Kinetics of Xe‐P3 release during pyrolysis of the coarse‐grained fractions of Orgueil (CI) meteorite nanodiamonds

The kinetics of the release of the Xe‐P3 component from coarse‐grained fractions of Orgueil (CI) meteorite nanodiamonds has been investigated using stepped and isothermal pyrolysis. It has been shown that a first‐order chemical reaction diffusion model with a single activation energy cannot provide a satisfactory explanation for the observed retention of Xe‐P3 during parent body thermal metamorphism and the kinetics of Xe‐P3 release from nanodiamonds during isothermal pyrolysis. Using the activation energy and frequency factor calculated according to this model, it is shown that in the course of thermal metamorphism of the Orgueil meteorite almost the entire Xe‐P3 component must have been lost in a very short time (<4 yr at approximately 100 °C). However, the calculated retention of Xe‐P3 increases significantly if a diffusion model with a spectrum of activation energies is used. In this case, the model can explain not only a high retention of Xe‐P3 in the Orgueil nanodiamonds but also the release pattern of the Xe‐P3 from Semarkona and Bishunpur nanodiamonds that have experienced a significant gas loss during parent body metamorphism as well as the release of Xe‐P3 during isothermal pyrolysis of the Orgueil nanodiamonds. The energetically complicated Xe‐P3 distribution is most likely caused by structural damage to the nanodiamond grains or a complex phase composition of carbon in the surface layer of the diamond grains. It is supposed that the structural damage of the diamond grains can have a radiation origin, while the variations of the carbon phase composition in the grain's mantle can be caused by the radiation‐induced reactions and/or a thermal effect.     

Populations of nanodiamond grains in meteorites from the data on isotopic composition and content of nitrogen

The present study has shown that the dependence of the isotopic composition of nitrogen on the N/C ratio, revealed from the data for bulk samples of meteoritic nanodiamond, can be obtained within the framework of the following model of the composition of populations of nanodiamond grains: (a) initial nanodiamond, i.e., the nanodiamond in the protoplanetary cloud before the accretion of the meteorite parent bodies, was composed mainly of grains of two populations (denoted as C_N and C_F), the ratio of which changed in meteorites depending on the degree of hydrothermal metamorphism; (b) only the grains of one of these populations (C_N) contain volume-bound nitrogen with δ¹⁵N = ?350‰; (c) the grains of both populations contain surface-bound nitrogen (δ¹⁵N ≡ 0). The calculations revealed the following properties of population grains in this model. (1) The grains of the C_N and C_F populations are most likely the same in isotopic composition of carbon and heterogeneous in distribution of its isotopes: the central part of grains is enriched with the δ¹²C isotope relative to the remainder of the grain. While the value of δ¹³C is ?37.3 ± 1.1‰ for carbon in the central part, it is ?32.8 ± 1.5‰ for the whole volume of the grains. (2) The noble gases of the HL component, specifically Xe-HL, are anomalous in isotopic composition and are most likely contained in the third population of nanodiamond grains (denoted as C_HL), the mass fraction of which is negligible relative to that for other grain populations. Only the grains of the C_HL population have an undoubtedly presolar origin, while the grains of the other nanodiamond populations could have formed at the early stages of the evolution of the protoplanetary cloud material before the accretion of the meteoritic parent bodies.     

Spallation recoil II: Xenon evidence for young SiC grains

Abstract— We have determined the recoil range of spallation xenon produced by irradiation of Ba glass targets with ?1190 and ?268 MeV protons, using a catcher technique, where spallation products are measured in target and catcher foils. The inferred range for ¹²⁶Xe produced in silicon carbide is ?0.19 μm, which implies retention of ?70% for ¹²⁶Xe produced in “typical” presolar silicon carbide grains of 1 μm size. Recoil loss of spallation xenon poses a significantly smaller problem than loss of the spallation neon from SiC grains. Ranges differ for the various Xe isotopes and scale approximately linearly as function of the mass difference between the target element, Ba, and the product. As a consequence, SiC grains of various sizes will have differences in spallation Xe composition. In an additional experiment at ?66 MeV, where the recoil ranges of ²²Na and ¹²⁷Xe produced on Ba glass were determined using γ‐spectrometry, we found no evidence for recoil ranges being systematically different at this lower energy. We have used the new data to put constraints on the possible presolar age of the SiC grains analyzed for Xe by Lewis et al. (1994). Uncertainties in the composition of the approximately normal Xe component in SiC (Xe‐N) constitute the most serious problem in determining an age, surpassing remaining uncertainties in Xe retention and production rate. A possible interpretation is that spallation contributions are negligible and that trapped ¹²⁴Xe/¹²⁶Xe is ?5% lower in Xe‐N than in Q‐Xe. But also for other reasonable assumptions for the ¹²⁴Xe/¹²⁶Xe ratio in Xe‐N (e.g., as in Q‐Xe), inferred exposure ages are considerably shorter than theoretically expected lifetimes for interstellar grains. A short presolar age is in line with observations by others (appearance, grain size distribution) that indicate little processing in the interstellar medium (ISM) of surviving (crystalline) SiC. This may be due to amorphization of SiC in the ISM on a much shorter time scale than destruction, with amorphous SiC not surviving processing in the early solar system. A large supply of relatively young grains may be connected to the proposed starburst origin (Clayton 2003) for the parent stars of the mainstream SiC grains.     

Spallation recoil and age of presolar grains in meteorites

Abstract— We have determined the recoil losses from silicon carbide (SiC) grain‐size fractions of spallation Ne produced by irradiation with 1.6 GeV protons. During the irradiation, the SiC grains were dispersed in paraffin wax in order to avoid reimplantation into neighboring grains. Analysis for spallogenic ²¹Ne of grain‐size separates in the size range 0.3 to 6 μm and comparison with the ²²Na activity of the SiC + paraffin mixture indicates an effective recoil range of 2–3 μm with no apparent effect from acid treatments, which are routinely used in the isolation of meteoritic SiC grains. Our results indicate that the majority of presolar SiC grains in primitive meteorites, which are micrometer‐sized, will have lost essentially all spallogenic Ne produced by cosmic‐ray interaction in the interstellar medium. This argues against the validity of previously published presolar ages of Murchison SiC (～10 to ～130 Ma, increasing with grain size; Lewis et al., 1994), where recoil losses had been based on calculated recoil energies. It is argued that the observed variations in meteoritic SiC grain‐size fractions of ²¹Ne/²²Ne ratios are more likely due to the effects of nucleosynthesis in the He‐burning shell of the parent AGB stars which imposes new boundary conditions on nuclear parameters and stellar models. It is suggested that spallation‐Xe produced on the abundant Ba and REE in presolar SiC, rather than spallogenic Ne, may be a promising approach to the presolar age problem. There is a hint in the currently available Xe data (Lewis et al., 1994) that the large (>1 μm) grains may be younger than the smaller (<1 μm) grains. The retention of spallogenic ²¹Ne produced by the bombardment of SiC grains of different grain sizes with 1.6 GeV protons, avoiding reimplantation into neighboring grains by dispersing the SiC grains in paraffin wax, has been derived from a comparison of mass spectrometrically determined ²¹Ne, retained in the grains, with the ²²Na activity of the grains‐plus‐paraffin mixture. Compared to estimates of retention used in previous attempts to determine presolar ages for SiC (Tang and Anders, 1988b; Lewis et al., 1990, 1994), the results indicate significantly lower values. They do, however, agree with retention as expected from previous measurements of recoil ranges in similar systems (Nyquist et al., 1973; Steinberg and Winsberg, 1974). The prime reason for the discrepancy must lie in the energy of the recoiling nuclei entering in the calculation of retention by Tang and Anders (1988b), which is based on considerations by Ray and Völk (1983). Based on the results, it appears questionable that spallation contributes significantly to the observed variations of ²¹Ne/²²Ne ratios among various SiC grain‐size separates (Lewis et al., 1994). We rather suggest that the variations, just as it has been observed for Kr and Ba already (Lewis et al., 1994; Prombo et al., 1993), have a nucleosynthetic origin. Confirmation needs input of improved nuclear data and stellar models into new network calculations of the nucleosynthesis in AGB stars of elements in the Ne region. Finally we argue that, to determine presolar system irradiation effects, spallation Xe is more favorable than is Ne, primarily because of smaller recoil losses for Xe. Although preliminary estimates hint at the possibility that the larger (>1 μm) grains are younger than the smaller (<1 μm) ones, the major uncertainty for a quantitative evaluation lies in the exact composition of the Xe‐N component thought to originate from the envelope of the SiC grains' parent stars.     

Solar wind and other gases in the regoliths of the Pesyanoe parent object and the moon

We report new data from Pesyanoe‐90,1 (dark lithology) on the isotopic signature of solar wind (SW) Xe as recorded in this enstatite achondrite which represents a soil‐breccia of an asteroidal regolith. The low temperature (≤800°C) steps define the Pesyanoe‐S xenon component, which is isotopically consistent with SW Xe reported for the lunar regolith. This implies that the SW Xe isotopic signature was the same at two distinct solar system locations and, importantly, also at different times of solar irradiation. Further, we compare the calculated average solar wind “SW‐Xe” signature to Chass‐S Xe, the indigenous Xe observed in SNC (Mars) meteorites. Again, a close agreement between these compositions is observed, which implies that a mass‐dependent differential fractionation of Xe between SW‐Xe and Chass‐S Xe is >1.5%_o per amu. We also observe fractionated (Pesyanoe‐F) Xe and Ar components in higher temperature steps and we document a fission component due to extinct ²⁴⁴Pu. Interestingly, the Pesyanoe‐F Xe component is revealed only at the highest temperatures (>1200°C). The Pesyanoe‐F gas reveals Xe isotopic signatures that are consistent with lunar solar energetic particles (SEP) data and may indicate a distinct solar energetic particle radiation as was inferred for the moon. However, we cannot rule out fractionation processes due to parent body processes. We note that ratios ³⁶Ar/³⁸Ar≤5 are also consistent with SEP data. Calculated abundances of the fission component correlate well with radiogenic ⁴⁰Ar concentrations, revealing rather constant ²⁴⁴Pu/K ratios in Pesyanoe, and separates thereof, and indicate that both components were retained. We identify a nitrogen component (δ¹⁵N = 44%_o) of non‐solar origin with an isotopic signature distinct from indigenous N (δ¹⁵N = ?33%^o). While large excesses at ¹²⁸Xe and ¹²⁹Xe are observed in the lunar regolith samples, these excesses in Pesyanoe are small. On the other hand, significant ¹²⁶Xe isotopic excesses, comparable to relative excesses observed in lunar soils and breccias, are prominent in the intermediate temperature steps of Pesyanoe‐90,1.     

Photoluminescence of silicon‐vacancy defects in nanodiamonds of different chondrites

Photoluminescence spectra show that silicon impurity is present in lattice of some nanodiamond grains (ND) of various chondrites as a silicon‐vacancy (SiV) defect. The relative intensity of the SiV band in the diamond‐rich separates depends on chemical composition of meteorites and on size of ND grains. The strongest signal is found for the size separates enriched in small grains; thus, confirming our earlier conclusion that the SiV defects preferentially reside in the smallest (≤2 nm) grains. The difference in relative intensities of the SiV luminescence in the diamond‐rich separates of individual meteorites are due to variable conditions of thermal metamorphism of their parent bodies and/or uneven sampling of nanodiamond populations. Annealing of separates in air eliminates surface sp²‐carbon; consequently, the SiV luminescence is enhanced. Strong and well‐defined luminescence and absorption of the SiV defect is a promising feature to locate cold (<250 °C) nanodiamonds in space.     

Isotopic compositions,nitrogen functional chemistry,and low‐loss electron spectroscopy of complex organic aggregates at the nanometer scale in the carbonaceous chondrite Renazzo

Organic matter (OM) was widespread in the early solar nebula and might have played an important role for the delivery of prebiotic molecules to the early Earth. We investigated the textures, isotopic compositions, and functional chemistries of organic grains in the Renazzo carbonaceous chondrite by combined high spatial resolution techniques (electron microscopy–secondary ion mass spectrometry). Morphologies are complex on a submicrometer scale, and some organics exhibit a distinct texture with alternating layers of OM and minerals. These layered organics are also characterized by heterogeneous ¹⁵N isotopic abundances. Functional chemistry investigations of five focused ion beam‐extracted lamellae by electron energy loss spectroscopy reveal a chemical complexity on a nanometer scale. Grains show absorption at the C‐K edge at 285, 286.6, 287, and 288.6 eV due to polyaromatic hydrocarbons, different carbon‐oxygen, and aliphatic bonding environments with varying intensity. The nitrogen K‐edge functional chemistry of three grains is shown to be highly complex, and we see indications of amine (C‐NH_x) or amide (CO‐NR₂) chemistry as well as possible N‐heterocycles and nitro groups. We also performed low‐loss vibrational spectroscopy with high energy resolution and identified possible D‐ and G‐bands known from Raman spectroscopy and/or absorption from C=C and C‐O stretch modes known from infrared spectroscopy at around 0.17 and 0.2 eV energy loss. The observation of multiglobular layered organic aggregates, heterogeneous ¹⁵N‐anomalous compositions, and indication of NH_x‐(amine) functional chemistry lends support to recent ideas that ¹⁵N‐enriched ammonia (NH₃) was a powerful agent to synthesize more complex organics in aqueous asteroidal environments.     

I‐Xe ages of Campo del Cielo silicates as a record of the complex early history of the IAB parent body

Using in situ laser analyses of a polished thin section from the IAB iron meteorite Campo del Cielo, we identified two silicate grains rich in radiogenic ^129*Xe, Cr‐diopside, and oligoclase, excavated them from the metal, and irradiated them with thermal neutrons for I‐Xe dating. The release profiles of ^129*Xe and ^128*Xe are consistent with these silicates being diopside and oligoclase, with activation energies, estimated using Arrhenius plots, of ～201 and ～171 kcal mole^?1, respectively. The 4556.4 ± 0.4 Ma absolute I‐Xe age of the more refractory diopside is younger than the 4558.0 ± 0.7 Ma I‐Xe age of the less refractory oligoclase. We suggest that separate impact events at different locations and depths on a porous initial chondritic IAB parent body led to the removal of the melt and recrystallization of diopside and oligoclase at the times reflected by their respective I‐Xe ages. The diopside and oligoclase grains were later brought into the studied inclusion by a larger scale catastrophic collision that caused breakup and reassembly of the debris, but did not reset the I‐Xe ages dating the first events. The metal melt most probably was <1250 °C when it surrounded studied silicate grains. This reassembly could not have occurred earlier than the I‐Xe closure in diopside at 4556.4 ± 0.4 Ma.     

Presolar grains in the CM2 chondrite Sutter's Mill

The Sutter's Mill (SM) carbonaceous chondrite is a regolith breccia, composed predominantly of CM2 clasts with varying degrees of aqueous alteration and thermal metamorphism. An investigation of presolar grains in four Sutter's Mill sections, SM43, SM51, SM2‐4, and SM18, was carried out using NanoSIMS ion mapping technique. A total of 37 C‐anomalous grains and one O‐anomalous grain have been identified, indicating an abundance of 63 ppm for presolar C‐anomalous grains and 2 ppm for presolar oxides. Thirty‐one silicon carbide (SiC), five carbonaceous grains, and one Al‐oxide (Al₂O₃) were confirmed based on their elemental compositions determined by C‐N‐Si and O‐Si‐Mg‐Al isotopic measurements. The overall abundance of SiC grains in Sutter's Mill (55 ppm) is consistent with those in other CM chondrites. The absence of presolar silicates in Sutter's Mill suggests that they were destroyed by aqueous alteration on the parent asteroid. Furthermore, SM2‐4 shows heterogeneous distributions of presolar SiC grains (12–54 ppm) in different matrix areas, indicating that the fine‐grained matrix clasts come from different sources, with various thermal histories, in the solar nebula.     

Galactic cosmic ray‐produced 129Xe and 131Xe excesses in troilites of the Cape York iron meteorite

Abstract— The flux of galactic cosmic rays (GCR) in the solar system appears to change with time. Based on the abundances in iron meteorites of cosmogenic nuclides of different half lives, Lavielle et al. (1999) found that the GCR flux increased in recent times (<100 Ma) by about 38% compared to average flux in the past 150 Ma to 700 Ma ago. A promising technique for calibrating the GCR flux during the past ?50 Ma, based on the ¹²⁹I and ¹²⁹Xe pair of nuclides, was discussed earlier (Marti 1986; Murty and Marti 1987). The ¹²⁹I‐¹²⁹Xe_n chronometer provides a shielding‐independent system as long as the exposure geometry remained fixed. It is especially suitable for large iron meteorites (Te‐rich troilite) because of the effects by the GCR secondary neutron component. Although GCR‐produced Xe components were identified in troilites, several issues require clarifications and improvements; some are reported here. We developed a procedure for achieving small Xe extraction blanks which are required to measure indigenous Xe in troilites. The ¹²⁹Xe and ¹³¹Xe excesses (¹²⁹Xe_n, ¹³¹Xe_n) due to neutron reactions in Te are correlated in a stepwise release run during the troilite decomposition. Our data show that indigenous Xe in troilite of Cape York has isotopic abundances consistent with ordinary chondritic Xe (OC‐Xe), in contrast to a terrestrial signature which was reported earlier. Two methods are discussed which assess and correct for an interfering radiogenic ¹²⁹Xe_r component from extinct ¹²⁹I. The corrected ¹²⁹Xe_n concentration in troilite D4 of Cape York yields a cosmic ray exposure (CRE) age of 82 ± 7 Ma consistent, within uncertainties, with reported data (Murty and Marti 1987; Marti et al. 2004).     

Light dement geochemistry of the Tagish Lake CI2 chondrite: Comparison with CI1 and CM2 meteorites

Abstract— We have studied the carbon and nitrogen stable isotope geochemistry of a small pristine sample of the Tagish Lake carbonaceous chondrite by high‐resolution stepped‐combustion mass spectrometry, and compared the results with data from the Orgueil (CI1), Elephant Moraine (EET) 83334 (CM1) and Murchison (CM2) chondrites. The small chip of Tagish Lake analysed herein had a higher carbon abundance (5.81 wt%) than any other chondrite, and a nitrogen content (?1220 ppm) between that of CI1 and CM2 chondrites. Owing to the heterogeneous nature of the meteorite, the measured carbon abundance might be artificially high: the carbon inventory and whole‐rock carbon isotopic composition (δ¹³C ? +24.4%_o) of the chip was dominated by ¹³C‐enriched carbon from the decomposition of carbonates (between 1.29 and 2.69 wt%; δ¹³C ? +67%_o and δ¹⁸O ? +35%_o, in the proportions ?4:1 dolomite to calcite). In addition to carbonates, Tagish Lake contains organic carbon (?2.6 wt%, δ¹³C ? ?9%_o; 1033 ppm N, δ¹⁵N ? +77%_o), a level intermediate between CI and CM chondrites. Around 2% of the organic material is thermally labile and solvent soluble. A further ?18% of the organic species are liberated by acid hydrolysis. Tagish Lake also contains a complement of presolar grains. It has a higher nanodiamond abundance (approximately 3650–4330 ppm) than other carbonaceous chondrites, along with ?8 ppm silicon carbide. Whilst carbon and nitrogen isotope geochemistry is not diagnostic, the data are consistent with classification of Tagish Lake as a CI2 chondrite.     

Isotopic compositions of different presolar silicon carbide size fractions from the Murchison meteorite

Abstract— We report measurements of isotopic ratios of C, N, Mg, Si, Ca, Ti, Cr, and Fe in bulk samples (aggregates of many grains) of up to seven different fractions of silicon carbide (SiC), ranging from 0.38 to 3.0μm in diameter, from the Murchison CM2 carbonaceous chondrite. Ratios of ¹²C/¹³C range from 37 to 42 and ¹⁴N/¹⁵N ratios from 370 to 520, within the range of single‐grain measurements on coarser samples and in agreement with an asymptotic giant branch (AGB) star origin of most of the grains. Variations among size fractions do not show any simple trend and can be explained by varying contamination with isotopically normal material. Silicon isotopic ratios vary only little and, with one exception, lie to the right of the singlegrain mainstream correlation line. This might indicate a higher percentage of the minor populations Y and Z among finer grain‐size fractions. All bulk samples have large ²⁶Mg excesses attributed to the presence of short‐lived ²⁶Al at the time of grain formation. Inferred ²⁶Al/²⁷Al ratios are much larger than those measured in single larger mainstream grains. This is probably because of the presence of SiC grains of type X; we obtain an estimate of 0.4 for their ²⁶Al/²⁷Al ratio. Our Ca‐isotopic measurements, the first made on presolar SiC grains, show excesses in ⁴²Ca and ⁴³Ca, which is in general agreement with theoretical expectations for AGB stars. Calcium‐44 excesses are much larger than expected and are probably because of X grains, which have high⁴⁴Ca excesses because of the decay of short‐lived ⁴⁴Ti produced in supernova explosions. We arrive at an estimate of 0.014 for the initial ⁴⁴Ti/⁴⁸Ti ratio of the X grains, within the range obtained from previous single X grain measurements. The Ti‐isotopic ratios of the bulk samples show a V‐shaped pattern with excesses of all isotopes relative to ⁴⁸Ti. Isotopes ⁴⁶Ti, ⁴⁷Ti, and ⁵⁰Ti show excesses relative to the correlation between Ti and Si ratios for single grains and are in general agreement with theoretical models of s‐process nucleosynthesis in AGB stars. In contrast, ⁴⁹Ti does not show any excess relative to the singlegrain data; it also fails to agree with theory, which predicts much larger excesses than observed. Measured ⁵³Cr/⁵²Cr and ⁵⁷Fe/⁵⁶Fe ratios are normal within errors. The first result is expected even for Cr in AGB star envelopes, but the second result suggests that most of the Fe analyzed originates from contamination. We have found no simple trends in isotopic composition with respect to grain size that can be interpreted in terms of nucleosynthetic origin, unlike the results for Kr, Xe, Ba, and Sr.     

Interpreting the I‐Xe system in individual silicate grains from Toluca IAB

Abstract— Detailed isotopic and mineralogical studies of silicate inclusions separated from a troilite nodule of the Toluca IAB iron meteorite reveal the presence of radiogenic ¹²⁹Xe in chlorapatite, plagioclase, perryite, and pyroxene grains. Subsequent I‐Xe studies of 32 neutron‐irradiated pyroxene grains indicate that high‐Mg and low‐Mg pyroxenes have distinctive I‐Xe signatures. The I‐Xe system in high‐Mg pyroxenes closed at 4560.5 ± 2.4 Ma, probably reflecting exsolution of silicates from the melt, while the low‐Mg pyroxenes closed at 4552.0 ± 3.7 Ma, 8.5 Ma later, providing a means for determining the cooling rate at the time of exsolution. If the host Toluca graphite‐troilite‐rich inclusion formed after the breakup and reassembly of the IAB parent body as has been suggested, the I‐Xe ages of the high‐Mg pyroxenes separated from this inclusions indicate that this catastrophic impact occurred not later than 4560.5 Ma, 6.7 Ma after formation of CAIs. The cooling rate at the time of silicates exsolution in Toluca is 14.5 ± 10.0 °C/Ma.     

Heterogeneous histories of Ni‐bearing pyrrhotite and pentlandite grains in the CI chondrites Orgueil and Alais

Compositional and structural analyses of CI chondrite iron–nickel sulfide grains reveal heterogeneity both across and within the Orgueil and Alais meteorites. Orgueil grains with the 4C monoclinic pyrrhotite structure have variable metal‐to‐sulfur ratios and nickel contents. These range from the nominal ratio of 0.875 for Fe₇S₈ with <1 atom% nickel to a high metal‐to‐sulfur ratio of 0.97 with 15 atom% nickel. These data reveal a previously unrecognized low‐temperature solid solution between Fe₇S₈ and Fe₅Ni₃S₈. We have also identified 6C monoclinic pyrrhotite among the Orgueil iron–nickel sulfides. The occurrence of pentlandite in Orgueil is confirmed for the first time crystallographically. In contrast, sulfide grains in Alais do not show the same spread in composition and structure; rather they represent the endmembers: low‐Ni 4C monoclinic pyrrhotite and pentlandite. We investigate possible formation/alteration scenarios: crystallization from a melt, solid‐state diffusion and/or exsolution, oxidation of pre‐existing sulfides, and precipitation from a fluid. Sulfide grains are sensitive to alteration conditions; these data suggest that the structures and compositions of the sulfide assemblages in Orgueil and Alais were established by late‐stage parent body aqueous alteration, followed in some cases by low‐temperature solid‐state processes. The samples record different alteration histories, with Orgueil experiencing lower equilibration temperatures (25 °C) than Alais (100–135 °C). We conclude that millimeter‐scale heterogeneity existed in alteration conditions (e.g., temperature, pH, oxygen fugacity, sulfur fugacity, duration of alteration) on the parent body. This variability is evidenced by the diversity among sulfide grains located within millimeters of one another.     

Isotopic properties of silicon carbide X grains from the Murchison meteorite in the size range 0.5‐1.5 μm

Abstract— We report isotopic abundances for C, N, Mg‐Al, Si, Ca‐Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 μm. Carbon was measured in 41 X grains, n in 37 grains, Mg‐Al in 18 grains, Si in 87 grains, Ca‐Ti in 25 grains, and Fe in 8 grains. These X grains have ¹²C/¹³C ratios between 18 and 6800, ¹⁴N/¹⁵n ratios from 13 to 200, δ²⁹Si/²⁸Si between ?750 and +60%₀, δ³⁰Si/²⁸Si from ?770 to ?10%₀, and ⁵⁴Fe/⁵⁶Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic ²⁶Mg (from the radioactive decay of ²⁶Al, half‐life ? 7 times 10⁵ years) and radiogenic ⁴⁴Ca (from the radioactive decay of ⁴⁴Ti, half‐life = 60 years). While all X grains but one have radiogenic ²⁶Mg, only ～20% of them have detectable amounts of radiogenic ⁴⁴Ca. Initial ²⁶Al/²⁷Al ratios of up to 0.36 and initial ⁴⁴Ti/⁴⁸Ti ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf‐Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct ⁴⁴Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for ²⁶Al/²⁷Al ratios, except for the highest ²⁶Al/²⁷Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed ²⁹Si/³⁰Si ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured ¹⁴n/¹⁵n ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 M_odot; type II SN if rotational mixing, preferential trapping of N, or both from ¹⁵n‐rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial ²⁶Al/²⁷Al ratios in small X grains are remarkably similar to those of large X grains (2–10 μm). Titanium‐44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti‐bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain‐size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources.     

The Isheyevo meteorite: Mineralogy,petrology, bulk chemistry,oxygen, nitrogen,carbon isotopic compositions,and 40Ar‐39Ar ages

Abstract— Isheyevo is a metal‐rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7–90 vol%). The metal‐rich lithologies with 50–60 vol% of Fe,Ni metal are dominant. The metal‐rich and metal‐poor lithologies are most similar to the CB_b and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar components—Fe,Ni metal, chondrules, refractory inclusions (Ca, Al‐rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clasts—but show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non‐porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in ¹⁵N (δ¹⁵N = 1122‰). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non‐magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CB_b chondrites, ～20% of Fe,Ni‐metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni‐rich (>20 wt% Ni). In contrast to CB_b and CH chondrites, most metal grains are thermally decomposed into Ni‐rich and Ni‐poor phases. Similar to CH chondrites, chondrules have porphyritic and non‐porphyritic textures and ferromagnesian (type I and II), silica‐rich, and aluminum‐rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite‐, grossite‐, and melilite‐rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico‐chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine‐grained dust. Such a scenario is consistent with the presence of solar wind—implanted noble gases in Isheyevo and with its comparatively old K‐Ar age. We cannot exclude that the K‐Ar system was affected by a later collisional event. The cosmic‐ray exposure (CRE) age of Isheyevo determined by cosmogenic ³⁸Ar is ～34 Ma, similar to that of the Bencubbin (CB_a) meteorite.     

Nitrogen components in IAB/IIICD iron meteorites

Abstract— Isotopic variations have been reported for many elements in iron meteorites, with distinct N signatures found in the metal and graphite of IAB irons. In this study, a dozen IAB/IIICD iron meteorites (see Table 1 for new classifications) were analyzed by stepwise pyrolysis to resolve nitrogen components. Although isotopic heterogeneity has been presumed to be lost in thermally processed parent objects, the high‐resolution nitrogen isotopic data indicate otherwise. At least one reservoir has a light nitrogen signature, δ¹⁵N = ?(74 ± 2)‰, at 900 °C to 1000 °C, with a possible second, even lighter, reservoir in Copiapo (δ¹⁵N ≤ ?82‰). These releases are consistent with metal nitride decomposition or low‐temperature metal phase changes. Heavier nitrogen reservoirs are observed in steps ≤700 °C and at 1200 °C to 1400 °C. The latter release has a δ¹⁵N signature with a limit of ≥?16‰. Xenon isotopic signatures are sensitive indicators for the presence of inclusions because of the very low abundances of Xe in metal. The combined high‐temperature release shows ¹³¹Xe and ¹²⁹Xe excesses to be consistent with shifts expected for Te(n,γ) reaction in troilite by epithermal neutrons, but there are also possible alterations in the isotopic ratios likely due to extinct ¹²⁹I and cosmic‐ray spallation. The IAB/IIICD iron data imply that at least one light N component survived the formation processes of iron parent objects which only partially exchanged nitrogen between phases. Preservation of separate N reservoirs conflicts with neither the model of impact‐heating effects for these meteorites nor reported age differences between metal and silicates.     

Kronos: exploring the depths of Saturn with probes and remote sensing through an international mission

Kronos is a mission aimed to measure in situ the chemical and isotopic compositions of the Saturnian atmosphere with two probes and also by remote sensing, in order to understand the origin, formation, and evolution of giant planets in general, including extrasolar planets. The abundances of noble gases, hydrogen, carbon, nitrogen, oxygen, sulfur and their compounds, as well as of the D/H, ⁴He/³He, ²²Ne/²¹Ne/²⁰Ne, ³⁶Ar/³⁸Ar, ¹³C/¹²C, ¹⁵N/¹⁴N, ¹⁸O/(¹⁷O)/¹⁶O, ¹³⁶Xe/¹³⁴Xe/¹³²Xe/¹³⁰Xe/¹²⁹Xe isotopic ratios will be measured by mass spectrometry on two probes entering the atmosphere of Saturn at two different locations near mid-latitudes, down to a pressure of 10 Bar. The global composition of Saturn will be investigated through these measurements, together with microwave radiometry determination of H₂O and NH₃ and their 3D variations. The dynamics of Saturn’s atmosphere will be investigated from: (1) measurements of pressure, temperature, vertical distribution of clouds and wind speed along the probes’ descent trajectories, and (2) determination of deep winds, differential rotation and convection with combined probe, gravity and radiometric measurements. Besides these primary goals, Kronos will also measure the intensities and characteristics of Saturn’s magnetic field inside the D ring as well as Saturn’s gravitational field, in order to constrain the abundance of heavy elements in Saturn’s interior and in its central core. Depending on the preferred architecture (flyby versus orbiter), Kronos will be in a position to measure the properties of Saturn’s innermost magnetosphere and to investigate the ring structure in order to understand how these tiny structures could have formed and survived up to the present times. An erratum to this article can be found at     

The Influence of Radiation Defects in Meteoritic Nanodiamond on the Elemental Composition of the P3 Noble Gas Component

Radiation defects in meteoritic nanodiamond produced during noble gas implantation have been ignored so far. At the same time, since the density of the defects depends on the mass of implanted ions with the same energy, graphitization of diamonds with a high density of defects during their thermal metamorphism could be an additional cause of the elemental fractionation of noble gases in the nanodiamond remaining after metamorphism. It has been shown that this factor allows one to explain the variations observed in the elemental composition of one of the main components (P3) of noble gases in the nanodiamond of meteorites thermally metamorphosed to a variable degree. It is also supposed that the bimodal release of the P3 noble-gas component upon pyrolysis of the nanodiamond can be caused by a different density of the radiation defects produced in the crystal lattice of nanodiamond grains during noble gas implantation. In this case, a low-temperature peak of gas release results from the destruction of the grains with a defect density higher than the critical one, at which diamond graphitization occurs at a relatively low pyrolysis temperature, while a high-temperature peak of release arises from the destruction of the nanodiamond grains, where the radiation defects have a weak influence on their graphitization temperature.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 342–352.Original Russian Text Copyright © 2005 by Fisenko, Semjonova.     

Isotopically anomalous neon in meteoritic nanodiamonds: Formation during type II supernova explosions

We hypothesize the formation of neon associated with isotopically anomalous xenon (Xe-HL) in meteoritic nanodiamonds and designated as Ne-X through the mixing of the Ne-HL and Ne-S subcomponents. The Ne-HL subcomponent is neon from the helium (He/C) zone of a type II supernova or a mixture of neon from this zone and its hydrogen zone, while the Ne-S subcomponent is spallation neon formed during a supernova explosion in nuclear spallation reactions induced by high-energy protons. Based on this hypothesis and the presumed abundances of neon isotopes in the zones of a high-mass (25M _⊙) supernova after its explosion, we have calculated the abundances of neon components in nanodiamond separates and its grain-size fractions. Our calculations have shown the following. (1) The main source of Ne-HL is neon from the helium zone of the supernova; as a result, the ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios for Ne-X are 0.26 ± 0.03 and 0.19 ± 0.04, respectively. The isotopic composition of Ne-X is identical to that for Ne-A2 if Ne-HL is produced by the mixing of neon from the helium and hydrogen zones in proportion 1: 1.06. (2) In meteoritic nanodiamonds, the main neon abundance is determined by neon of the P3 component (Ne-P3). Ne-P3 is retained during thermal metamorphism, because it is sited in traps of the crystal lattice of diamond with a high energy of its activation. (3) The Ne-X/Ne-P3 ratio increases with nanodiamond grain size; as a result, there is no need to invoke an additional neon component (Ne-P6) to interpret the data on neon in meteoritic nanodiamonds.