U. Ott

Noble gases in SNC meteorites: Shergotty, Nakhla, Chassigny

Abstract I report the elemental and isotopic composition of the noble gases in various fragments of the Shergotty, Nakhla, and Chassigny meteorites. Conclusions presented earlier (OTT and Begemann, 1985), which were based on results for trapped Ar, Kr and Xe only, are reaffirmed by the complete set of data. These include: 1. 1) Shergotty contains trapped gases that can be described as a mixture of the gas type found in the glass of shergottite EETA 79001 (SPB-type gases; possibly introduced by shock from the Martian atmosphere) and a second component characterized by low 36 Ar 132 Xe , 84 Kr 132 Xe and 129 Xe 132 Xe . Trapped gases found in Chassigny ( 36 Ar 132 Xe ~ 10 ; 84 Kr 132 Xe ~ 1 ; 129 Xe 132 Xe = 1.03 ) could be a pure sample of this second component. 2. 2) Trapped gases in Nakhla cannot be described as such a simple mixture. 3. 3) Xe in Chassigny has a solar-like isotopic composition, with 129 Xe 132 Xe significantly lower than in SPB-Xe (~ 1.03 vs . ~ 2.4). If these values are characteristic for interior and atmosphere, respectively, of the SPB, the evolution of the SPB and Earth must have differed greatly. Arguments are presented that Chassigny-type gases are a better choice than (fractionated) terrestrial air for the second component, that, when mixed with SPB-gases produces the Shergotty composition. The ratios 40 Ar 36 Ar t and 40 Ar 129 Xe ∗ (where 129 Xe ∗ is 129 Xe in excess of a 129 Xe 132 Xe ratio of ~ 1) in Shergotty agree with the respective values for EETA 79001 glass. This is taken as evidence for a late introduction of the supposedly shock implanted atmospheric component into Shergotty; it is at odds with scenarios that introduce the atmospheric component ~180 m.y. ago. Based on the variable ratio 40 Ar 129 Xe ∗ and its unshocked nature, Nakhla acquired its excess 129 Xe by a different process. 21 Ne-based cosmic-ray exposure ages agree well with values in the literature, but do not readily agree with the younger ages based on the abundance of cosmogenic 126 Xe. Neutron capture products are present in Nakhla Kr and there is evidence for a fission-like component in Xe of both Shergotty and Nakhla. GCR interactions with the SPB are unlikely to be the reason for the low 36 Ar 38 Ar ratio in SPB-Ar.

Trapped noble gases in unequilibrated ordinary chondrites

The abundances and isotopic compositions of noble gases were determined in bulk samples and acid-resistant residues of eight unequilibrated (type 3.0-3.8) ordinary chondrites (two LLs; four Ls, with two from a paired fall; two Hs) including the most primitive one, Semarkona. HL-Xe was found in all cases except Dhajala (H 3.8). The H-part of HL-Xe is isotopically indistinguishable from that in Allende; the same they find true for the ratio H-Xe/L-Xe. Thus, their data do not confirm previous reports of variations of this ratio among ordinary chondrites nor do they confirm that in ordinary chondrites this ratio is any different from that in Allende. Constraints on the origin of HL-Xe as well as the trapping mechanism are discussed. Also present in all meteorites except Dhajala is an HF/HCl soluble component with a high Ar/Xe ratio. The abundances of both isotopically ordinary Xe and HL-Xe correlate with petrological subtype. Combustion of a Dhajala acid-resistant residue yielded Ne with abundance ratios {sup 20}Ne/{sup 22}Ne = 10.1 {plus minus} 0.2 and {sup 21}Ne/{sup 22}Ne {le} 0.04 for that part of the Ne (Q-Ne) which is associated with the bulk of the (ordinary) heavy noble gases. The noble gases released in this experiment weremorexa0» associated with the combustion of only a minor portion of C; the highest {sup 132}Xe/C atom ratio of 1.3 {times} 10{sup {minus}8} was found for the first combustion step and is only 10 times lower than the solar ratio. In Semarkona (LL 3.0) there is evidence for the presence of Ne-E; in ALH A 77278 about half the Ne is of solar origin (SEP-Ne).«xa0less

On neutron-induced and other noble gases in Allende inclusions

Abstract Noble gases have been measured in one bulk sample, six light inclusions, and one black xenolith from four specimens of the carbonaceous chondrite Allende which cover a range in their 60Co content of about a factor of ten. The release pattern obtained upon heating the samples at different temperature steps shows: 1. 1.) The parent nuclides of radiogenic 4He are finely dispersed throughout the inclusions in host minerals with grain sizes smaller than the typical range of α-particles within them; 2. 2.) Neon from the inclusions contains a component deficient in 20,21Ne which is most conspicuous at low temperatures. The apparent excess 22Ne∗ is attributed to spallation reactions on sodium although the reasons for excluding NeE are weak; 3. 3.) 36Ar/38Ar ratios up to 90 are observed at release temperatures around 800°C. For inclusions from the same specimens the excess 36Ar∗ is proportional to the chlorine content, for different specimens the chlorine-normalized amounts of 36Ar∗ tend to correlate with 60Co; 4. 4.) 80,82Kr are found enriched about 80 fold, the ratio of the excess amounts 80 Kr ∗ 82 Kr ∗ is 2.68 ± 0.06; 5. 5.) Xenon is dominated by 129Xe, the overabundances are accompanied by much smaller excesses of 128Xe. The ratio of the excess amounts 128 Xe ∗ 129 Xe ∗ is constant for inclusions from the same specimens while for different specimens there is a range of a factor of four. The abundance anomalies in Ar and Kr as well as that of 128Xe can be explained as being due to the capture of neutrons by Cl, Br, and I, respectively, during the exposure of the meteoroid to the cosmic radiation. There is no compelling evidence for a contribution from an early neutron irradiation nor, in the case of 36Ar, for a contribution from extinct 36Cl. The ratio of the specific excesses 36 Ar ∗ 128 Xe ∗ as well as the 80 Kr ∗ 82 Kr ∗ ratio indicate no more than ~20 percent of the neutrons to have been thermal neutrons.

Noble gases in ureilites: cosmogenic, radiogenic, and trapped components

Abstract Abundances and isotopic compositions of Ne (in bulk samples only), Ar, Kr, and Xe have been investigated in 6 monomict, 3 polymict, and the diamond-free ureilite ALH78019 and their acid-resistant, C-rich residues. Isotopic ratios of Kr and Xe are very uniform and agree with data for ureilites from the literature. The measured ratio 38 Ar/ 36 Ar showed large variations due to an experimental artifact. This is shown to be connected to the pressure dependence of the instrumental mass discrimination, which for ureilites with their low abundance of 40 Ar is different from that of the usual air standard. This observation necessitates a reassessment for the recently reported 36 Ar excesses due to possible decay of extinct 36 Cl in the Efremovka meteorite. Trapped 22 Ne in the range of (1.4–2.5) × 10 −8 cc STP/g is present in bulk ureilites. A Ne three-isotope plot for polymict ureilites indicates the presence of solar Ne. 21 Ne-based cosmic ray exposure ages for the 10 ureilites studied range from 0.1 Ma (for ALH78019) to 46.8 Ma (for EET83309) All ureilites may have started with nearly the same initial elemental ratio ( 132 Xe/ 36 Ar) 0 , established in the nebula during gas trapping into their carbon carrier phases (diamond, amorphous C) by ion implantation. Whereas diamonds are highly retentive, amorphous C has suffered gas loss due to parent body metamorphism. The correlation of the elemental ratios 132 Xe/ 36 Ar and 84 Kr/ 36 Ar along the mass fractionation line could be understood as a two-component mixture of the unaffected diamond gases and the fractionated (to varying degrees) gases from amorphous C. In this view, the initial ratio ( 132 Xe/ 36 Ar) 0 is a measure of the plasma temperature in the nebula at the formation location of the carbon phases. Its lack of correlation with Δ 17 O (a signature of the silicate formation location) indicates that carbon phases and silicates formed independently in the nebula, and not from a carbon-rich magma The elemental ratios 132 Xe/ 36 Ar and 84 Kr/ 36 Ar in carbon-rich acid residues show a decreasing trend with depth (inferred from carbon consumption during combustion), which can be interpreted as a consequence of the ion implantation mechanism of gas trapping that leads to greater depth of implantation for lighter mass ion The similarity between trapped gases in phase Q in primitive chondrites and the C phases in ureilites—for both elemental and isotopic compositions—strongly suggests that phase Q might also have received its noble gases by ion implantation from the nebula. The slight differences in the elemental ratios can be explained by a plasma temperature at the location of phase Q gas loading that was about 2000 K lower than for ureilite C phases. This inference is also consistent with the finding that the trapped ratio 129 Xe/ 132 Xe (1.042 ± 0.002) in phase Q is slightly higher, compared to that of ureilite C phases (1.035 ± 0.002), as a consequence of in situ decay of 129 I, and becomes observable due to higher value of I/Xe in phase Q as a result of ion implantation at about 2000 K lower plasma temperature.

Nitrogen components in ureilites

Abundances and isotopic compositions of nitrogen and argon have been investigated in bulk samples as well as in acid-resistant C-rich residues of a suite of ureilites consisting of six monomict (Havero, Kenna, Lahrauli, ALH81101, ALH82130, LEW85328), three polymict (Nilpena, EET87720, EET83309), and the diamond-free ureilite ALH78019. Nitrogen in bulk ureilites varies from 6.3 ppm (in ALH 78019) to ∼55 ppm (in ALH82130), whereas C-rich acid residues have ∼65 to ∼530 ppm N, showing approximately an order of magnitude enrichment, compared with the bulk ureilites, somewhat less than trapped noble gases. Unlike trapped noble gases that show uniform isotopic composition, nitrogen shows a wide variation in δ15N values within a given ureilite as well as among different ureilites. The variations observed in δ15N among the ureilites studied here suggest the presence of at least five nitrogen components. The characteristics of these five N components and their carrier phases have been identified through their release temperature during pyrolysis and combustion, their association with trapped noble gases, and their carbon (monitored as CO + CO2 generated during combustion). Carrier phases are as follows: 1) Amorphous C, as found in diamond-free ureilite ALH78019, combusting at ≤500°C, with δ15N = –21‰ and accompanied by trapped noble gases. Amorphous C in all diamond-bearing ureilites has evolved from this primary component through almost complete loss of noble gases, but only partial N loss, leading to variable enrichments in 15N. 2) Amorphous C as found in EET83309, with similar release characteristics as component 1, δ15N ≥ 50‰ and associated with trapped noble gases. 3) Graphite, as clearly seen in ALH78019, combusting at ≥700°C, δ15N ≥ 19‰ and devoid of noble gases. 4) Diamond, combusting at 600–800°C, δ15N ≤ –100‰ and accompanied by trapped noble gases. 5) Acid-soluble phases (silicates and metal) as inferred from mass balance are expected to contain a large proportion of nitrogen (18 to 75%) with δ15N in the range –25‰ to 600‰. Each of the ureilites contains at least three N components carried by acid-resistant C phases (amorphous C of type 1 or 2, graphite, and diamond) and one acid-soluble phase in different proportions, resulting in the observed heterogeneity in δ15N. In addition to these five widespread components, EET83309 needs an additional sixth N component carried by a C phase, combusting at <700°C, with δ15N ≥ 153‰ and accompanied by noble gases. It could be either noble gas–bearing graphite or more likely cohenite. Some excursions in the δ15N release patterns of polymict ureilites are suggestive of contributions from foreign clasts that might be present in them. n nNitrogen isotopic systematics of EET83309 clearly confirm the absence of diamond in this polymict ureilite, whereas the presence of diamond is clearly indicated for ALH82130. Amorphous C in ALH78019 exhibits close similarities to phase Q of chondrites. n nThe uniform δ15N value of −113 ± 13 ‰ for diamond from both monomict and polymict ureilites and its independence from bulk ureilite δ15N, Δ17O, and %Fo clearly suggest that the occurrence of diamond in ureilites is not a consequence of parent body–related process. The large differences between the δ15N of diamond and other C phases among ureilites do not favor in situ shock conversion of graphite or amorphous C into diamond. A nebular origin for diamond as well as the other C phases is most favored by these data. Also the preservation of the nitrogen isotopic heterogeneity among the carbon phases and the silicates will be more consistent with ureilite formation models akin to “nebular sedimentation” than to “magmatic” type.

Comment on "The nature and origin of ureilites" by J. L. Berkley et al.

Abstract The record of primordial noble gases in ureilites imposes serious constraints on models of ureilite petrogenesis. It is argued that the inhomogeneous distribution between different mineral phases and the low 40 Ar 36 Ar ratio in particular can only be satisfied with difficulties in the model proposed by Berkley et al.

Potassium, stardust and the last supernova

No isotopic anomalies have yet been reported for K, but the relevant published literature is sparse and error limits for the scarce (0.01% abundance) isotope 40K are relatively large, 0.5 to 1%. We have developed thermal ionization mass spectrometric procedures by which error limits on 40K are an order of magnitude lower and applied them to analysis of a series of sequential dissolution fractions of the carbonaceous chondrites Orgueil (CI) and Murchison (CM), a sampling procedure known to reveal pervasive isotopic anomalies in Cr. Most of the fractions analyzed have 40K abundances that are normal (i.e., consistent with terrestrial composition) within analytical error limits. Whole–rock 40K abundances of Orgueil and Murchison are normal within about 1 permil or less. However, some dissolution fractions do exhibit evident isotopic anomalies, excesses of 40K up to about 35 ϵ. For K, as for Cr, the most plausible interpretation is that the anomalies reflect the presence of presolar grains that have not been thoroughly mixed with other solar system materials. In detail, the K anomalies do not correlate with the Cr anomalies, and thus probably represent different mineral carriers. Neither carrier phase is yet identified, but they differ from known and well-studied forms of presolar grains in that they are not acid-resistant. The isotopes of K are likely co-synthesized with some short-lived radionuclides, notably 26Al, the presence of which in the early solar system demands a “late” nucleosynthetic injection into the interstellar molecular cloud from which the solar system formed, no more than about 1 Ma before its collapse. It has been suggested that the distribution of 26Al (and other short-lived radionuclides) in early solar system materials was radically heterogeneous, perhaps because of the late injection. Because of its relatively short half-life (1.25 Ga), not in the “extinct radionuclides” range but still short compared to the age of the galaxy, 40K provides a usefully sensitive measure of the distribution of late nucleosynthetic additions to the solar system or its antecedent cloud. As a specific quantitative illustration, if a model 25-solar-mass supernova is invoked to account for observed levels of 26Al it will also provide about 1% of nebular 39K and about 3% of nebular 40K; the difference in the proportions of the K isotopes simply reflects the circumstance that by the time of solar system formation most of the 40K ever added to the sun’s precursor materials over the history of the galaxy had already decayed. There would be a 24‰ 40K anomaly between materials that did or did not incorporate such a contribution. The absence of so large a difference between the earth and the carbonaceous chondrites implies that they incorporate nearly the same amounts of any freshly synthesized K component of this magnitude. Unless some efficient mechanism for nebular scale chemical separation is postulated, the same should be true for co-synthesized nuclides such as 26Al.

The reactions 25Mg(a,n)28Si, 26Mg(a,n)29Si and their possible impact on nucleosynthesis

At the time neon burning is activated in massive stars, large amounts of 25Mg and 26Mg have been accumulated from the previous evolutionary phases. In particular, at typical temperatures for neon shell burning, the reactions 25Mg(α,n)28Si and 26Mg(α,n)29Si become efficient providing additional neutron fluxes and affecting the nucleosynthesis distribution. Good knowledge of the reaction rates is required to study their effect on nucleosynthesis during neon shell burning conditions (T9≳1.5, where T9 is the temperature in gigakelvin). At the Nuclear Science Laboratory, University of Notre Dame, IN, USA we developed a neutron detector to investigate the 25Mg(α,n)28Si and 26Mg(α,n)29Si reactions as possible neutron sources during neon shell burning. In the present paper we briefly describe the experimental procedures developed to measure those reactions. Finally, we discuss the impact of 25Mg(α,n)28Si and 26Mg(α,n)29Si on the s‐process nucleosynthesis for the barium isotopes.