnaja, and Dimmitt, respectively. No correlation of (²⁰Ne/²²Ne)_Q with the classification or the alteration history of the meteorites has been found. The Ar, Kr, and Xe isotopic ratios for all six samples are identical within their uncertainties and similar to earlier Q determinations as well as to Ar‐Xe in ureilites. Thus, an unknown process probably accounts for the alteration of the originally incorporated Ne‐Q. The noble gas elemental compositions provide evidence that Q consists of at least two carbonaceous carrier phases “Q1” and “Q2” with slightly distinct chemical properties. Ratios (Ar/Xe)_Q and (Kr/Xe)_Q reflect both thermal metamorphism and aqueous alteration. These parent‐body processes have led to larger depletions of Ar and Kr relative to Xe. In contrast, meteorites that suffered severe aqueous alteration, such as the CM chondrites, do not show depletions of He and Ne relative to Ar but rather the highest (He/Ar)_Q and (Ne/Ar)_Q ratios. This suggests that Q1 is less susceptible to aqueous alteration than Q₂. Both subphases may well have incorporated noble gases from the same reservoir, as indicated by the nearly constant, though very large, depletion of the lighter noble gases relative to solar abundances. However, the elemental ratios show that Q₁ and Q₂ must have acquired (or lost) noble gases in slightly different element proportions. Cold Bokkeveld suggests that Q₁ may be related to presolar graphite. Phases Q₁ and Q₂ might be related to the subphases that have been suggested by Gros and Anders (1977). The distribution of the ²⁰Ne/²²Ne ratios cannot be attributed to the carriers Q₁ and Q₂. The residues of Chainpur and Cold Bokkeveld contain significant amounts of Ne‐E(L), and the data confirm the suggestion of Huss (1997) that the ²²Ne‐E(L) content, and thus the presolar graphite abundances, are correlated with the metamorphic history of the meteorites.     

Comparison of cosmic‐ray exposure ages and trapped noble gases in chondrule and matrix samples of ordinary,enstatite, and carbonaceous chondrites

Abstract— We performed a comprehensive study of the He, Ne, and Ar isotopic abundances and of the chemical composition of bulk material and components of the H chondrites Dhajala, Bath, Cullison, Grove Mountains 98004, Nadiabondi, Ogi, and Zag, of the L chondrites Grassland, Northwest Africa 055, Pavlograd, and Ladder Creek, of the E chondrite Indarch, and of the C chondrites Hammadah al Hamra 288, Acfer 059, and Allende. We discuss a procedure and necessary assumptions for the partitioning of measured data into cosmogenic, radiogenic, implanted, and indigenous noble gas components. For stone meteorites, we derive a cosmogenic ratio ²⁰Ne/²²Ne of 0.80 ± 0.03 and a trapped solar ⁴He/³He ratio of 3310 ± 130 using our own and literature data. Chondrules and matrix from nine meteorites were analyzed. Data from Dhajala chondrules suggest that some of these may have experienced precompaction irradiation by cosmic rays. The other chondrules and matrix samples yield consistent cosmic‐ray exposure (CRE) ages within experimental errors. Some CRE ages of some of the investigated meteorites fall into clusters typically observed for the respective meteorite groups. Only Bath's CRE age falls on the 7 Ma double‐peak of H chondrites, while Ogi's fits the 22 Ma peak. The studied chondrules contain trapped ²⁰Ne and ³⁶Ar concentrations in the range of 10^?6–10^?9 cm³ STP/g. In most chondrules, trapped Ar is of type Q (ordinary chondritic Ar), which suggests that this component is indigenous to the chondrule precursor material. The history of the Cullison chondrite is special in several respects: large fractions of both CR‐produced ³He and of radiogenic ⁴He were lost during or after parent body breakup, in the latter case possibly by solar heating at small perihelion distances. Furthermore, one of the matrix samples contains constituents with a regolith history on the parent body before compaction. It also contains trapped Ne with a ²⁰Ne/²²Ne ratio of 15.5 ± 0.5, apparently fractionated solar Ne.     

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.