J. W. Arden

The chronology of the Nakhla achondritic meteorite

Abstract A well-defined internal Rb Sr isochron has been determined for the unbrecciated Nakhla achondrite, defining an age T = (1.24 ± 0.01) AE and an initial 87 Sr/ 86 Sr ratio of 0.70254 ± 0.00003. The excellence of fit to the isochron shows that a very thorough Sr isotopic equilibration took place at this time between all phases of the meteorite. Recent petrological observations indicate that the age most probably represents the time of an igneous crystallization event on the parent body of Nakhla. The age shows that we may no longer assume that major chemical and physical differentiation processes took place only in early solar system history. The Rb Sr systematics are shown not to require late formation of the Nakhla parent body. Geochemical similarities between Nakhla and the earth are discussed.

Evidence for Multiple Sources of Diamond from Primitive Chondrites

Fine-grained diamonds, the most abundant form of circumstellar dust isolated from primitive meteorites, have elemental and isotopic characteristics that are dependent on the host meteorite type. Carbon isotopic compositions vary from –32 to –38 per mil, and nitrogen associated with the diamond changes in overall abundance by over a factor of four from 0.2 to 0.9 weight percent, between ordinary and CM2-type chondrites. Although the ratio of carbon to nitrogen evolves in a distinctive way during combustion of diamond separates, metamorphic degassing of nitrogen is not the main cause of the differences in nitrogen content. The data suggest that intrinsic differences must have been inherited by the diamonds at the time of their formation and that the diamonds were distributed heterogeneously in the solar nebula during condensation. However, the hypothesis that a distinct nitrogen carrier remains hidden within the diamond cannot be ruled out.

Presolar components in the ordinary chondrites

Abstract As part of the search for presolar material in primitive meteorites eleven ordinary chondrites, ranging from petrologic type 3 to 6, have been analysed for C and N isotopes by stepped combustion. Isotopically anomalous components have been found only in those meteorites of petrologic type 3.6 or below. In terms of their isotopic composition and combustion temperature these components are similar to, but may not be identical with, those in carbonaceous chondrites. The presence of presolar components in ordinary chondrites may have important implications for the degree of reprocessing their precursor materials can have experienced in the nebula. Although the lack of isotopically anomalous components above type 3.6 may be due to heterogeneous accretion of the ordinary chondrite parent bodies, their destruction during metamorphism provides a more straightforward explanation.

Terrestrial carbon and nitrogen isotopic ratios from cretaceous-tertiary boundary nanodiamonds.

One hypothesis for the origin of the nanometer-size diamonds found at the Cretaceous-Tertiary (K-T) boundary is that they are relict interstellar diamond grains carried by a postulated asteroid. The 13C/12C and 15N/14N ratios of the diamonds from two sites in North America, however, show that the diamonds are two component mixtures differing in carbon and nitrogen isotopic composition and nitrogen abundance. Samples from a site from Italy show no evidence for either diamond component. All the isotopic signatures obtained from the K-T boundary are material well distinguished from known meteoritic diamonds, particularly the fine-grain interstellar diamonds that are abundant in primitive chondrites. The K-T diamonds were most likely produced during the impact of the asteroid with Earth or in a plasma resulting from the associated fireball.

A NEW TYPE OF METEORITIC DIAMOND IN THE ENSTATITE CHONDRITE ABEE

Diamonds with δ13C values of –2 per mil and less than 50 parts per million (by mass) nitrogen have been isolated from the Abee enstatite chondrite by the same procedure used for concentrating Cδ, the putative interstellar diamond found ubiquitously in primitive meteorites and characterized by δ13C values of –32 to –38 per mil, nitrogen concentrations of 2,000 to 12,500 parts per million, and δ15N values of –340 per mil. Because the Abee diamonds have typical solar system isotopic compositions for carbon, nitrogen, and xenon, they are presumably nebular in origin rather than presolar. Their discovery in an unshocked meteorite eliminates the possibility of origins normally invoked to account for diamonds in ureilites and iron meteorites and suggests a low-pressure synthesis. The diamond crystals are ∼100 nanometers in size, are of an unusual lath shape, and represent ∼100 parts per million of Abee by mass.

Xenon and neon from acid-resistant residues of Inman and Tieschitz

Abstract Xenon and neon from highly oxidized acid residues of the unequilibrated ordinary chondrites Inman and Tieschitz are dominated by Xe-HL and Ne-A2, although Xe-S is also present in the oxidized Inman residue. The isotopic composition of Xe released at lower temperatures from the Inman residues most likely represents mixtures of trapped Xe and Xe-HL at all isotopes except 129Xe and 128Xe where significant and correlated excesses are observed. These are clearly both derived from I, the former from decay of extinct 129I and the latter from neutron capture by 127I, requiring a thermal equivalent fluence of ~3 × 1017 n/cm2, or 2–3 × 1016 n/cm2 in the resonance region, confirmed by artificial irradiation of an aliquot. The 129 I 127 I ratio at Xe closure is about 1 × 10−4, similar to the canonical meteoritic value which, given the short half-life of 129I, suggests closure times similar to other meteorites and subsequent irradiation within the solar system. In Tieschitz there may be some evidence for separation of Xe-L from Xe-H as would be expected if the neutron-deficient end of the Xe-HL spectrum is not produced by the same processes as the neutronrich end and is not completely homogenized with it. Ne-E(H) is measured in oxidized residues from both of these meteorites, but no evidence is found for Ne-E(L). Correlations are observed between cosmogenic 21Ne and Ne-E(H) in the higher temperature fractions of both Inman and Tieschitz. These may not be due to a common host phase, but could instead be due to high temperature reactions between spinel and SiC.

Thallium and lead in the Allende C3V carbonaceous chondrite. A study of the matrix phase

Abstract Tl and Pb isotopic abundances have been measured in various phases from Allende and the distribution and siting of these elements in the matrix phase investigated. Matrix fractions, prepared by sieving, sedimentation, magnetic separation and acid etching, were further characterised by X-ray diffraction and SEM. Tl concentrations range from 1 ppb in coarse grained inclusions to 1560 ppb in the acid-etched carbon residues and from 32 ppb to 194 ppb in the 16 matrix fractions. Pb concentrations which range from 0.1 ppm to 3.1 ppm, are enhanced in magnetic phases and depleted in Allende pentlandite relative to the whole meteorite. The Tl-204Pb abundance diagram is described near the origin by the inclusions and chondrules and extends through sulphide to the non-magnetic and magnetic matrix fractions. Abundances in the finest grained matrix fractions form a linear trend which passes through the origin and the Orgueil and Murchison whole meteorite data. The deviation of magnetic matrix fractions from the above linear trend is probably related to the presence of an intimate association of an awaruite-sulphide-carbon species. 92% of the Tl in the carbon residues, which is released on hydrolysis with HCl, probably resides in an organic host molecule(s) on the macromolecular carbonaceous material, whereas the surface film of organics on the matrix grains shows no apparent enrichment of Tl. Tl and 204Pb abundances revealed an inverse correlation with grain size and a distribution within the grains rather than as a surface layer, is indicated. Constraints placed by the data on the formational environment of the matrix grains are considered. Interstellar shock heating and rapid radiative cooling is a possible mechanism for the establishment of the observed inverse correlation of volatile abundance with grain size. Matrix data do not lend support for a recent redistribution of lead as an explanation for the apparent excess Pb in Allende. The apparent initial Pb isotopic compositions of the matrix fractions are heterogeneous and not attributable to terrestrial contamination. The very magnetic fractions have high abundances of Pb and the least radiogenic apparent initial Pb compositions, whereas the non-magnetic fractions have lower Pb abundances but more radiogenic apparent initial Pb compositions. The data also indicate that use of the predicted Tl-204Pb cosmothermometer, to infer accretion temperatures, is apparently not valid for individual phases of Allende.

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

Abstract 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 H 2 O, 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 of 206 Pb 204 Pb= 10.00and 207 Pb 204 Pb= 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 is 4562 ± 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 terrestrial 205 Tl/ 203 Tl ratio. Measurement of a terrestrial reagent standard in the range 1–10 ng Tl gave, for 20 analyses, a mean 205 Tl/ 203 Tl ratio of 2.38907 ± 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 be 206 Pb 204 Pb= 9.57and 207 Pb 204 Pb= 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.

Uranium-lead age of the Bruderheim L6 chondrite and the 500-Ma shock event in the L-group parent body

Abstract Measured Pb-Pb whole meteorite data for the Bruderheim L6 chondrite scatter slightly about a line passing above Can˜on Diablo lead and yielding an age of 4.482 + 0.017 Ga, using the terrestrial 238 U/ 235 U ratio of 137.88. The measured U isotopic composition for Bruderheim, using the dissolution procedures employed for these U-Pb studies, is near the terrestrial composition. In the concordia diagram the U-Pb data chiefly plot above the concordia curve and define a line which intersects the concordia curve at 4.536 + 0.006 Ga and 0.495 Ga, but the data for Bruderheim cannot be understood at all in terms of the more usual two- or three-stage episodic U-Pb models involving a fixed μ-value in the first stage. Most samples show an apparent excess of radiogenic lead for single-stage (closed system) evolution when Can˜on Diablo troilite is used for the initial lead composition. Evidence is presented to show that the apparent excess radiogenic lead cannot be explained by terrestrial contamination alone. A different U-Pb model is presented which describes qualitatively and quantitatively most features of the U-Pb data for Bruderheim. If this model correctly describes the U-Pb evolution of Bruderheim then the “formation” age is given as 4.536 Ga by both U-Pb and Pb-Pb data, the meteorite U-Pb system was disturbed by a later (shock?) event at about 500 Ma ago, and the data are consistent with (though do not require) a Can˜on Diablo initial lead composition. This interpretation suggests that the classical phenomenon of apparent excess radiogenic lead reflects the application of a single-stage model to a meteorite that has evidently experienced at least a two-stage history. The explanation of the observation that in the concordia diagram most meteorite samples (corrected for Can˜on Diablo lead) plot in the lead excess region remains obscure, though this may be due to the wrong choice of initial lead for Bruderheim.

Uranium-lead abundances and isotopic studies in the chondrites Richardton and Farmington

Abstract 207 Pb/ 204 Pb- 206 Pb/ 204 Pb whole meteorite isochrons for Richardton (H5) and Farmington (L5) are presented and give Pb-Pb ages of 4.545 ± 0.010 and 4.620 ± 0.010 Ga respectively (errors ± 2σ). The Pb-Pb isochron for Farmington passes below the Can˜on Diablo troilite composition, which may therefore not be the initial Pb composition for this meteorite. All samples show an apparent excess radiogenic lead for single-stage (closed-system) evolution when Can˜on Diablo troilite is used for the initial lead composition. Evidence is presented to show that the apparent excess Pb cannot be explained by terrestrial contamination. There is no unique isotopic composition for initial lead that yields concordant ages at 4.55 Ga for all samples for either meteorite. The data likewise cannot be reconciled to Can˜on Diablo initial lead through any of the conventional two- and three-stage evolution models. The apparent excess Pb, with respect to a Can˜on Diablo troilite composition and a single-stage closed-system history, and the apparent inhomogeneous initial Pb isotopic compositions, appear to be real. This may be an indication that the U-Pb systems in these meteorites are disturbed, but this disturbance cannot be described consistently by any of the conventional episodic evolution models.