Carbon,Oxygen, Silicon,Sulphur, Calcium,and Iron Determinations in Twenty-four Dark Clasts and One Dark Inclusion of the Allende Meteorite by 12C(d,p)13C Nuclear Reaction and PIXE

Abstract— A study of carbon contents of millimeter-sized dark clasts and of one dark inclusion of the Allende meteorite with the ¹³C(d,P₀)¹³C method has been undertaken. The results show that C in dark Allende areas varies, perhaps in a “clustered” fashion, from near 0.3%, which is essentially the bulk C-content of the meteorite, to at least 2.1% in the dark rim of a dark inclusion. At least one cluster has now been identified with C-contents in the range 0.4%-0.5%. Raman spectra of C in the dark inclusion ADIR 4E1-fu and in matrix have also been determined. These are identical. Our results are consistent with a comparatively low temperature origin of essentially all elemental carbon in Allende.     

Comparison of cosmic‐ray exposure ages and trapped noble gases in chondrule and matrix samples of ordinary,enstatite, and carbonaceous chondrites

Abstract— We performed a comprehensive study of the He, Ne, and Ar isotopic abundances and of the chemical composition of bulk material and components of the H chondrites Dhajala, Bath, Cullison, Grove Mountains 98004, Nadiabondi, Ogi, and Zag, of the L chondrites Grassland, Northwest Africa 055, Pavlograd, and Ladder Creek, of the E chondrite Indarch, and of the C chondrites Hammadah al Hamra 288, Acfer 059, and Allende. We discuss a procedure and necessary assumptions for the partitioning of measured data into cosmogenic, radiogenic, implanted, and indigenous noble gas components. For stone meteorites, we derive a cosmogenic ratio ²⁰Ne/²²Ne of 0.80 ± 0.03 and a trapped solar ⁴He/³He ratio of 3310 ± 130 using our own and literature data. Chondrules and matrix from nine meteorites were analyzed. Data from Dhajala chondrules suggest that some of these may have experienced precompaction irradiation by cosmic rays. The other chondrules and matrix samples yield consistent cosmic‐ray exposure (CRE) ages within experimental errors. Some CRE ages of some of the investigated meteorites fall into clusters typically observed for the respective meteorite groups. Only Bath's CRE age falls on the 7 Ma double‐peak of H chondrites, while Ogi's fits the 22 Ma peak. The studied chondrules contain trapped ²⁰Ne and ³⁶Ar concentrations in the range of 10^?6–10^?9 cm³ STP/g. In most chondrules, trapped Ar is of type Q (ordinary chondritic Ar), which suggests that this component is indigenous to the chondrule precursor material. The history of the Cullison chondrite is special in several respects: large fractions of both CR‐produced ³He and of radiogenic ⁴He were lost during or after parent body breakup, in the latter case possibly by solar heating at small perihelion distances. Furthermore, one of the matrix samples contains constituents with a regolith history on the parent body before compaction. It also contains trapped Ne with a ²⁰Ne/²²Ne ratio of 15.5 ± 0.5, apparently fractionated solar Ne.     

Oxygen isotope signatures in bulk chondrules: Implications for the aqueous alteration and thermal metamorphism on the Allende CV3 parent body

Precise triple oxygen isotope compositions of 32 Allende bulk chondrules (ABCs) are determined using laser‐assisted fluorination mass spectrometry. Various chemically characterized chondrule types show ranges in δ¹⁸O that vary from ?4.80‰ to +1.10‰ (porphyritic olivine; PO, N = 15), ?3.10‰ to +1.50‰ (porphyritic olivine pyroxene; POP, N = 9), ?3.40‰ to +2.60‰ (barred olivine; BO, N = 4), and ?3.60‰ to +1.30‰ (porphyritic pyroxene; PP, N = 3). Oxygen isotope data of these chondrules yield a regression line referred to as the Allende bulk chondrule line (ABC line, slope = 0.86 ± 0.02). Most of our data fall closer to the primitive chondrule minerals line (PCM line, slope = 0.987 ± 0.013) and the carbonaceous chondrite anhydrous mineral line (CCAM line, slope = 0.94 ± 0.02) than the Allende anhydrous mineral line (AAML, slope = 1.00 ± 0.01) with a maximum δ¹⁸O value (+2.60‰) observed in a BO chondrule and a minimum δ¹⁸O value (?4.80‰) shown by a PO chondrule. Similarly, these chondrules depict variable ?¹⁷O values that range from ?5.65‰ to ?3.25‰ (PO), ?4.60‰ to ?2.80‰ (POP), ?4.95‰ to ?3.00‰ (BO), ?5.30‰ to ?3.20‰ (PP), and ?4.90‰ (CC). A simple model is proposed for the Allende CV3 chondrite with reference to the AAML and PCM line to illustrate the isotopic variations occurred due to the aqueous alteration processes. The estimated temperature ranging from 10 to 130 °C (mean ~60 °C) implies that the secondary mineralization in Allende happened in a warmer and relatively dry environment compared to Murchison. We further propose that thermal metamorphism could have dehydrated the Allende matrix at temperatures between >150 °C and <600 °C.     

A XANES and Raman investigation of sulfur speciation and structural order in Murchison and Allende meteorites

Insoluble organic matter (IOM) and hydrothermally treated IOM extracted from two carbonaceous chondrites, Murchison and Allende, was studied using sulfur K‐edge XANES (X‐ray absorption near edge structure) and μ‐Raman spectroscopy, with the aim to understand their IOM's sulfur speciation and structural order, and how aqueous alteration or thermal metamorphism may have transformed these materials. We found that the sulfur‐functional group chemistry of both the Murchison IOM and hydrothermally treated IOM samples have a large chemical variability ranging from oxidation states of S^?2 to S⁺⁶, and exhibit a transformation in their oxidation state after the hydrothermal treatment (HT) to produce thiophenes and thiol compounds. Sulfoxide and sulfite peaks are also present in Murchison. Sulfates considered intrinsic to Murchison are most likely preaccretionary in nature, and not a result of reactions with water at high temperatures on the asteroid parent body. We argue that the reduced sulfides may have formed in the CM parent body, while the thiophenes and thiol compounds are a result of the HT. Micro‐Raman spectra show the presence of aliphatic and aromatic moieties in Murchison's material as observed previously, which exhibits no change after HT. Because the Murchison IOM was modified, as seen by XANES analysis, absence of a change observed using micro‐Raman indicated that although the alkyl carbons of IOM were cleaved, the aromatic network was not largely modified after HT. By contrast, Allende IOM contains primarily disulfide and elemental sulfur, no organic sulfur, and shows no transformation after HT. This nontransformation of Allende IOM after HT would indicate that parent body alteration of sulfide to sulfate is not feasible up to temperatures of 300°C. The reduced sulfur products indicate extreme secondary chemical processing from the precursor compounds in its parent body at temperatures as high as 624°C, as estimated from μ‐Raman D band parameters. The Raman parameters in Allende IOM that was interpreted in terms of amorphous carbon with regions of large clusters of benzene rings, was transformed after the HT to those with fewer benzene rings.     

Cr isotopes in physically separated components of the Allende CV3 and Murchison CM2 chondrites: Implications for isotopic heterogeneity in the solar nebula and parent body processes

Chromium isotopic data of physically separated components (chondrules, CAIs, variably magnetic size fractions) of the carbonaceous chondrites Allende and Murchison and bulk rock data of Allende, Ivuna, and Orgueil are reported to evaluate the origin of isotopic heterogeneity in these meteorites. Allende components show ε⁵³Cr and ε⁵⁴Cr from ?0.23 ± 0.07 to 0.37 ± 0.05 and from ?0.43 ± 0.08 to 3.7 ± 0.1, respectively. In components of Murchison, ε⁵³Cr and ε⁵⁴Cr vary from ?0.06 ± 0.08 to 0.5 ± 0.1 and from 0.7 ± 0.2 to 1.7 ± 0.1, respectively. The non‐systematic variations of ε⁵³Cr and ⁵⁵Mn/⁵²Cr in the components of Allende and Murchison were likely caused by small‐scale, alteration‐related redistribution of Mn >20 Ma after formation of the solar system. Chondrule fractions show the lowest ⁵⁵Mn/⁵²Cr and ε⁵⁴Cr values of all components, consistent with evaporation of Mn and ε⁵⁴Cr‐rich carrier phases from chondrule precursors. Components other than the chondrules show higher Mn/Cr and ε⁵⁴Cr, suggestive of chemical and isotopic complementarity between chondrules and matrix‐rich fractions. Bulk rock compositions calculated based on weighted compositions of components agree with measured Cr isotope data of bulk rocks, in spite of the Cr isotopic heterogeneity reported by the present and previous studies. This indicates that on a sampling scale comprising several hundred milligrams, these meteorites sampled isotopically and chemically homogeneous nebular reservoirs. The linear correlation of ⁵⁵Mn/⁵²Cr with ε⁵³Cr in bulk rocks likely was caused by variable fractionation of Mn/Cr, subsequent mixing of phases in nebular domains, and radiogenic ingrowth of ⁵³Cr.     

40Ar39Ar ages of Allende

KAr and/or ⁴⁰Ar³⁹Ar plateau ages of Allende samples—whole rock, matrix, chondrules, white inclusions–range from 3.8 AE for matrix of ?5 AE for some white inclusions, but cluster strongly near 4.53 AE. This age marks the dominant KAr resetting of Allende materials. Age spectra show disturbances due to ³⁹Ar recoil or some other argon redistribution processes. Possible explanations for the apparent presolar ages (>4.6 AE) include: ?20% loss of ³⁹Ar; ?40% loss of ⁴⁰K ～3.8 AE ago with no loss of ⁴⁰Arl trapped argon of unique ⁴⁰Ar/³⁶Ar isotopic composition; admixture of “very old” presolar grains.     

Evidence for an impact‐induced magnetic fabric in Allende,and exogenous alternatives to the core dynamo theory for Allende magnetization

We conducted a paleomagnetic study of the matrix of Allende CV3 chondritic meteorite, isolating the matrix's primary remanent magnetization, measuring its magnetic fabric and estimating the ancient magnetic field intensity. A strong planar magnetic fabric was identified; the remanent magnetization of the matrix was aligned within this plane, suggesting a mechanism relating the magnetic fabric and remanence. The intensity of the matrix's remanent magnetization was found to be consistent and low (~6 μT). The primary magnetic mineral was found to be pyrrhotite. Given the thermal history of Allende, we conclude that the remanent magnetization was formed during or after an impact event. Recent mesoscale impact modeling, where chondrules and matrix are resolved, has shown that low‐velocity collisions can generate significant matrix temperatures, as pore‐space compaction attenuates shock energy and dramatically increases the amount of heating. Nonporous chondrules are unaffected, and act as heat‐sinks, so matrix temperature excursions are brief. We extend this work to model Allende, and show that a 1 km/s planar impact generates bulk porosity, matrix porosity, and fabric in our target that match the observed values. Bimodal mixtures of a highly porous matrix and nominally zero‐porosity chondrules make chondrites uniquely capable of recording transient or unstable fields. Targets that have uniform porosity, e.g., terrestrial impact craters, will not record transient or unstable fields. Rather than a core dynamo, it is therefore possible that the origin of the magnetic field in Allende was the impact itself, or a nebula field recorded during transient impact heating.     

Isotopic Hg in an Allende carbon-rich residue

Abstract— A carbon-rich residue from Allende subjected to stepwise heating yielded two isotopically resolvable types of Hg: one with a ¹⁹⁶Hg/²⁰²Hg concentration ratio the same as terrestrial (monitor) Hg; the other enriched in ¹⁹⁶Hg relative to ²⁰²Hg by ～60%. Hg with the 202 isotope enriched relative to 196, as is found in bulk Allende, was not observed. Whether the result of mass fractionation or nucleosynthesis, the distinct types of Hg entered different carrier phases and were not thermally mobilized since the accretion of the Allende parent body.     

Pb isotopic age of the Allende chondrules

Abstract— We have studied Pb‐isotope systematics of chondrules from the oxidized CV3 carbonaceous chondrite Allende. The chondrules contain variably radiogenic Pb with a ²⁰⁶Pb/²⁰⁴Pb ratio between 19.5–268. Pb‐Pb isochron regression for eight most radiogenic analyses yielded the date of 4566.2 ± 2.5 Ma. Internal residue‐leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566.6 ± 1.0 Ma, our best estimate for an average age of Allende chondrule formation. This Pb‐Pb age is consistent with the range of model ²⁶Al‐²⁶Mg ages of bulk Allende chondrules reported by Bizzarro et al. (2004) and is indistinguishable from Pb‐Pb ages of Ca‐Al‐rich inclusions (CAIs) from CV chondrites (4567.2 ± 0.6 Ma) (Amelin et al. 2002) and the oldest basaltic meteorites. We infer that chondrule formation started contemporaneously with or shortly after formation of CV CAIs and overlapped in time with formation of the basaltic crust and iron cores of differentiated asteroids. The entire period of chondrule formation lasted from 4566.6 ± 1.0 Ma (Allende) to 4564.7 ± 0.6 Ma (CR chondrite Acfer 059) to 4562.7 ± 0.5 Ma (CB chondrite Gujba) and was either continuous or consisted of at least three discrete episodes. Since chondrules in CB chondrites appear to have formed from a vapor‐melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated (Krot et al. 2005), there were possibly a variety of processes in the early solar system occurring over at least 4–5 Myr that we now combine under the umbrella name of “chondrule formation.”     

Isotope anomalies in tellurium and palladium from Allende nanodiamonds

Abstract— ‐The elemental and isotopic abundances of Te and Pd have been measured by thermal ionization mass spectrometry in a purified sample of interstellar nanodiamonds from the Allende meteorite, after combustion of the diamonds in an oxygen plasma. Small positive anomalies were found in ¹²⁸Te (4.0 ± 1.5 %0) and ¹³⁰Te (9.3 ± 2.8 %0) from three analyses of the Allende nanodiamond sample EB, and in ¹¹⁰Pd (9.4 ± 5.7 %0) from two analyses of the same sample. No other anomalies of a significant nature were found in either Te or Pd. These results are consistent with the neutron burst model (Meyer et al., 2000), and the r‐process based rapid separation model of Ott (1996) in that ^{128, 130}Te and ¹¹⁰Pd are enhanced relative to their solar abundances. However, in the framework of the neutron burst model, some separation between stable products and radioactive precursors may be required in order to be consistent with the full data set of Te isotopes. In the framework of the rapid separation scenario, our data suggests a separation time of approximately 4000 s, based on the magnitudes of the precursor life‐times for ¹²⁸Te and ¹³⁰Te. The elemental abundance ratio Te‐H/Xe‐H agrees with the prediction of the rapid separation model, provided little fractionation occurred during trapping of the exotic components by the diamonds, but differs significantly from expectations based on the neutron burst model. Differences in the inferred ¹²⁸Te/¹³⁰Te ratio between our work and that of Richter et al. (1998) point to the need for further investigations.     

Origin of fayalitic olivine rims and lath-shaped matrix olivine in the CV3 chondrite Allende and its dark inclusions

Abstract— We have studied an Allende dark inclusion by optical microscopy, scanning electron microscopy, electron microprobe analysis and transmission electron microscopy. The inclusion consists of chondrules, isolated olivines and matrix, which, as in the Allende host, is mainly composed of 5–20 μm long lath-shaped fayalitic grains with a narrow compositional range (Fa_{42 ± 2}) and nepheline. Olivine phenocrysts in chondrules and isolated olivine grains show various degrees of replacement by 5–10 μm wide fayalitic rims (Fa_{39 ± 2}) and 100–1000 μm wide translucent zones, which consist of 5–20 μm long lath-shaped fayalitic grains (Fa_{41 ± 1}) intergrown with nepheline. These fayalitic olivines, like those in the matrix of the dark inclusion, contain 10–20 nm sized inclusions of chromite, hercynite, and Fe-Ni sulfides. The fayalitic rims around remnant olivines are texturally and compositionally identical to those in Allende host, suggesting that they have similar origins. Chondrules are surrounded by opaque rims consisting of tiny lath-shaped fayalitic olivines (<1–3 μm long) intergrown with nepheline. As in the Allende host, fayalitic olivine veins may crosscut altered chondrules, fine-grained chondrule rims and extend into the matrix, indicating that alteration occurred after accretion. We infer that fayalitic olivine rims and lath-shaped fayalites in Allende and its dark inclusions formed from phyllosilicate intermediate phases. This explanation accounts for (1) the similarity of the replacement textures observed in the dark inclusion and Allende host to aqueous alteration textures in CM chondrites; (2) the anomalously high abundances of Al and Cr and the presence of tiny inclusions of spinels and sulfides in fayalitic olivines in Allende and Allende dark inclusions; (3) abundant voids and defects in lath-shaped fayalites in the Allende dark inclusion, which may be analogous to those in partly dehydrated phyllosilicates in metamorphosed CM/CI chondrites. We conclude that the matrix and chondrule rims in Allende were largely converted to phyllosilicates and then completely dehydrated. The Allende dark inclusions experienced diverse degrees of aqueous/hydrothermal alteration prior to complete dehydration. The absence of low-Ca pyroxene in the dark inclusion and its significant replacement by fayalitic olivine in Allende is consistent with the lower resistance of low-Ca pyroxene to aqueous alteration relative to forsteritic olivine. Hydro-thermal processing of Allende probably also accounts for the low abundance of planetary noble gases and interstellar grains, and the formation of nepheline, sodalite, salite-hedenbergite pyroxenes, wollastonite, kirschsteinite and andradite in chondrules and Ca,Al-rich inclusions.     

REE abundances in the matrix of the Allende (CV) meteorite: Implications for matrix origin

Abstract— The trace element distributions in the matrix of primitive chondrites were examined using four least‐contaminated matrix specimens from the polished sections of the Allende (CV) meteorite. Analysis of rare earth element (REE), Ba, Sr, Rb, and K abundances by isotope dilution mass spectrometry revealed that the elemental abundances of lithophile elements except for alkali metals (K, Rb) in the specimens of the Allende matrix studied here are nearly CI (carbonaceous Orgueil) chondritic (～1 × CI). Compared to refractory elements, all the matrix samples exhibited systematic depletion of the moderately volatile elements K and Rb (0.1–0.5 × CI). We suggest that the matrix precursor material did not carry significant amounts of alkali metals or that the alkalis were removed from the matrix precursor material during the parent body process and/or before matrix formation and accretion. The matrix specimens displayed slightly fractionated REE abundance patterns with positive Ce anomalies (CI‐normalized La/Yb ratio = 1.32–1.65; Ce/Ce* = 1.16–1.28; Eu/Eu* = 0.98–1.10). The REE features of the Allende matrix do not indicate a direct relationship with chondrules or calcium‐aluminum‐rich inclusions (CAIs), which in turn suggests that the matrix was not formed from materials produced by the breakage and disaggregation of the chondrules or CAIs. Therefore, we infer that the Allende matrix retains the REE features acquired during the condensation process in the nebula gas.     

Whole‐rock 26Al‐26Mg systematics of amoeboid olivine aggregates from the oxidized CV3 carbonaceous chondrite Allende

Abstract– We report on mineralogy, petrography, and whole‐rock ²⁶Al‐²⁶Mg systematics of eight amoeboid olivine aggregates (AOAs) from the oxidized CV chondrite Allende. The AOAs consist of forsteritic olivine, opaque nodules, and variable amounts of Ca,Al‐rich inclusions (CAIs) of different types, and show evidence for alteration to varying degrees. Melilite and anorthite are replaced by nepheline, sodalite, and grossular; spinel is enriched in FeO; opaque nodules are replaced by Fe,Ni‐sulfides, ferroan olivine and Ca,Fe‐rich pyroxenes; forsteritic olivine is enriched in FeO and often overgrown by ferroan olivine. The AOAs are surrounded by fine‐grained, matrix‐like rims composed mainly of ferroan olivine and by a discontinuous layer of Ca,Fe‐rich silicates. These observations indicate that AOAs experienced in situ elemental open‐system iron‐alkali‐halogen metasomatic alteration during which Fe, Na, Cl, and Si were introduced, whereas Ca was removed from AOAs and used to form the Ca,Fe‐rich silicate rims around AOAs. The whole‐rock ²⁶Al‐²⁶Mg systematics of the Allende AOAs plot above the isochron of the whole‐rock Allende CAIs with a slope of (5.23 ± 0.13) × 10^?5 reported by Jacobsen et al. (2008) . In contrast, whole‐rock ²⁶Al‐²⁶Mg isotope systematics of CAIs and AOAs from the reduced CV chondrite Efremovka define a single isochron with a slope of (5.25± 0.01) × 10^?5 ( Larsen et al. 2011 ). We infer that the excesses in ²⁶Mg* present in Allende AOAs are due to their late‐stage open‐system metasomatic alteration. Thus, the ²⁶Al‐²⁶Mg isotope systematics of Allende CAIs and AOAs are disturbed by parent body alteration processes, and may not be suitable for high‐precision chronology of the early solar system events and processes.     

Transmission electron microscope study of compact Type A calcium-aluminum-rich inclusions from CV3 chondrites: Clues to their origin

Abstract— A transmission electron microscope (TEM) study of three coarse-grained Type A Ca, Al-rich inclusions (CAIs) from Allende, Acfer 082 and Acfer 086 (all CV3 chondrites) was performed in order to decipher their origin and effects of possible metamorphism. The constituent minerals of the CAIs are found to exhibit very similar microstructural characteristics in each of the inclusions studied. In general, the minerals show a well-developed equilibrium texture with typical 120° triple junctions. Melilites are clearly considerably strained and characterized by high dislocation densities up to 3 × 10¹¹ cm^?2. The dislocations have Burgers vectors of [001], [110] or [011] and often form subgrain boundaries subparallel {100}. Melilite in the Allende CAI additionally contains thin amorphous lamellae mostly oriented parallel to {001}. Fassaite (Al-Ti-diopside) is almost featureless even on the TEM scale. Only a few subplanar dislocation walls composed of dislocations with Burgers vectors [001] and 1/2 [110] were detected. Although enclosed within the highly strained melilites, the euhedral spinels contain only low dislocation densities (<2 × 10⁴ cm^?2). In the Allende CAI, spinels were found twinned on {111}. Perovskite is also characterized by a low number of linear lattice defects. All grains possess orthorhombic symmetry and are commonly twinned according to a 90° rotation around [101]. Many crystals exhibit typical domain structures as well as curved twin walls where two orthogonal sets intersect. In addition to the mineral phases described above, tiny inclusions of the simple oxides CaO and TiO₂ were found within melilite (CaO), spinel (CaO, TiO₂) and perovskite (CaO, TiO₂). Based on these observations, it is assumed that at the beginning of the formation of the CAIs a condensed solid precursor was present. Euhedral spinels poikilitically enclosed within melilites suggest that this solid aggregate was then molten. If the pure oxides represent relict condensates, their presence proves that this melting was incomplete. While still plastic, the CAIs were shocked by microimpacts causing the high dislocation densities in melilite as well as diaplectic melilite glass and twinned spinels in the Allende CAI. In Acfer 082 and 086, the deformation took place at elevated temperatures, preventing the solid phase transition and mechanical twinning. The absence of linear lattice defects in spinel, fassaite and perovskite most probably reflects inhomogeneous pressure distribution in the polycrystalline CAI as well as the different strengths of the minerals. According to cooling-rate experiments on perovskite by Keller and Buseck (1994), the dominating (101) twins in the CAI perovskites point to cooling rates ≤50 °C/min. Finally, after crystallization of the CAI was complete, mild thermal metamorphism caused the formation of subgrain boundaries, 120° triple junctions and chemical homogenization of the melilites.     

Axtell and Allende: A Mössbauer spectroscopic study

Abstract— Mössbauer spectra showed the CV3 oxidized subgroup meteorites Allende and Axtell to be similar in olivine content and in a surprising lack of pronounced magnetic components, but different in Fe³⁺ phases. One atypical Allende sample showed an 8% area under the curve for magnetite, which is still less than a reported value for a CV3 reduced subgroup member. Allende's unusual Fe³⁺ spectral region distinguishes it from most other stony meteorites and is difficult to fit to known mineral parameters.     

CONSTRAINTS ON CHONDRULE ORIGIN FROM PETROLOGY OF ISOTOPICALLY CHARACTERIZED CHONDRULES IN THE ALLENDE METEORITE

Petrographic and chemical features of Allende ferromagnesian chondrules previously analyzed for oxygen and silicon isotopes by Clayton et al. (1983a) provide additional information on chondrule origin. Allende, like other carbonaceous chondrites, contains two chondrule populations, but one of these is represented by only one chondrule in this isotopically characterized set. All Allende chondrules fall along an isotopic mixing line, probably defined by an ¹⁶O-rich solid component and an isotopically heavier oxygen gaseous exchange component. Differences in the amounts of isotopic exchange for porphyritic and barred chondrules presumably resulted from varying degrees of melting. Those porphyritic chondrules containing abundant relict grains experienced the least isotopic exchange. Chondrules with high bulk FeO/(FeO + MgO) ratios apparently persisted longer as liquids and contain more of the exchange component. The distinct directions of oxygen isotopic exchange in chondrules from carbonaceous, ordinary, and enstatite chondrites indicate that each formed from different solid precursor materials. Silicon isotopic variations in Allende chondrules probably reflect evaporative loss of lighter isotopes; however, silicon loss is also controlled by chondrule sizes, which are unknown. Observed correlations point to the importance of kinetic factors in a gaseous nebula for chondrule genesis, and are not consistent with models that explain chondrules as mixtures of several solids with distinct oxygen and silicon isotopic compositions.     

Neodymium,strontium and chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the chronology of the angrite parent body

Abstract— Neodymium, strontium, and chromium isotopic studies of the LEW86010 angrite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar nebula. Pyroxene and phosphate were found to contain ～98% of its Sm and Nd inventory. A conventional ¹⁴⁷Sm-¹⁴³Nd isochron yielded an age of 4.53 ± 0.04 Ga (2 σ) and ?¹⁴³ _Nd = 0.45 ± 1.1. An ¹⁴⁶Sm-¹⁴²Nd isochron gives initial ¹⁴⁶Sm/¹⁴⁴Sm = 0.0076 ± 0.0009 and ?¹⁴³ _Nd = ?2.5 ± 0.4. The Rb-Sr analyses give initial ⁸⁷Sr/⁸⁶Sr (I⁸⁷_Sr) = 0.698972 ± 8 and 0.698970 ± 18 for LEW and ADOR, respectively, relative to ⁸⁷Sr/⁸⁶Sr = 0.71025 for NBS987. The difference, ΔI⁸⁷_Sr, between I⁸⁷_Sr for the angrites and literature values for Allende CAIs, corresponds to ～9 Ma of growth in a solar nebula with a CI chondrite value of ⁸⁷Rb/⁸⁶Sr = 0.91, or ～5 Ma in a nebula with solar photospheric ⁸⁷Rb/⁸⁶Sr = 1.51. Excess ⁵³Cr from extinct ⁵³Mn (t1/2 = 3.7 Ma) in LEW86010 corresponds to initial ⁵³Mn/⁵⁵Mn = 1.44 ± 0.07 × 10^?6 and closure to Cr isotopic homogenization 18.2 ± 1.7 Ma after formation of Allende inclusions, assuming initial ⁵³Mn/⁵⁵Mn = 4.4 ± 1.0 × 10^?5 for the inclusions as previously reported by the Paris group (Birck and Allegre, 1988). The ¹⁴⁶Sm/¹⁴⁴Sm value found for LEW86010 corresponds to solar system initial (¹⁴⁶Sm/¹⁴⁴Sm)_o = 0.0080 ± 0.0009 for crystallization 8 Ma after Allende, the difference between Pb-Pb ages of angrites and Allende, or 0.0086 ± 0.0009 for crystallization 18 Ma after Allende, using the Mn-Cr formation interval. The isotopic data are discussed in the context of a model in which an undifferentiated “chondritic” parent body formed from the solar nebula ～2 Ma after Allende CAIs and subsequently underwent differentiation accompanied by loss of volatiles. Parent bodies with Rb/Sr similar to that of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic systematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, ～2.6 Ma after the CAIs, to satisfy the Sr and Cr isotopic systematics.     

Axtell,a new CV3 chondrite find from Texas

Abstract— We describe a previously unreported meteorite found in Axtell, Texas, in 1943. Based on the mineralogical composition and texture of its matrix and the sizes and abundance of chondrules, we classify it as a CV3 carbonaceous chondrite. The dominant opaque phase in the chondrules is magnetite, and that in refractory inclusions is Ni-rich NiFe metal (awaruite). Axtell, therefore, belongs to the oxidized subgroup of CV3 chondrites, although unlike Allende it escaped strong sulfidation. The meteorite bears a strong textural resemblance to Allende, and its chondrule population and matrix appear to be quite similar to those of Allende, but its refractory inclusions, thermoluminescence properties, and cosmogenic ⁶⁰Co abundances are not. Our data are consistent with a terrestrial age for Axtell of ～100 years and a metamorphic grade slightly lower than that of Allende.     

GAS RELEASE AND ORDERING OF CARBON IN THE ALLENDE METEORITE

Mass-spectrometric determinations of the inert gas release from samples of the Allende carbonaceous meteorite heated in the range 300°C to 1100°C, followed by Raman spectroscopic studies of the 1360 cm^?1 and 1580 cm^?1 bands of carbon support the hypothesis that the gas release at high temperatures is causally related with ordering of carbon.     

MgAl2O4 spinels from Allende and NWA 763 carbonaceous chondrites: Structural refinement,cooling history,and trace element contents

MgAl₂O₄ spinels from Allende and NWA 763 carbonaceous chondrites were studied by X‐ray single crystal diffraction, SEM, electron microprobe, LA‐ICP‐MS, and Raman spectroscopy. Those from Allende are almost pure, but, in one case, we found a strong FeO_tot zonation. Spinels from NWA 763 show Mg‐Fe²⁺ substitutions. Almost pure MgAl₂O₄ spinels from both meteorites underwent slow cooling and reached their intracrystalline closure temperature (T_c) in the range 460–520 °C. The NWA 763 spinel with higher FeO content shows a T_c of about 720 °C. X‐ray single crystal diffraction and Raman spectroscopy suggest a slow cooling and an ordered structure with trivalent cations in M site and divalent in T site. Among the trace elements, Ti and Co are enriched with respect to the terrestrial analogs, while Mn, Ni, and Sn show intermediate values between different terrestrial occurrences. Vanadium cannot be used as a tracer of extraterrestrial origin as for Cr‐spinels, because its content is similar in extraterrestrial and terrestrial spinels. In the zoned crystal from Allende, Co show a strong zonation similar to that of FeO.