Microscopic search for the carrier phase Q of the trapped planetary noble gases in Allende,Leoville and Vigarano

Abstract— High‐resolution transmission electron microscopy micrographs of acid‐resistant residues of the Allende, Leoville, and Vigarano meteorites show a great variety of carbon structures: curved and frequently twisted and intertwined graphene sheets, abundant carbon black‐like particles, and hollow “sacs”. It is suggested that perhaps all of these are carriers for the planetary Q‐noble gases in these meteorites. Most of these materials are pyrocarbons that probably formed by the pyrolysis of hydrocarbons either in a gas phase, or on hot surfaces of minerals. An attempt was made to analyze for argon with particle‐induced x‐ray emission in 143 spots of grains of floating and suspended matter from freeze‐dry cycles of an Allende bulk sample in water, and floating “black balls” from sonication in water of samples from the Allende meteorite. The chemical compositions of these particles were obtained, but x‐ray signals at the wavelength of argon were obtained on only a few spots.     

On unfractionated solar noble gases in the H3–6 meteorite Acfer 111

Abstract Solar noble gases He, Ne, Ar and Kr implanted in the H3–6 meteorite regolith breccia Acfer 111 agree in their elemental composition with that in present-day solar wind and, except for a 25% deficit of ⁴He, also with adopted solar abundances. The presence of such unfractionated solar gases makes Acfer 111 unique (until now). Closed system stepped etching releases noble gases that can be explained as mixtures of two distinct types of He, Ne, and Kr of isotopic compositions as they have been derived previously from meteorites and lunar samples that contain heavily fractionated solar gases. Since the same putative end members, ascribed to the solar wind (SW) and supra-thermal solar energetic particles (SEP), are also present in Acfer 111, we argue that these end members represent two truly independent components. We discount the possibility that one isotopic composition derived from the other by diffusion of the gases within, or upon their release from, their host phases. The isotopic signatures of noble gases in Acfer 111 agree with those in a lunar ilmenite of young antiquity ?100 Ma) but are in disagreement with the noble gases in lunar ilmenite 79035 of 1–2 Ga antiquity. Systematic changes are discussed of the nuclide abundance ratios as etching proceeds; they are ascribed to differences in trapping efficiency and in penetration depth of the different noble gas ion species upon their implantation.     

Abundance and isotopic composition of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite

Abstract— Abundances and isotopic compositions of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite were measured. The abundance ratios of spallogenic components in metal reveal a ³He deficiency which is due to the diffusive loss of parent isotopes, that is, tritium (Tilles, 1963; Schultz, 1967). The diffusive loss likely has been induced by thermal heating by the Sun during cosmic‐ray exposure (～160 Ma; Lavielle et al, 1999). Thermal process such as impact‐induced partial loss may have affected the isotopic composition of spallogenic Ne. The ¹²⁹Xe/¹³¹Xe ratio of cosmogenic components in the metal indicates an enhanced production of epi‐thermal neutrons. The abundance ratios of spallogenic components in the graphite reveal that it contained small amounts of metal and silicates. The isotopic composition of heavy noble gases in graphite itself was obtained from graphite treated with HF/HCl. The isotopic composition of the etched graphite shows that it contains two types of primordial Xe (i.e., Q‐Xe and El Taco Xe). The isotopic heterogeneity preserved in the Bohumilitz graphite indicates that the Bohumilitz graphite did not experience any high‐temperature event and, consequently, must have been emplaced into the metal at subsolidus temperatures. This situation is incompatible with an igneous model as well as the impact melting models for the IAB‐IIICD iron meteorites as proposed by Choi et al. (1995) and Wasson et al (1980).     

Primordial noble gases in a graphite‐metal inclusion from the Canyon Diablo IAB iron meteorite and their implications

Abstract— We have carried out noble gas measurements on graphite from a large graphite‐metal inclusion in Canyon Diablo. The Ne data of the low‐temperature fractions lie on the mixing line between air and the spallogenic component, but those of high temperatures seem to lie on the mixing line between Ne‐HL and the spallogenic component. The Ar isotope data indicate the presence of Q in addition to air, spallogenic component and Ar‐HL. As the elemental concentration of Ne in Q is low, we could not detect the Ne‐Q from the Ne data. On the other hand, we could not observe Xe‐HL in our Xe data. As the Xe concentration and the Xe/Ne ratio in Q is much higher than that in the HL component, it is likely that only the contribution of Q is observed in the Xe data. Xenon isotopic data can be explained as a mixture of Q, air, and “El Taco Xe.” The Canyon Diablo graphite contains both HL and Q, very much like carbonaceous chondrites, retaining the signatures of various primordial noble gas components. This indicates that the graphite was formed in a primitive nebular environment and was not heated to high, igneous temperatures. Furthermore, a large excess of ¹²⁹Xe was observed, which indicates that the graphite was formed at a very early stage of the solar system when ¹²⁹I was still present. The HL/Q ratios in the graphite in Canyon Diablo are lower than those in carbonaceous chondrites, indicating that some thermal metamorphism occurred on the former. We estimated the temperature of the thermal metamorphism to about 500–600 °C from the difference of thermal retentivities of HL and Q. It is also noted that “El Taco Xe” is commonly observed in many IAB iron meteorites, but its presence in carbonaceous chondrites has not yet been established.     

Mineralogy of a refractory inclusion in the Allende (C3V) meteorite

Abstract— A spherical, 220-μm diameter, spinel-hibonite-perovskite inclusion from the Allende C3V meteorite contains a central hibonite cluster with an angular boundary. This central hibonite is enclosed within spinel that is zoned from Mg-rich at the hibonite boundary to more Fe-rich at the inclusion boundary. This spinel zone includes lath-shaped hibonites usually oriented subradial to the central hibonites. Two textural types of perovskites are present as exsolution from the central hibonite and as equidimensional grains within both the central hibonite and spinel. These second perovskites have exsolution lamellae of Al₂O₃. Within the central hibonite and adjacent to some equidimensional perovskites, a fine porous phase interpreted as alteration has a composition of nearly pure Al₂O₃ with minor amounts of Na and Si. This is possibly either an intergrowth of corundum and nepheline or a modified Al₂O₃, β-alumina. The central hibonites and equidimensional perovskites are considered relict grains on which the spinel-hibonite layer crystallized. The relict material had undergone slow cooling in a previous event to produce exsolution of original high-temperature compositions. Later alteration caused breakdown of hibonite to give an Al₂O₃-rich phase. This inclusion represents a composite body which formed in a Ca-Al-rich environment.     

Transmission electron microscopy of a refractory inclusion from the Allende meteorite: Anatomy of a pyroxene

Abstract— A crystal of clinopyroxene from the coarse-grained refractory inclusion Egg 6 of the Allende meteorite has been studied in detail by transmission electron microscopy. The pyroxene crystal contains euhedral, dislocation-free inclusions of pure spinel MgAl₂O₄, without any topotactic relation to the host. Extensive dislocation walls at equilibrium, characteristic of high-temperature anneal, are present in the crystal. Alteration products are occasionally observed at the spinel-pyroxene interface close to regions where dislocation walls decorated with bubbles (or voids) are present. The bubbles, often in the shape of tubes along the dislocation lines, are thought to be due to the precipitation of a fluid migrating along the dislocations. The observations are compatible with crystallization of the refractory inclusions from the melt and with the existence of a later stage of metasomatism.     

Rare-earth elements in matrix,inclusions, and chondrules of the Allende meteorite

Rare-earth elements in a whole-rock sample and in major components of the Allende meteorite were investigated; for a few samples, abundances of Ba, Sr, Ca, and Al were also determined. Of the materials investigated in the present work, CaAl-rich inclusions G and O seem to be of the greatest significance. In spite of the minor difference in mineralogy between them, the apparent chondrite-normalized RE pattern is much different between these two inclusions. (Yb and Eu in inclusion G appear exceptionally irregular). This observation is inferred to reflect a rather subtle difference in condition of condensation. It is also worthwhile to note that, while two portions (pink and white) of the inclusion G show similar aspects in the abundances of lithophile trace elements investigated, they show a remarkable difference at the same time. The white portion (G_w) of inclusion G can be considered to be a mixture of chondritic material and highly fractionated material like the faintly pink portion (G_p) picked from the same inclusion. This would suggest the possibility that the G_p-like material was produced from chondritic dust.The “matrix” separated from Allende was found to be fractionated with respect to the RE abundances relative to representative chondrite. It has also a very high value for the Ba abundance.     

Noble gases in individual chondrules of the Allende CV3 chondrite

We analyzed noble gases in nine individual chondrules, an assemblage of small chondrules, and four whole‐rock samples of the Allende CV3 chondrite. Major elements were also determined for five chondrules. The cosmic ray exposure ages are calculated from cosmogenic ³He to be 5.17 ± 0.38 and 5.15 ± 0.25 Myr for the averages of the chondrules and whole rocks, respectively, showing no significant pre‐exposure evidence for the studied chondrules. Large amounts of ³⁶Ar, ^80,82Kr, and ¹²⁸Xe produced by neutron capture are observed in most samples; the abundances of these nuclides are correlated among the samples. The epithermal neutron flux and neutron slowing down density are calculated based on [⁸⁰Kr]_n, from which a sample depth of about 30 cm can be calculated. The measured chondrules contain variable amounts of radiogenic ¹²⁹Xe. The abundance ratios of radiogenic ¹²⁹Xe to neutron capture–produced ¹²⁸Xe are rather constant among the studied chondrules; four chondrules give more precise ratios at the high‐temperature fractions, ranging from 1920 ± 80 to 2280 ± 140, which corresponds to a time difference of 3.9 ± 2.4 Myr. It is noticeable that most chondrules also contain ²⁴⁴Pu‐derived fission Xe. The average ²⁴⁴Pu/²³⁸U ratio for nine chondrules is 0.0069 ± 0.0018, which agrees well with the preferred ratio reported for chondrites.     

Birth of the presolar nebula: The sequence of condensation revealed in the Allende meteorite

This work applies the well-known supernova-trigger hypothesis for solar system formation to explain in detail many properties of the Allende meteorite. The Allende carbonaceous chondrite meteorite is an assemblage of millimetre- to centimetre-sized Ca-Al-rich inclusions (CAI's), fine-grained alkali-rich spinel aggregates, amoeboid olivine aggregates, olivine chondrules and sulfide chondrules set in an extremely fine-grained black matrix. Detailed isotopic, chemical and textural properties show that these components formed in the above order as independent cosmic grains. Some CAI's containmicron-sized metal nuggets in which the normally incompatible refractory (Mo, Re, W) and platinum group (Pt, Os, Ir, Ru) metals are alloyed together in approximately cosmic proportions, suggesting that these nuggets also condensed as cosmic grains.From the consistent pattern of enclosure of earlier components on the above list within later ones, it appears that in the environment where these materials formed, condensation moved inexorably in the direction of increasing olivine and decreasing refractory element and¹⁶O content (from 4% excess¹⁶O to normal terrestrial oxygen isotopic composition). Condensation sequences are all short and incomplete, from which it is concluded that condensing materials were soon separated from the condensing environment and isolated until all were brought together in a final snowstorm of fine-grained, olivine crystals constituting the meteorite matrix.These major properties can be accounted for in a model in which a supernova remnant (SNR) in the snowplow phase, whose oxygen was initially pure¹⁶O, pushes into a dark interstellar cloud. In the model, condensation of CAI's begins in the SNR shell when it has been diluted with 2500 times its mass of matter from the cloud, which also in part explains the rarity of observed isotopic anomalies in CAI's. The retardation of the SNR by the cloud propels condensed grains ahead toward the cloud under their own momentum. Continuing dilution by the cloud and continuing removal of the most refractory elements in grains can explain the evolving patterns of fractionation and depletion of refractory elements, including REE's, in successive condensates. Features such as rims on CAI's and concentric zonation of fine-grained aggregates can also be satisfied in the model. A presolar origin and a short ( 10 000 years) formation time for inclusions in carbonaceous chondrites are major implications of the model.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

REE abundances in the matrix of the Allende (CV) meteorite: Implications for matrix origin

Abstract— The trace element distributions in the matrix of primitive chondrites were examined using four least‐contaminated matrix specimens from the polished sections of the Allende (CV) meteorite. Analysis of rare earth element (REE), Ba, Sr, Rb, and K abundances by isotope dilution mass spectrometry revealed that the elemental abundances of lithophile elements except for alkali metals (K, Rb) in the specimens of the Allende matrix studied here are nearly CI (carbonaceous Orgueil) chondritic (～1 × CI). Compared to refractory elements, all the matrix samples exhibited systematic depletion of the moderately volatile elements K and Rb (0.1–0.5 × CI). We suggest that the matrix precursor material did not carry significant amounts of alkali metals or that the alkalis were removed from the matrix precursor material during the parent body process and/or before matrix formation and accretion. The matrix specimens displayed slightly fractionated REE abundance patterns with positive Ce anomalies (CI‐normalized La/Yb ratio = 1.32–1.65; Ce/Ce* = 1.16–1.28; Eu/Eu* = 0.98–1.10). The REE features of the Allende matrix do not indicate a direct relationship with chondrules or calcium‐aluminum‐rich inclusions (CAIs), which in turn suggests that the matrix was not formed from materials produced by the breakage and disaggregation of the chondrules or CAIs. Therefore, we infer that the Allende matrix retains the REE features acquired during the condensation process in the nebula gas.     

Nitrogen and noble gases in a glass sample from the LEW88516 shergottite

Abstract— A glass separate from the LEW88516 shergottite was analyzed by step-wise combustion for N and noble gases to determine whether it contained trapped gas similar in composition to the martian atmosphere-like component previously observed in lithology C of EETA79001. Excesses of ⁴⁰Ar and ¹²⁹Xe were in fact observed in this glass, although the amounts of these excesses are ≤20% of those seen in the latter meteorite, and are comparable to the amounts seen in whole-rock analyses of LEW88516. The isotopic composition of N in LEW88516 does not show an enrichment in ¹⁵N commensurate with the amount of isotopically-heavy N expected from the noble gases excesses. One must posit some extreme assumptions about the nature of the N components present in LEW88516 in order to allow the presence of the trapped nitrogen component. Alternatively, the N has somehow been decoupled from the noble gases, and was either never present or has been lost.     

Noble gases in metal and schreibersite of the Acuña (IIIAB) iron meteorite

Abstract— We measured abundances and isotopic compositions of noble gases in metal and schreibersite of the Acuña (IIIAB) iron meteorite. The concentrations of noble gases in Acuña metal are very low compared to those reported so far for other iron meteorites. The isotopic ratios of He, Ne and Ar indicate that they are mostly of cosmogenic origin. Cosmogenic components are even present in Kr and Xe, which could not have been produced from Fe, Ni and P and are probably due to the spallation of trace elements of higher masses. The high ⁴He/²¹Ne ratio of 420 in Acuña metal indicates that the samples were at a deep position within a very large meteoroid. The exposure ages of Acuña were estimated to be 50–200 Ma from ³He, ²¹Ne and ³⁸Ar abundances and by utilizing the diagrams of production rates vs. the ⁴He/²¹Ne ratio based on the Signer-Nier model. The low exposure age of Acuña may indicate a history different from that of other IIIAB irons whose exposure ages cluster at ～670 Ma. Otherwise, Acuña may be one of the samples with the low production rate, which can not be estimated from the diagrams of the Signer-Nier model.     

Petrography,classification, oxygen isotopes,noble gases,and cosmogenic records of Kamargaon (L6) meteorite: The latest fall in India

A single piece of meteorite fell on Kamargaon village in the state of Assam in India on November 13, 2015. Based on mineralogical, chemical, and oxygen isotope data, Kamargaon is classified as an L‐chondrite. Homogeneous olivine (Fa: 25 ± 0.7) and low‐Ca pyroxene (Fs: 21 ± 0.4) compositions with percent mean deviation of <2, further suggest that Kamargaon is a coarsely equilibrated, petrologic type 6 chondrite. Kamargaon is thermally metamorphosed with an estimated peak metamorphic temperature of ~800 °C as determined by two‐pyroxene thermometry. Shock metamorphism studies suggest that this meteorite include portions of different shock stages, e.g., S3 and S4 (Stöffler et al. 1991 ); however, local presence of quenched metal‐sulfide melt within shock veins/pockets suggest disequilibrium melting and relatively higher shock stage of up to S5 (Bennett and McSween 1996 ). Based on noble gas isotopes, the cosmic‐ray exposure age is estimated as 7.03 ± 1.60 Ma and nitrogen isotope composition (δ¹⁵N = 18‰) also correspond well with the L‐chondrite group. The He‐U, Th, and K‐Ar yield younger ages (170 ± 25 Ma 684 ± 93, respectively) and are discordant. A loss of He during the resetting event is implied by the lower He‐U and Th age. Elemental ratios of trapped Ar, Kr, and Xe can be explained through the presence of a normal Q noble gas component. Relatively low activity of ²⁶Al (39 dpm/kg) and the absence of ⁶⁰Co activity suggest a likely low shielding depth and envisage a small preatmospheric size of the meteoroid (<10 cm in radius). The Kr isotopic ratios (⁸²Kr/⁸⁴Kr) further argue that the meteorite was derived from a shallow depth.     

Light noble gases in Jilin: More of the same and something new

Abstract— Results are reported for documented samples from two drill cores and for specimens from the strewn field of the largest known stone meteorite, the H chondrite Jilin. Core B was found to have been parallel to the surface of Jilin during its first stage of irradiation, in 2π geometry. Core A was normal to the 2π surface; in it the mean attenuation length for the production by galactic cosmic rays of ³⁸Ar from metal and of ²¹Ne from Mg, Al, Si in silicates was found to be the same. A numerical value for the mean attenuation length of (71 ± 4) cm or (246 ± 14) g × cm^?2 follows if corrections for the contribution from the second stage of exposure are based on T₂ = 0.32 Ma; agreement with the lower values of ～180 g × cm^?2 obtained from lunar studies and target data requires T₂ to be about twice as long. Previous results are confirmed that in specimens with high contents of stable spallogenic gases the ratio ²¹Ne_bulk/³⁸Ar_metal is low. The suggestion had been, and is, mat this is a transition effect in near-surface samples where the secondary cascade of nuclear-active particles, and hence the production of ²¹Ne, was not yet fully developed. This suggestion is borne out by the present results on two samples that, based on cosmic-ray tracks and ⁶⁰Co content, are certified near-surface samples (although, strictly speaking, this is true only for the depth of burial during the second stage of irradiation). Cosmic-ray produced ⁶⁰Co is positively correlated with ⁴He content, indicating that significant losses of ⁴He occurred when the Jilin meteoroid had acquired already its final size and shape and that the losses were more severe for near-surface samples than for such from the 4π interior. Presumably, the losses were caused by a thermal spike associated with the excavation of Jilin from its parent body. The same event caused losses of part of the ³He produced during the first irradiation stage. From the systematics of the ³He/²¹Ne vs. ⁴He correlation, we derive for the second 4π irradiation a production ratio ³He/²¹Ne = 3.2 ± 0.4.     

Production of noble gases near the surface of Europa and the prospects for in situ chronology

The age of the surface of Europa is probably tens of Myr or less, but is poorly constrained. Two different geochronology schemes could potentially be applied to near-surface samples to provide far more precise ages. First, the surface salts apparently contain enough potassium to make potassium-argon dating feasible. Second, the bombardment of the surface with both galactic cosmic rays and protons trapped within the jovian magnetosphere will cause nuclear reactions that can lead to measurable buildups of cosmogenic noble gases, which can be used to determine both cosmic-ray exposure ages and erosion, deposition, or mixing rates for surface modification. The major differences between Europa's salt-rich ice and the rocks (meteorites, lunar samples and terrestrial rocks) in which cosmogenic noble gases are normally measured are that the abundance of target elements for nuclear reactions creating neon and argon are lower (because of the high water content), but neutron-capture reactions, particularly ³⁵Cl(n,γ)³⁶Cl(β⁻)³⁶Ar, are enhanced because of the thermalizing effects of the water. As well as presenting calculations of noble gas production near the surface of Europa, we also show that the measurements required to determine ages are within reach of technology in the near future, if an instrument can be landed on the surface of Europa.     

Rapid effects of terrestrial alteration on highly siderophile elements in the Sutter's Mill meteorite

The ¹⁸⁷Re-¹⁸⁷Os isotopic systematics of many bulk chondrites plot well beyond analytical uncertainties of a primordial isochron. Limited variations in ¹⁸⁷Os/¹⁸⁸Os, coupled with large variations in Re/Os ratios among chondrites, suggest that this apparently open-system behavior is a result of the comparatively recent gain or loss of Re and/or Os. In order to assess whether or not rapid alteration in the terrestrial environment could be responsible for open-system behavior in chondrites, four pieces of the Sutter's Mill meteorite were examined for Os isotopic systematics and abundances of highly siderophile elements. Pieces SM1 and SM2 were collected prior to a rain event, within 2 days of the fall. Pieces SM51 and SM53 were collected after a rain event. There are significant but minor relative and absolute variations in the abundances of the highly siderophile elements, as well as ¹⁸⁷Os/¹⁸⁸Os among the four pieces. Rhenium-Os isotopic data for SM1 and SM2 plot within analytical uncertainties of a primordial isochron, while powders made from SM51 and SM53 do not. These results suggest that interactions with rain caused some redistribution of Re, and to a lesser extent Os, within small pieces of the meteorite. Thus, Re-Os isotopic systematics of <dm-size pieces of chondrites must be considered susceptible to modification after only a short time on the surface, where exposed to rain.     

The history of ordinary chondrites from the data on stable isotopes of noble gases (a review)

The results of studying the distributions of stable radiogenic and cosmogenic isotopes of noble gases in ordinary chondrites are reviewed here: the distributions of gas-retention and exposure ages are analyzed, the chronology and consequences of catastrophic destruction of the parent bodies of H and L chondrites are discussed, the fallen and preatmospheric masses of Antarctic and non-Antarctic meteorites are compared, the diffusion losses of noble gases in meteorites are considered, the ablation of meteorites and their mean lifetime after separation from the parent bodies are estimated, and a number of other problems are discussed.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 2, 2005, pp. 141–168.Original Russian Text Copyright © 2005 by Alexeev.     

Formation of a small impact structure discovered within the Agoudal meteorite strewn field,Morocco

A relic impact structure was recognized within the strewn field of the Agoudal iron meteorite. The heavily eroded structure has preserved shatter cones in a limestone basement, and remnants of autochthonous and allochthonous breccias. Fragments of iron incorporated into the allochthonous breccia have a chemical composition (Ni = 5.16 wt%, Ir = 0.019 ppm) similar to that of the Agoudal meteorite, supporting a syngenetic origin of the strewn field and the impact structure. The total recovered mass of Agoudal meteorite fragments is estimated at approximately 500 kg. The estimated size of the SE–NW‐oriented strewn field is 6 × 2 km. Model calculations with minimal preatmospheric size show that a similar meteorite strewn field plus one small crater with observed shock effects could be formed by fragmentation of a meteoroid approximately 1.4 m in diameter with an impact angle of approximately 60° from the horizontal. However, the most probable is an impact of a larger, 3–4 m diameter meteoroid, resulting a strewn field with approximately 10 craters, 10–30 m in diameter each, plus numerous meteorite fragments. The calculated scattering area of meteorite shrapnel ejected from these impact craters could completely cover the observed strewn field of the Agoudal meteorite.     

Orphan radiogenic noble gases in lunar breccias: evidence for planet pollution of the Sun?

Surface-correlated noble gases in lunar soils are primarily implanted SW (solar wind) noble gases. However, they also include apparently orphan radiogenic ⁴⁰Ar, ¹²⁹Xe, and ²⁴⁴Pu-derived fission Xe in excess of plausible primordial solar origin. These orphan radiogenic components are usually assigned a lunar origin, in a scenario in which radiogenic noble gases produced in the lunar interior were degassed into the transient atmosphere and then re-implanted to the lunar surface together with SW. There are some quantitative difficulties with this scenario, however, and it requires special constraints on the degassing history of the Moon that have not emerged from more general thermal history models. We therefore urge consideration of alternative hypotheses. As a possible source for the orphan radiogenic noble gases, we have examined planetary pollution of the Sun, as suggested by studies of extrasolar planetary systems (e.g., Murray et al., 2001, Astrophys. J. 555, 801-815; Israelian et al., 2001, Nature 411, 163-166). Pollution of the Sun by 2M_⊕ (two Earth mass) planetary materials (Murray et al., 2001, Astrophys. J. 555, 801-815) is likely not significant for Ar but could be important to account for orphan Xe in the Moon.     

Effects of aqueous alteration on primordial noble gases and presolar SiC in the carbonaceous chondrite Tagish Lake

Effects of aqueous alteration on primordial noble gas carriers were investigated by analyzing noble gases and determining presolar SiC abundances in insoluble organic matter (IOM) from four Tagish Lake meteorite (C2‐ung.) samples that experienced different degrees of aqueous alteration. The samples contained a mixture of primordial noble gases from phase Q and presolar nanodiamonds (HL, P3), SiC (Ne‐E[H]), and graphite (Ne‐E[L]). The second most altered sample (11i) had a ~2–3 times higher Ne‐E concentration than the other samples. The presolar SiC abundances in the samples were determined from NanoSIMS ion images and 11i had a SiC abundance twice that of the other samples. The heterogeneous distribution of SiC grains could be inherited from heterogeneous accretion or parent body alteration could have redistributed SiC grains. Closed system step etching (CSSE) was used to study noble gases in HNO₃‐susceptible phases in the most and least altered samples. All Ne‐E carried by presolar SiC grains in the most altered sample was released during CSSE, while only a fraction of the Ne‐E was released from the least altered sample. This increased susceptibility to HNO₃ likely represents a step toward degassing. Presolar graphite appears to have been partially degassed during aqueous alteration. Differences in the ⁴He/³⁶Ar and ²⁰Ne/³⁶Ar ratios in gases released during CSSE could be due to gas release from presolar nanodiamonds, with more He and Ne being released in the more aqueously altered sample. Aqueous alteration changes the properties of presolar grains so that they react similar to phase Q in the laboratory, thereby altering the perceived composition of Q.