Leo Alaerts

Isotopic anomalies of noble gases in meteorites and their origins—VI. Presolar components in the Murchison C2 chondrite

Noble gases were analyzed by stepped heating in 5 fractions of a chemically resistant residue (0.27%) from Murchison that had been separated according to grain size and resistance to HClO4. Nine gas components were recognized, of which 3 appear to be presolar. 1. (1) Ne-E(H): (Ne20Ne22 < 0.80, Ne21Ne22 < 0.0075), released at 1000–1600° and located in spinel. 2. (2) Ne-E(L): (Ne20Ne22 2.6 and > 0.7), and may represent monoisotopic, monoelemental Ne22 components from the decay of Na22. 3. (3) s-process Xe and Kr: (Xe130Xe132 = 0.46 ± 0.07; Kr86Kr82 = 2.5 ± 1.0), released at 1200° 1600° and located in a poorly characterized, possibly carbonaceous phase distinct from the host phase of Ne-E(L). It is accompanied by relatively small amounts of other noble gases (He3Xe5130 ~ 1400; NeE22Xe5130 ≤ 500). None of the three presolar carriers shows a clear excess of He3 or Ne21 that could be attributed to a presolar irradiation. A new composition of s-Xe has been derived from 17 Murchison or Orgueil fractions enriched in this component. It is similar to an earlier estimate by Srinivasan and Anders (1978), and shows the characteristic dominance of isotopes 128, 130 and 132. The spectrum of s-Kr is more tentative, but shows a clear enrichment of Kr86. This indicates that the neutron capture lifetime for Kr85 during the s-process was comparable to its 10.76 yr β-decay half life. Components of solar-system origin include two varieties of Ne-A, differing in host phase, release temperature, 2122 ratio, and associated Xe component: ComponentMineralT(°C)Ne20/Ne22Ne21/Ne22Xe136/Xe132Ne-A1Polymer≤8008.70 ± 0.110.024 ± 0.0010.313Ne-A2Chromite1000−12008.54 ± 0.080.035 ± 0.001≥ 0.54 The high 2122 ratio of Ne-A2 is responsible for the ostensible ‘excesses’ of spallogenic Ne21 in chromiterich separates from Murchison and Allende (Srinivasanet al., 1977; Lewiset al., 1977a). The ArKrXe components in Murchison ferrichromite could not be measured in pure form, due to contamination with polymer. However, mixing calculations suggest that they are identical to their counterparts in Allende. Apparently Murchison and Allende ferrichromite formed under very similar conditions, and hence have similar gas components, including CCFXe.

Isotopic anomalies of noble gases in meteorites and their origins—III. LL-chondrites

Nine LL-chondrites were studied by a selective etching technique, to characterize the noblegas components in three mineral fractions: HF-HCl-solubles (silicates, metal, troilite, etc.; comprising ∼ 99% of the meteorite), chromite and carbon (∼ 0.3–0.7%) and Q (a poorly characterized mineral defined by its solubility in HNO3, comprising ∼ 0.05% of the meteorite but containing most of the Ar, Kr, Xe and a neon component of 20Ne22Ne = 10.9 ± 0.8). The 20Ne36Ar ratio in Q falls wi petrologic type and rising 36Ar content, as expected for condensation from a cooling solar nebula, but contrary to the trend expected for metamorphic losses. Chondrites of different petrologic types therefore cannot all be derived from the same volatile-rich ancestor, but must have formed over a range of temperatures, with correspondingly different intrinsic volatile contents. The CCFXe (carbonaceous chondrite fission) component varies systematically with petrologic type. The most primitive LL3s (Krymka, Bishunpur, Chainpur) contain substantial amounts of CCFXe in chromite-carbon, enriched relative to primordial Xe as shown by high 136Xe132Xe (0.359–0.459, vs 0.310 for primordial Xe). These are accompanied by He and by Ne with 20Ne22Ne ≈ 8.0 and by variable amounts of a xenon component enriched in the light isotopes. The chromite in these meteorites is compositionally peculiar, containing substantial amounts of Fe(III). These meteorites, as well as Parnallee (LL3) and Hamlet (LL4) also contain CCFXe in phase Q, heavily diluted by primordial Xe (136Xe132Xe = 0.317–0.329). On the other hand, LL5s and 6s (Olivenza, St. Severin, Manbhoom and Dhurmsala) contain no CCFXe in either mineral. This deficiency must be intrinsic rather than caused by metamorphic loss, because Q in these meteorites still contains substantial amounts of primordial Ne. If CCFXe comes from a supernova, then its distribution in LL-chondrites requires three presolar carrier minerals of the right solubility properties, containing three different xenon components in certain combinations. These minerals must be appropriately distributed over the petrologic types, together with locally produced Q containing primordial gases, and they must be isotopically normal, in contrast to the gases they contain. On the other hand, if CCFXe comes from fission of a volatile superheavy element, then its decrease from LL3 to LL6 can be attributed to progressively less complete condensation from the solar nebula. Ad hoc assumptions must of the host phase Q, its association with ferrichromite and the origin of the associated xenon component enriched in the light isotopes. Soluble minerals in LL3s and LL4s contain a previously unobserved, solar xenon component, which, however, is not derived from the solar wind. Three types of ‘primordial’ xenon thus occur side-by-side in different minerals of the same meteorite: strongly fractionated Xe in ferrichromite and carbon, lightly fractionated Xe in phase Q, and ‘solar’ Xe in solubles. Because the first two can apparently be derived from the third by mass fractionation, it seems likely that all were trapped from the same solar nebula reservoir, but with different degrees of mass fractionation.

Isotopic anomalies of noble gases in meteorites and their origins—IV. C3 (Ornans) carbonaceous chondrites

Abstract Noble gases were measured in bulk samples of the C3V chondrites Grosnaja, Vigarano, and Leoville, and in HF,HCl-insoluble residues before and after etching with HNO 3 . The residues were characterized by INAA and SEM. Gas components were determined, directly or by subtraction, for the following fractions: HF , HCl - solubles (⪢98% of the meteorite), ‘ sphase Q ’, a poorly characterized trace mineral that is insoluble in HCl-HF but soluble in HNO 3 , and an insoluble residue , consisting of ferrichromite, carbonaceous matter, and spinel. Bulk meteorites show some correlation of the noble-gas pattern with McSweens subclasses: two ‘oxidized’ C3Vs—Allende (LEWIS et al , 1975) and Grosnaja— have lower Ar/Xe but higher Ne/Xe ratios than the ‘reduced’ C3Vs—Vigarano and Leoville—which are transitional to LL3s and C3O chondrites in both respects. An HCl-soluble mineral of high Ar/Xr ratio seems to be responsible. In other respects, the 3 C3Vs of this study resemble Allende, with only moderate differences. Phase Q contains most of the Ar, Kr, Xe, but only small amounts of Ne; the etched residues contain planetary Ne ( Ne 20 Ne 22 ≅ 8.5 ) and the controversial CCFXe component, enriched in the heavy Xe isotopes ( Xe136 Xe132 ≅ 0.4–0.5 ). The CCFXe is accompanied by an ‘L-Xe’ component that is enriched in the light Xe isotopes. The proportion of the two is virtually constant in C3Vs. as in all other C-chondrites. in contrast to the ~ 2-fold variation in ordinary chondrites. C3Vs have systematically higher Xe 136 Xe 132 ratios, and hence higher ratios of CCFXe to planetary Xe, than do other chondrite classes. This may reflect some peculiarity in their formation conditions, favoring uptake of CCFXe.

Primordial noble gases in chondrites: the abundance pattern was established in the solar nebula

Ordinary chondrites, like carbonaceous chondrites, contain primordial noble gases mainly in a minor phase comprising ≤0.05 percent of the meteorite, probably an iron-chromium sulfide. The neon-20/argon-36 ratios decrease with increasing argon-36 concentration, as expected if the gas pattern was established by condensation from the solar nebula, and was negligibly altered by metamorphism in the meteorite parent bodies. Meteoritic and planetary matter apparently condensed over a substantial range of temperatures.

A carbonaceous inclusion from the Krymka LL-chondrite - Noble gases and trace elements

Abstract A black inclusion from the Krymka LL3 chondrite was analyzed for 20 trace elements and five noble gases, by radiochemical neutron activation and mass spectrometry. The trace element pattern somewhat resembles that of C1 or C2 chondrites, but with several unique features. Elements of nebular condensation T ≳ 1000 K (U, Re, Os, Ir, Ni, Pd, Au, Sb and Ge) are essentially undepleted, as in C1 chondrites, but Re Ir is 1.49 × higher than the characteristic Cl value. Among elements condensing below 1000 K, Cs, Se, Te, and In are depleted to approximately C2 levels (~0.6 × C1), whereas Ag, Bi, Tl are enriched to ~ 1.6 × C1. Such enrichments are thought to be characteristic of late nebular condensates. The noble-gas pattern also is unique. Gas contents are higher than in C1s, by factors of 2.6 to 19 for Ne through Xe. The Ar 36 Xe 132 ratio of 500 is higher than mean values for C1s or C2s (109 or 89) and exceeds even the highest value seen in C3Os, 420, whereas the He 4 Ne 20 ratio of 62 is much lower than the values for C1s and C2s (200–370). The Xe 129 Xe 132 and Xe 136 Xe l32 ratios of 1.040 and 0.320 resemble those of C1 chondrites, and seem to imply typical proportions of radiogenic Xe129 and ‘fissiogenic’ xenon. It appears that the inclusion represents a new primitive meteorite type, similar to C-chondrites, but probably a late condensate from a region of higher nebular pressure.

Isotopic anomalies in meteorites and their origins—V. Search for fission fragment recoils in Allende sulfides

Abstract Three troilite- and pentlandite-rich samples from the Allende C3 chondrite were analyzed for Xe (and in one case Ne and Ar) by mass spectrometry, in 13–22 temperature steps. All samples released a small ‘CCFXe’ component (enriched in the heavy isotopes Xe 134, 136 ) at the relatively low temperature of 700–800°C, ahead of adsorbed atmospheric Xe (~900°C), radiogenic Xe 129 (1000°C), and primordial Xe (1250°C). Though such a labile component suggests implanted fission recoils, the simultaneous release of Ne, Ar, and Xe 124, 126 shows that it instead comes from carbon and perhaps chromite, two major host phases of CCFXe. Apparently small amounts of these phases are occluded in sulfides, and decompose by chemical reaction upon heating. Thus the experiment fails to resolve the nature of CCFXe. A marked enrichment of Xe 124 , without corresponding enrichments in Xe 126 or Xe 131–136 , was observed in the 550–650° and 1400–1500° fractions. Though requiring confirmation, it supports earlier evidence for the complexity and variability of the light xenon component, contrary to claims that it is an integral part of CCFXe.

On the kinetics of volatile loss from chondrites

We have re-examined data by Lipschutz and coworkers on thermal release of T1, Bi, In from primitive chondrites, in order to obtain information on the nature and activation energy (E) of the release processes: desorption, volume diffusion, and decomposition of the host phase. Plausible though not definitive choices may be made in some cases. For the Allende C3 chondrite, the main release for Bi and T1 (80 and 86%) between 400 and 700°C appears to be due to desorption of a surface layer, coupled with grain boundary diffusion as the slow step. The main release of In (80%) above 600°C and the small (10–20%) tails of Bi and T1 between 700 and 1000°C probably represent volume diffusion, with activation energies near 30 kcal/mol. The much smaller Es (2–5 kcal/mol) found for this interval by the Purdue group are artifacts, resulting from their failure to correct the initial concentration for the material lost in the preceding peak. Finally, the residual Bi and T1 remaining at 1000°C seem to represent solid solutions in temperature-resistant phases, such as ‘Q’, the principal carrier of planetary noble gases in the meteorite. This distribution—a small amount in solid solution and a large amount in a surface film—qualitatively agrees with that predicted by Larimer (1973, Geochim. Cosmochim. Acta 37, 1603–1623) for condensation from the solar nebula, though some of the substrates may have been sulfides rather than metal. Results for Abee and other primitive meteorites are essentially similar, except for a very abrupt 500°C release of T1 from Krymka (81%) and Bi from Tieschitz (70%). This release may represent decomposition of a thermolabile phase in a late condensate, such as organic matter or phyllosilicates. The presence of such a condensate (‘mysterite’) was inferred previously from the apparent overabundance of T1 and Bi in these meteorites.