Nitrogen abundances and isotopic compositions in stony meteorites

Nitrogen contents range from a few parts per million in ordinary chondrites and achondrites to several hundred parts per million in enstatite chondrites and carbonaceous chondrites. Four major isotopic groups are recognized: (1) C1 and C2 carbonaceous chondrites have δ¹⁵N of+30to+50%.; (2) enstatite chondrites have δ¹⁵N of?30to?40‰; (3) C3 chondrites have low δ¹⁵N with large internal variations; (4) ordinary chondrites have δ¹⁵N of?10to+20‰. The major variations are primary, representing isotopic abundances established at the time of condensation and accretion. Secondary processes, such as spallation reactions, solar wind implantation and metamorphic loss may cause small but observable isotopic variations in particular cases. The large isotopic difference between enstatite chondrites and carbonaceous chondrites cannot be accounted for by equilibrium condensation from a homogeneous nebular gas, and requires either unusually large kinetic effects, or a temporal or spatial variation of isotopic composition of the nebula. Nitrogen isotopic heterogeneity in the nebula due to nuclear processes has not been firmly established, but may be required to account for the large variations found within the Allende and Leoville meteorites. The unique carbonaceous chondrite, Renazzo, has δ¹⁵N of+170%., which is well beyond the range of all other data, and also requires a special source. It is not yet possible, from the meteoritic data, to establish the mode of accretion of nitrogen onto the primitive Earth.     

Nucleosynthetic osmium isotope anomalies in acid leachates of the Murchison meteorite

We present osmium isotopic results obtained by sequential leaching of the Murchison meteorite, which reveal the existence of very large internal anomalies of nucleosynthetic origin (ε¹⁸⁴Os from ? 108 to 460; ε¹⁸⁶Os from ? 14.1 to 12.6; ε¹⁸⁸Os from ? 2.6 to 1.6; ε¹⁹⁰Os from ? 1.7 to 1.1). Despite these large variations, the isotopic composition of the total leachable osmium (weighted average of the leachates) is close to that of bulk chondrites. This is consistent with efficient large-scale mixing of Os isotopic anomalies in the protosolar nebula. The Os isotopic anomalies are correlated, and can be explained by the variable contributions of components derived from the s, r and p-processes of nucleosynthesis. Surprisingly, much of the s-process rich osmium is released by relatively mild leaching, suggesting the existence of an easily leachable s-process rich presolar phase, or alternatively, of a chemically resistant r-process rich phase. Taken together with previous evidence for a highly insoluble s-process rich carrier, such as SiC, these results argue for the presence of several presolar phases with anomalous nucleosynthetic compositions in the Murchison meteorite. The s-process composition of Os released by mild leaching diverges slightly from that released by aggressive digestion techniques, perhaps suggesting that the presolar phases attacked by these differing procedures condensed in different stellar environments. The correlation between ε¹⁹⁰Os and ε¹⁸⁸Os can be used to constrain the s-process ¹⁹⁰Os/¹⁸⁸Os ratio to be 1.275 ± 0.043. Such a ratio can be reproduced in a nuclear reaction network for a MACS value for ¹⁹⁰Os of ~ 200 ± 22 mbarn at 30 keV. More generally, these results can help refine predictions of the s-process in the Os mass region, which can be used in turn to constrain the amount of cosmoradiogenic ¹⁸⁷Os in the solar system and hence the age of the Galaxy.We also present evidence for extensive internal variation of ¹⁸⁴Os abundances in the Murchison meteorite. A steep anti-correlation is observed between ε¹⁸⁴Os and ε¹⁸⁸Os. Since ¹⁸⁴Os is formed uniquely by the p-process, this anti-correlation cannot be explained by variable addition or subtraction of s-process Os to average solar system material. Instead, this suggests that p-process rich presolar grains (e.g., supernova condensates) may be present in meteorites in sufficient quantities to influence the Os isotopic compositions of the leachates. Nevertheless, ¹⁸⁴Os is a low abundance isotope and we cannot exclude the possibility that the measured anomalies for this isotope reflect unappreciated analytical artifacts.     

Stable calcium isotopic composition of meteorites and rocky planets

New measurements of mass-dependent calcium isotope effects in meteorites, lunar and terrestrial samples show that Earth, Moon, Mars, and differentiated asteroids (e.g., 4-Vesta and the angrite and aubrite parent bodies) are indistinguishable from primitive ordinary chondritic meteorites at our current analytical resolution (± 0.07‰ SD for the ⁴⁴Ca/⁴⁰Ca ratio). In contrast, enstatite chondritic meteorites are slightly enriched in heavier calcium isotopes (ca. + 0.5‰) and primitive carbonaceous chondritic meteorites are depleted in heavier calcium isotopes (ca. ? 0.5‰). The calcium isotope effects cannot be easily ascribed to evaporation or intraplanetary differentiation processes. The isotopic variations probably survive from the earliest stages of nebular condensation, and indicate that condensation occurred under non-equilibrium (undercooled nebular gas) conditions. Some of this early high-temperature calcium isotope heterogeneity is recorded by refractory inclusions (Niederer and Papanastassiou, 1984) and survived in planetesimals, but virtually none of it survived through terrestrial planet accretion. The new calcium isotope data suggest that ordinary chondrites are representative of the bulk of the refractory materials that formed the terrestrial planets; enstatite and carbonaceous chondrites are not. The enrichment of light calcium isotopes in bulk carbonaceous chondrites implies that their compositions are not fully representative of the solar nebula condensable fraction.     

Genetic relations between iron and stony meteorites

The main group pallasites and the mesosiderites fall within the oxygen isotope group previously determined for the calcium-rich achondrites (eucrites, howardites and diogenites), consistent with derivation from a common source material, and perhaps a common parent body. The group IIE iron meteorites were derived from the same source material as H-group ordinary chondrites. The chondrite-like silicate inclusions in group IAB iron meteorites are not related to the ordinary chondrites, but may be related to the enstatite chondrites. Several meteorites previously considered “anomalous” fall into these groups: Pontlyfni and Winona with the IAB irons, and Netschaëvo possibly with the H chondrites and IIE irons. The unusual pallasites Eagle Station and Itzawisis have remarkable oxygen isotopic compositions, and have more of the ¹⁶O-rich component than any other meteorite. Bencubbin and Weatherford are also unusual in their isotopic compositions, and may bear some relationship to the C2 carbonaceous chondrites. Lodran and Enon are isotopically similar to one another and are close to the achondrite-mesosiderite-pallasite group.     

Nitrogen isotope fractionations in the Fischer-Tropsch synthesis and in the Miller-Urey reaction

Nitrogen isotope fractionations have been measured in Fischer-Tropsch and Miller-Urey reactions in order to determine whether these processes can account for the large¹⁵N/¹⁴N ratios found in organic matter in carbonaceous chondrites. Polymeric material formed in the Fischer-Tropsch reaction was enriched in¹⁵N by only 3‰ relative to the starting material (NH₃). The¹⁵N enrichment in polymers from the Miller-Urey reaction was 10–12‰. Both of these fractionations are small compared to the 80–90‰ differences observed between enstatite chondrites and carbonaceous chondrites. These large differences are apparently due to temporal or spatial variations in the isotopic composition of nitrogen in the solar nebula, rather than to fractionation during the production of organic compounds.     

Lithium isotope composition of ordinary and carbonaceous chondrites, and differentiated planetary bodies: Bulk solar system and solar reservoirs

In order to better constrain the Li isotope composition of the bulk solar system and Li isotope fractionation during accretion and parent body processes, Li isotope compositions and concentrations were determined on a number of meteorite falls and finds. This is the first comprehensive study that systematically investigates a representative set of samples from carbonaceous chondrites (CI, CM2, CO3, CV3, CK4 and one ungrouped member), enstatite chondrites (EH, EL), ordinary chondrites (H, L, LL), and achondrites (one eucrite, diogenites, one pallasite, and a silicate inclusion from a IAB iron).

Carbonaceous chondrites have an average isotope composition of δ⁷Li = + 3.2‰ ± 1.9 (2σ) which agrees with the average composition of relatively pristine olivines (representative for the bulk composition) from the Earth primitive upper mantle (PUM). This is lighter than the average δ⁷Li of the basaltic differentiates of the Earth, Moon and Mars and the achondrites. It is an important observation, however, that the lighter end of the isotopic range of the differentiates always coincides with the averages of the mantle olivines and the carbonaceous chondrites. From this we conclude that the bulk of the inner solar system consists mostly of material from carbonaceous chondrites and that the variation seen in the differentiates is due to planetary body processes. Ordinary chondrites are significantly lighter than carbonaceous chondrites. No significant differences in δ⁷Li exist between enstatite chondrites (n = 3) and carbonaceous or ordinary chondrites. The difference between carbonaceous and ordinary chondrites and the variability within the chondrites could indicate the existence of distinct Li isotope reservoirs in the early solar nebula.     

The solar Na/Ca and S/Ca ratios: A close comparison with carbonaceous chondrites

Solar abundances based on recent laboratory oscillator strengths confirm the relationship between solar matter and carbonaceous chondrites. Within spectroscopic uncertainties (typically±40%) these meteorites contain refractory and volatile elements in solar proportions. Significant improvement of accuracy at present seems restricted to a few abundant elements having reliable quantum-mechanical oscillator strengths, and necessitates strictly differential spectrum analysis. Taking this into account, the solar abundance ratios Na/Ca and S/Ca have been determined to an accuracy of±15%. The results are:Na/Ca= 0.91and S/Ca= 6.8. These volatile/refractory ratios just match type 1 carbonaceous chondrites, but contrast with other types.These and related interstellar abundance features place constraints on the condensation process and a potential heterogeneity of the solar nebula. There is evidence that no drastic pre-solar separation of interstellar gas and grains has occurred, but minor imbalance may be a common mechanism co-determining stellar metal content.     

Oxygen isotopic heterogeneities,their petrological correlations,and implications for melt origins of chondrules in unequilibrated ordinary chondrites

Ten whole chondrules separated from the Dhajala (H3, 4), Hallingeberg (L3), and Semarkona (LL3) chondrites were individually analyzed for bulk element composition by instrumental neutron activation with half of each chondrule subsequently sacrificed for oxygen isotopic analysis and half retained for petrographic and electron microprobe analysis. On a three-isotope plot (δ¹⁷O vs. δ¹⁸O), the chondrules neither cluster near their respective chondrite hosts nor in the vicinities of previously recognized chondrite group averages. Instead, they define a trend resolvable into mixing and fractionation components but dominated by mixing in a manner similar to that previously observed for clasts from the LL3 chondrite ALHA76004. Covariations of chondrule isotopic mixing and fractionation parameters with petrological parameters were sought by two-variable linear least-squares regression analyses. However, the only two isotopic/petrological correlations significant at the 95% confidence level were δ¹⁷O vs. total bulk Fe (r = ?0.68) and mixing parameter,m¹⁸, vs. bulk weight ratio (CaO + Al₂O₃)/MgO (r = +0.67). Other correlations of apparent statistical significance were found by treating the chondrules as separate porphyritic (3 porphyritic olivine-pyroxene, 1 porphyritic olivine, 1 barred olivine) and non-porphyritic (4 radial pyroxene, 1 granular pyroxene/cryptocrystalline) textural subgroups. The reliability of the trends, based on so few samples, is not clear but the results at least indicate that possible existence of distinct isotopic/petrological subgroups of chondrules should be further investigated. Absence of certain isotopic/petrological trends expected as condensation effects argues against direct nebular condensation as the dominant process of chondrule formation. Instead, a model involving melting of heterogeneous solids, followed by various degrees of liquid/gas exchange, is favored. In any case, chondrule oxygen isotopic evolution was dominated by two-component mixing; fractional vaporization was, at most, a second-order effect. In addition to chondrules, parent bodies of unequilibrated ordinary chondrites must have also incorporated a¹⁶O-rich component which might have been fine-grained “matrix”.     

Lu-Hf and Sm-Nd isotopic systematics in chondrites and their constraints on the Lu-Hf properties of the Earth

Sm-Nd and Lu-Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm-Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean ¹⁴⁷Sm/¹⁴⁴Nd of 0.1960±0.0005, and a mean ¹⁴³Nd/¹⁴⁴Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm-Nd reference parameters, ¹⁴⁷Sm/¹⁴⁴Nd=0.1966 and ¹⁴³Nd/¹⁴⁴Nd=0.512638 (when fractionation-corrected to ¹⁴⁶Nd/¹⁴⁴Nd=0.7219).Lu-Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of ¹⁷⁶Lu/¹⁷⁷Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10⁻¹¹ year⁻¹ is used for the decay constant of ¹⁷⁶Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311-324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with ¹⁷⁶Lu/¹⁷⁷Hf less than 0.030 are employed, no combination of the main population of chondrite Lu-Hf data yields a regression with sufficiently low error to constrain the decay constant of ¹⁷⁶Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu-Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu-Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu-Hf characteristics results in a BSE Hf-Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial ε_Hf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu-Hf.     

The interstellar dust as a precursor of Ca,Al-rich inclusions in carbonaceous chondrites

(1) The observed anomalies in meteoritic oxygen isotope compositions are not due to an incomplete mixing of several dust or gas-plus-dust components in the solar nebula. If they were, other elements would display similar anomalies. (The FUN inclusions in Allende appear to be exceptions to this premise.) (2) The anomalies must therefore stem from differing degrees of incomplete exchange of oxygen isotopes between the primordial gas and dust components of the nebula. The dust is more likely to have been the¹⁶O-enriched component. (3) Since the isotopic difference between dust and gas probably could not have been preserved if the dust was ever completely vaporized in the nebula, the Ca,Al-rich inclusions (CAI's) in carbonaceous chondrites are unlikely to be condensates, but instead are distillation residues. (4) If so, the observed depletion of super-refractory elements in the Group II CAI's cannot have been accomplished by fractional condensation in the solar nebula. (5) Then this depletion, and a number of other properties of the components of primitive meteoritic material, must be relics of pre-solar system fractionations among different populations of interstellar dust grains.     

Primordial129Xe in meteorites

Xenon isotopic analyses by stepwise heating are presented for two neutron-irradiated chondrites, Arapahoe (L5) and Bjurböle (L4). The iodine-xenon formation age of Arapahoe is the oldest yet observed, 9.9 ± 0.8 m.y. before that of Bjurböle. It is thus unlikely that younger ages found in carbonaceous chondrite magnetite record the condensation of the solar nebula. The composition of trapped xenon in Arapahoe is normal except for a deficiency of¹²⁹Xe, where we infer ¹²⁹/Xe¹³²Xe= 0.56 ? 0.04, well below the apparent primordial solar system value. This need not conflict with higher values in other metamorphosed meteorites since growth of¹²⁹Xe from decay of¹²⁹I in xenon-depleted environments can be substantial. The contrast with apparent average solar system composition cannot be easily explained, however, since there is no way to generate one composition from the other. The simplest way to achieve low¹²⁹Xe seems to be to suppose that before decay to¹²⁹Xe r-process production at mass 129 condensed into dust as¹²⁹I, and that Arapahoe's parent body formed in a region of the solar system substantially depleted of this dust before any isotopic homogenization by vaporization of the remaining dust. Arapahoe is not unique in having trapped¹²⁹Xe-deficient xenon, nor in any other respect yet observed, so some such history evidently characterizes major groups of meteorites.     

Distribution of lead and thallium in the matrix of the Allende meteorite and the extent of terrestrial lead contamination in chondrites

Selective chemical dissolution has been used to study the distribution of Pb and Tl in an ultrafine ?20-μm matrix separate of Allende. The matrix was exposed to high-purity reagents ranging from H₂O, then HCl of increasing concentration and finally HF-HCl mixtures. A total of 17 extractions were obtained, each for a minimum period of 10 days. The isotopic compositions of the Pb released during the slow dissolution of the matrix fall into four distinct groups. The first, consisting of four extractions, released a component of terrestrial Pb isotopic composition with a total abundance of about 1 ppb. The next six extractions, which contained the bulk of the indigenous Pb and Tl corresponding to 96% and 94%, respectively, of the total matrix abundance, was of a reasonably homogeneous Pb isotopic composition with mean ratios of²⁰⁶Pb²⁰⁴Pb= 10.00and²⁰⁷Pb²⁰⁴Pb= 10.74. In the final seven extractions, the released Pb falls into two higher isotopic groupings and probably results from the dissolution of debris from chondrules and inclusions. The apparent age of the internal matrix isochron is4562 ± 14 My. The release of Pb and Tl shows a reasonable correlation with the matrix dissolution. This indicates that the Pb and Tl reside predominantly within the matrix phases rather than as a localised phase. The Tl isotopic composition of two matrix fractions and whole meteorite were measured and found to be indistinguishable from the terrestrial²⁰⁵Tl/²⁰³Tl ratio. Measurement of a terrestrial reagent standard in the range 1–10 ng Tl gave, for 20 analyses, a mean²⁰⁵Tl/²⁰³Tl ratio of2.38907 ± 0.00102 (2σ).The estimate of terrestrial Pb contamination is considerably lower than the 6–300 ppb assumed in some recent studies in order to explain the phenomenon of apparent excess radiogenic Pb in chondrites. The problem of terrestrial Pb pollution and the evidence which argues against a relatively severe and homogeneous Pb contamination of meteorites, is briefly considered. The apparent initial isotopic composition of the bulk of the indigenous Pb in the Allende matrix was found to be²⁰⁶Pb²⁰⁴Pb= 9.57and²⁰⁷Pb²⁰⁴Pb= 10.47. This is of a higher composition than the Pb in the Can?on Diablo troilite phase and further indicates that the phenomenon of apparent excess radiogenic Pb in chondrites is real.     

Cosmogenic neon produced from sodium in meteoritic minerals

Cosmogenic neon in sodium-rich oligoclase feldspar from the ordinary chondrites St. Severin and Guaren?a is characterized by an unusually high²²Ne/²¹Ne = 1.50 ± 0.02. This high ratio is due to the cosmogenic²²Ne/²¹Ne production ratio in sodium which is 2.9 ± 0.3, two to three times the production ratio in any other target element. The relative production rate of²¹Ne per gram sodium is one quarter the production rate per gram magnesium. The striking enrichment of²²Ne relative to²¹Ne in sodium arises from enhanced indirect production from²³Na via²²Na.The unusual composition of cosmogenic neon in sodium and sodium-rich minerals explains the high²²Ne/²¹Ne ratios observed in inclusions of the Allende carbonaceous chondrite, and observed during low-temperature extraction of neon from ordinary chondrites. The isotopic composition of cosmogenic neon released during the stepwise heating of a trapped gas-rich meteorite containing sodium-rich phases can be expected to vary, and use of a constant cosmogenic neon composition to derive the composition of the trapped gas may not be justified. Preferential loss of this²²Ne-enriched cosmogenic neon from meteoritic feldspar can result in a 2–3% drop in the measured cosmogenic²²Ne/²¹Ne ratio in a bulk meteorite sample. This apparent change in composition can lead to overestimation of the minimum pre-atmospheric mass of the meteorite by a factor of two.     

Al–Mg and Pb–Pb systematics of Allende CAIs: Canonical solar initial Al/Al ratio reinstated

The precise knowledge of the initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] is crucial if we are to use the very first solid objects formed in our Solar System, calcium–aluminum-rich inclusions (CAIs) as the “time zero” age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of ²⁷Al/²⁴Mg ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three micro-drilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined ²⁶Al–²⁶Mg fossil isochron with an (²⁶Al/²⁷Al)₀ of (5.23 ± 0.13) × 10^− 5. Internal mineral isochrons obtained for three of these CAIs (A44A, AJEF, and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with (²⁶Al/²⁷Al)₀ = (4.96 ± 0.25) × 10^− 5, anchored to our precisely determined absolute ²⁰⁷Pb–²⁰⁶Pb age of 4567.60 ± 0.36 Ma for the same mineral separates, reinstate the “canonical” (²⁶Al/²⁷Al)₀ of 5 × 10^− 5 for the early Solar System. The uncertainty in (²⁶Al/²⁷Al)₀ corresponds to a maximum time span of ± 20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the ²⁶Al–²⁶Mg and U–Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not support the “supra-canonical” ²⁶Al/²⁷Al ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supra-canonical ²⁶Al/²⁷Al ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS.The radiogenic ²⁰⁸Pb/²⁰⁶Pb ratio obtained as a by-product from the Pb–Pb age dating is used to estimate time-integrated ²³²Th/²³⁸U ratio (κ value) of CAIs. Limited κ variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the κ value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed.     

Isotopic composition of carbonaceous-chondrite kerogen: evidence for an interstellar origin of organic matter in meteorites

Stepwise combustion has revealed systematic patterns of isotopic heterogeneity for C, H and N in the insoluble organic fraction (m-kerogen) from the Orgueil and Murray carbonaceous chondrites. Those patterns are essentially identical for both meteorites, indicating a common source of m-kerogen. The data cannot be reconciled with a single mass-fractionation process acting upon a single precursor composition. This indicates either a multi-path history of mass-dependent processing or a significant nucleogenetic contribution, or both. If mass-fractionation were the dominant process, the magnitude of the observed isotopic variability strongly suggests that ion-molecule reactions at very low temperatures, probably in interstellar clouds, were responsible. In any case, an interstellar, rather than solar nebular, origin for at least some of the meteoritic organic matter is indicated. This has interesting implications for the origin of prebiotic molecules, temperatures in the early solar system, and the isotopic compositions of volatiles accreted by the terrestrial planets.     

Isotopic lead ages of chondritic meteorites

The isotopic composition of lead was determined for two carbonaceous, two H, and two L chondrites. All are falls. The²⁰⁶Pb/²⁰⁴Pb ratios cover a range from 9.45 to 37.33; the²⁰⁷Pb/²⁰⁴Pb ratios range from 10.39 to 26.10. The isotopic data define a²⁰⁷Pb/²⁰⁶Pb age of 4.635 AE. Uranium and lead concentration data indicate that the isotopic lead ages for the Bruderheim chondrite are concordant within approximately 20%. This contrasts with lead data in the literature for chondrites, which consistently indicate discordant isotopic lead ages due to large excesses of radiogenic lead by factors of two or more. The isotopic lead ages for Pultusk may be concordant; those for a sample of Richardton are not. The lack of concordance for Richardton is not due to the analytical procedures, rather it is likely a result of the handling history of the chondrite prior to analysis.The L-3 chondrite, Mezo-Madaras contains such a high concentration of lead - 5.27 PPM - that accurate ratios for primordial lead can be obtained. These are ²⁰⁶Pb/²⁰⁴Pb= 9.310;²⁰⁷Pb/²⁰⁴Pb= 10.296, values which are in close agreement with a recently reported measurement on lead in troilite from the Canyon Diablo iron meteorite.     

Ubiquitous isotopic anomalies in Ti from normal Allende inclusions

A newly developed technique for high-precision isotopic analyses of titanium was applied to terrestrial rocks and course- and fine-grained Allende inclusions. Repeated analyses of three terrestrial rocks gave excellent agreement (usually less than 2 × 10^?4 deviations) with a Ti metal standard. All seven Allende inclusions studied here were previously determined to contain isotopically normal Nd and/or Sm, indicating that none belongs to a small group of peculiar inclusions, dubbed as FUN inclusions. Yet, every inclusion showed a clearly resolvable excess at⁵⁰Ti, with ε(50/46) [deviation from the Ti standard in parts in 10⁴] ranging from +7 to +10 for five of the inclusions, while two pink fine-grained inclusions gave larger excesses of +15 and +28. Six inclusions also exhibited greater than 2σ deficits at⁴⁷Ti/⁴⁶Ti, with an average value greater than ?2 ε-units. Analyses of pyroxene and melilite separates and bulk samples from one inclusion furnished no evidence for Ti isotopic disequilibrium within a single inclusion. However, the possibility that pyroxene contributed the bulk of the Ti in all samples renders this a rather insensitive test. A graphical presentation of Ti isotopic abundances for these normal Allende inclusions, two previously analyzed FUN inclusions, and terrestrial samples demonstrates that at least three distinct components are required.The discovery of widespread isotopic anomalies in Ti fromnormal Allende inclusions establishes Ti as the first non-noble-gas element studied since oxygen to show such isotopic heterogeneity. A survey of published nucleosynthetic origins of Ti isotopes suggests that the dominant⁵⁰Ti excesses in these inclusions are due to the relative enrichment of isotopes synthesized during hydrostatic burning in or near the core of a massive star. Such a source is seemingly consistent with the absence of isotopic anomalies in previously analyzed elements, and can be tested via its prediction of similar excesses for the neutron-rich isotopes of Cr, Fe and Ni and their respective radioactive decay products.     

Oxygen isotopic constraints on the origin of Mg-rich olivines from chondritic meteorites

Chondrules are the major high temperature components of chondritic meteorites which accreted a few millions years after the oldest solids of the solar system, the calcium–aluminum-rich inclusions, were condensed from the nebula gas. Chondrules formed during brief heating events by incomplete melting of solid dust precursors in the protoplanetary disk. Petrographic, compositional and isotopic arguments allowed the identification of metal-bearing Mg-rich olivine aggregates among the precursors of magnesian type I chondrules. Two very different settings can be considered for the formation of these Mg-rich olivines: either a nebular setting corresponding mostly to condensation–evaporation processes in the nebular gas or a planetary setting corresponding mostly to differentiation processes in a planetesimal. An ion microprobe survey of Mg-rich olivines of a set of type I chondrules and isolated olivines from unequilibrated ordinary chondrites and carbonaceous chondrites revealed the existence of several modes in the distribution of the ?¹⁷O values and the presence of a large range of mass fractionation (several ‰) within each mode. The chemistry and the oxygen isotopic compositions indicate that Mg-rich olivines are unlikely to be of nebular origin (i.e., solar nebula condensates) but are more likely debris of broken differentiated planetesimals (each of them being characterized by a given ?¹⁷O). Mg-rich olivines could have crystallized from magma ocean-like environments on partially molten planetesimals undergoing metal–silicate differentiation processes. Considering the very old age of chondrules, Mg-rich olivine grains or aggregates might be considered as millimeter-sized fragments from disrupted first-generation differentiated planetesimals. Finally, the finding of only a small number of discrete ?¹⁷O modes for Mg-rich olivines grains or aggregates in a given chondrite suggests that these shattered fragments have not been efficiently mixed in the disk and/or that chondrite formation occurred in the first vicinity of the breakup of these planetary bodies.     

I-Xe age and trapped Xe components of the Murray (C-2) chondrite

A neutron-irradiated bulk sample of the Murray (C-2) carbonaceous chondrite was etched with H₂O₂ and then divided into colloidal and non-colloidal fractions. The H₂O₂ treatment removed ～80% of the trapped Xe and greatly increased variations in the¹²⁹Xe/¹³²Xe ratio measured in stepwise heating. The colloid showed very little excess¹²⁹Xe, but the anti-colloid gave a fairly good I-Xe correlation corresponding to formation 3.7 ± 2.1 m.y. after Bjurböle.Variations in the trapped Xe component were also observed; most notably the 550°C anti-colloid fraction has large deficiencies relative to AVCC at the heavy isotopes. A tentative decomposition suggests U-Xe, a “primitive” trapped component, as the dominant component with minor contributions from H-Xe, L-Xe, and S-Xe (s-process nucleosynthesis). The identification of U-Xe rests primarily on the agreement of themeasured¹³⁴Xe/¹³⁶Xe ratio with U-Xe. This observation lends support to proposals for such a “primitive” trapped Xe component and demonstrates that at least some carbonaceous chondrite phases sampled a xenon reservoir nearly devoid of H-Xe.     

A classification of meteorites based on oxygen isotopes

On the basis of¹⁸O/¹⁶O and¹⁷O/¹⁶O ratios, meteorites and planets can be grouped into at least six categories, as follows: (1) the terrestrial group, consisting of the earth, moon, differentiated meteorites and enstatite chondrites; (2) types L and LL ordinary chondrites; (3) type H ordinary chondrites; (4) anhydrous minerals of C2, C3, C4 carbonaceous chondrites; (5) hydrous matrix minerals of C2 carbonaceous chondrites; (6) the ureilites. Objects of one category cannot be derived by fractionation or differentiation from the source materials of any other category.