Allende Meteorite Research Articles

Extinction of olivine and pyroxene in the mid- and far-infrared

The extinction coefficients Q ext /a of orthopyroxene, clinopyroxene, Fo 100 , Fo 90 , Fo 80 , Fo 60 , Fo 40 and the Allende meteorite have been obtained in the mid- and far-infrared ranges. These crystalline silicates have many absorption peaks. The main peaks in forsterite at 10, 11, 20, 24, 34, 50 and 70 μm shift to longer wavelength and weaken as the Mg/(Mg + Fe) ratio decreases. We discuss the composition of cometary dust grains in relation to our data

Oxygen isotopic composition of individual olivine grains from the Allende meteorite

The oxygen isotopic compositions of individual olivine grains from the Allende meteorite were determined by ion microprobe techniques. A variety of olivine grains were analysed: refractory forsterite grains (< 1 wt% FeO), cores of two isolated olivine grains (2–20 wt% FeO), their FeO-rich (> 26 wt%) rims, and twenty-eight individual matrix olivine grains (21–41 wt% FeO). In an oxygen three-isotope plot refractory forsterite grains ( δ 17 O ≈ −11%.; δ 18O ≈ −8‰ ) fall on the Allende mixing line but are significantly less enriched in 16O than spinel and pyroxene from Ca,Al-rich inclusions (CAIs), possibly indicating a later formation of forsteritic olivine. Cores of isolated olivine grains with somewhat higher FeO contents (IOGs) have oxygen isotopic compositions similar to those of the refractory forsterites( δ 17O ≈ −12‰ and δ 18O ≈ − 9‰ ). Both types of olivine grains have oxygen isotopic compositions significantly different from those of chondrules. This clearly demonstrates that these olivine grains are not the result of crystallization of chondrule melts. Fayalite-rich rims of the IOGs are less enriched in 16O ( δ 17O ≈ −3‰ ; δ 18O ≈ 0‰ ) than the forsteritic cores. Arguments are presented that this supports a condensation origin of the rims from a gas phase less enriched in 16O than the gas phase from which the forsteritic cores had formed. Matrix olivines have oxygen isotopic compositions similar to fayalite-rich rims. However, there are variations in the oxygen isotopic composition of matrix olivines that exceed analytical uncertainties, suggesting a more complex origin.

Decay properties of exotic N

Beta-decay half-lives and \ensuremath{\beta}-delayed neutron-emission probabilities of the very neutron-rich nuclei $^{44}\mathrm{S}$ and $^{45--47}\mathrm{Cl}$ have been measured. These isotopes, which lie at or close to the N=28 magic shell, were produced in interactions of a 60 MeV/u $^{48}\mathrm{Ca}$ beam from GANIL (Grand Acc\'el\'erateur National d'Ions Lourds) with a $^{64}\mathrm{Ni}$ target, and were separated by the doubly achromatic spectrometer LISE (Ligne d'Ions Super Epluch\'es). Their decay was studied by a \ensuremath{\beta}-n time correlation measurement. The results are compared to recent model predictions and indicate a rapid weakening of the N=28 shell effect below $_{20}^{48}\mathrm{Ca}_{28}$. The nuclear structure effects reflected in the decay properties of the exotic S and Cl isotopes may be the clue for the astrophysical understanding of the unusual $^{48}\mathrm{Ca}$${/}^{46}$Ca abundance ratio measured in the solar system as well as the Ca-Ti-Cr anomalies observed in E. King inclusions of the Allende meteorite.

A search for C 60 in carbonaceous chondrites

Analysis of interior samples of the Murchison meteorite by two routes yielded an upper limit of 2 ppb for its C 60 content, as compared to parts per million levels for individual polycyclic aromatic hydrocarbons (PAHs). Provided the samples contain an interstellar component, which is probable since Murchison hydrocarbons contain excess deuterium, this result argues against the ubiquitous presence of C 60 in the interstellar medium. A possible explanation for the absence of C 60 was found in experiments showing how PAHs replace fullerenes as stable end products when hydrogen is present during carbon condensation. As a secondary result we found high molecular weight PAHs in the Murchison and Allende meteorites. Coronene and its methyl derivatives are especially interesting since features in the coronene spectrum have been shown to match some of the unidentified interstellar infrared emission bands.

Self-diffusion of magnesium in spinel and in equilibrium melts: Constraints on flash heating of silicates

We have measured Mg self-diffusion in spinel and coexisting melt at bulk chemical equilibrium using an isotopic tracer. The diffusion coefficients were calculated from the measured isotope profiles using a model that includes the complementary diffusion of 24Mg, 25Mg, and 26Mg in both phases with the constraint that the Mg content of each phase is constant. This model also permits the calculation of the diffusion coefficient in one phase from the measured data in a coexisting phase. The activation energy and pre-exponential factor for Mg self-diffusion in spinel are, respectively, 384 ± 7kJ and 74.6 ± 1.1 cm 2/s. These data indicate Mg diffusion in spinel is much slower than previous estimates, probably because earlier measurements included grain boundary transport. The activation energy for Mg self-diffusion in coexisting melt is 343 ± 25 kJ and the pre-exponential factor is 7791.9 ± 1.3 2cm/s. The results from this study were applied to evaluate cooling rates of Plagioclase-Olivine Inclusions (POI) in the Allende meteorite. Given a maximum melting temperature for POIs of ~ 1500°C, these results show that a 10 μ radius spinel would equilibrate isotopically with a melt within about 60 min. To preserve Mg isotopic heterogeneity, the POIs must have initially cooled faster than 15 to 250°C/h depending on the initial temperature of flash heating. The cooling rate must also be slow enough to generate the observed basaltic textures. The inferred cooling rate appears to be comparable or up to ten times greater than those inferred from experimental and textural studies of synthetic CAI systems. The nature of the heating process is thus required to be short with relatively rapid cooling, such as flash heating. However, the relatively rapid cooling cannot be due to radiation into a cold, ~400°K nebula but would require radiation into a rather stable hot environment.

Plutonium-244 dating I. Initial ratio of plutonium to uranium in the allende meteorite

Re-examination of all known xenon isotope data for the carbonaceous chondrite Allende reveals that this meteorite contains as much as (22±1)·10−1 2 csSTP per gram of fissogenic136Xe (136fXe) from the extinct nuclide244Pu and it appears to have started to retain its xenon more than 4800 million years ago, when the244Pu to238U ratio in the solar system was 0.113±0.006 (atom/atom).

Plutonium-244 dating

Re-examination of all known xenon isotope data for the carbonaceous chondrites Murray and Murchison reveals that these meteorites contain (25±1)•10 12 and (23±1)•10 −12 ccSTP 136f Xe/g of 244 Pu fission xenon, respectively, and they started to retain their xenon at about the same time the Allende meteorite started to retain its xenon more than 4800 million years ago. Some of the mineral inclusions isolated from these meteorites contain xenon and barium isotopes which had been exposed to very high fluences of thermal neutrons

Transmission electron microscopy of a refractory inclusion from the Allende meteorite: Anatomy of a pyroxene

Abstract— A crystal of clinopyroxene from the coarse‐grained refractory inclusion Egg 6 of the Allende meteorite has been studied in detail by transmission electron microscopy.The pyroxene crystal contains euhedral, dislocation‐free inclusions of pure spinel MgAl2O4, without any topotactic relation to the host. Extensive dislocation walls at equilibrium, characteristic of high‐temperature anneal, are present in the crystal. Alteration products are occasionally observed at the spinel‐pyroxene interface close to regions where dislocation walls decorated with bubbles (or voids) are present. The bubbles, often in the shape of tubes along the dislocation lines, are thought to be due to the precipitation of a fluid migrating along the dislocations.The observations are compatible with crystallization of the refractory inclusions from the melt and with the existence of a later stage of metasomatism.

Calcic Micas in the Allende Meteorite: Evidence for Hydration Reactions in the Early Solar Nebula

Two calcic micas, clintonite and margarite, have been identified in alteration products in a calcium- and aluminum-rich inclusion (CAI) in the Allende meteorite. Clintonite replaces grossular in alteration veins, and margarite occurs as lamellae in anorthite. Their occurrence suggests that, in addition to undergoing high-temperature alteration by a volatile and iron-rich vapor that produced the grossular and anorthite, some CAIs underwent alteration at moderate temperatures (</=400 kelvin). Petrographic evidence suggests that the calcic micas formed before accretion but after the formation of the layered rim sequences that surround the CAL. These calcic micas provide strong evidence that, contrary to theoretical calculations, some hydration reactions occurred in the early solar nebula.

Correlated study of initial [formula omitted] and AlMg isotopic systematics and petrologic properties in a suite of refractory inclusions from the Allende meteorite

We have studied a suite of six coarse-grained Ca-Al-rich inclusions (CAIs) from the Allende meteorite, applying in concert detailed petrographic and chemical characterization, ion probe mass spectrometric analyses of the Al-Mg isotopic system to determine the abundance and distribution of 26A1, and thermal emission mass spectrometric analyses of the Rb-Sr system to determine initial 87 Sr 86 Sr ratios. The simplest inclusion is a Type Bl, USNM 3529-Z, that shows evidence of minor alteration and recrystallization since solidification from a melt droplet; e.g., some of the coarse-grained anorthite in the inclusion (especially near the inclusion rim) apparently formed by recrystallization or melting of fine-grained secondary anorthite that had replaced melilite. Four other Type Bs contain evidence of more widespread recrystallization in addition to evidence of local replacement of melilite by coarse anorthite; e.g., melilite zoning is complex and not igneous in origin, and in one inclusion (USNM 3529-21) melilite and pyroxene crystals enclose relicts of an earlier generation of the same phases. The sixth inclusion, USNM 3898, is a Type A whose properties appear to be largely metamorphic in origin. All six inclusions show variations in Mg isotopic compositions indicating that they incorporated 26A1. In 3529-Z the relative abundance of 26A1 ( 26 Al 27 Al = (4.0 ± 0.1) × 10 −5 ) is close to the “canonical” value ( 26 Al 27 Al = 5 × 10 −5 ) for CAIs, but both anorthite and melilite show minor departures from a strict isochron relationship between Mg and Al. In particular, the “coarse” anorthite near the inclusion rim has initial 26 Al 27 Al ratios of 1−2 × 10 −5, implying formation at least 1.5 million years after the “first-generation” anorthite. The other four Type Bs exhibit larger disturbances of the Al-Mg system that can be understood in terms of local recrystallization and isotopic exchange, primarily between anorthite and melilite. Only 3898 is consistent with a strict Al-Mg isochron relationship, but this CAI lacks primary anorthite, and the small range of Al/Mg among the phases analyzed may preclude resolution of any small isotopic disturbances. The RbSr isotopic system also typically indicates some level of disturbance, some of which must have been relatively recent. Nevertheless, phases characterized by very low Rb/Sr permit precise identification of initial 87 Sr 86 Sr . Our Al-Mg and Sr data are largely consistent with a simple chronological interpretation of both systems; i.e., the 26Al results suggest that all the initial 87 Sr 86 Sr ratios should have the same value and, with one exception, our initial 87 Sr 86 Sr ratios are consistent with a single value; this value is somewhat higher than the primitive value ALL reported by Gray et al. (1973), however. Also with one exception, the very low Rb/Sr CAI data reported by Gray et al. (1973) are consistent with the same value. The exception is the same inclusion in both studies; our analysis of USNM 3898 yields an initial 87 Sr 86 Sr higher than that of the other CAIs, while Gray et al. (1973) obtained an initial 87 Sr 86 Sr lower than that of the other CAIs for the same inclusion (labeled as D7, on which ALL is based). Neither result can be readily explained as a chronological aberration, since 3898 has nearly canonical 26 Al 27 Al . The present data are the first which establish a firm association between primitive 26 Al 27 Al and primitive 87 Sr 86 Sr by analysis of both isotopic systems in the same inclusions. The comparison of both isotopic systems, interpreted as simple nebular chronometers, does not reveal any chronological inconsistencies that demand resolution in terms of a grossly heterogeneous distribution of 26Al.

Organic chemical analysis on a microscopic scale using two-step laser desorption/laser ionization mass spectrometry

The distribution of polycyclic aromatic hydrocarbons (PAHs) in the Allende meteorite has been measured using two-step laser desorption/laser multiphoton ionization mass spectrometry. This method enables in situ analysis with a spatial resolution of 1 millimeter or better of selected organic molecules. We present results showing that PAH concentrations are locally high compared to the average concentration found by analysis of pulverized samples, and are found primarily in the fine grained matrix; no PAHs were detected in the interiors of individual chondrules at our detection limit of about 0.05 parts per million. We have recently extended the spatial resolution of this instrument to 40 microns.

The isotopic composition of zinc, palladium, silver, cadmium, tin, and tellurium in acid-etched residues of the Allende meteorite

The isotopic and elemental abundances of Zn, Pd, Ag, Cd, Sn and Te have been measured in three acid-resistant residues extracted from the Allende meteorite. High-efficiency, low-contamination ion-exchange procedures were developed to separate and purify the nanogram amounts of these elements present. Elemental-abundance determinations performed by Mass Spectrometric Isotope Dilution agree with previously published work for similarly derived residues. No isotope anomalies similar to those found for Xe (Xe-HL) in these samples were detected for any of these elements, which is consistent with the residues not being derived directly from the Xe-HL carriers. The lack of major Te-isotope anomalies does not support earlier reports of 126Te and 130Te excesses which were measured by neutron activation in similar samples. Small excesses were detected in the minor isotopes of Sn and Te, but these may be due to measurement problems associated with the small ion currents obtained for these samples. Two of the residue solutions contain Cd with up to several percent excesses for 106Cd and 108Cd. Interpretations of these results are limited by the unknown nature of the carrier minerals in the residues but may indicate the presence of a p-process component in Allende residues.

Zinc isotope anomalies in Allende meteorite inclusions

The isotopic compositions of Zn, Cr, Ti, and Ca have been measured in a number of CAIs from the Allende meteorite. The aim was to test astrophysical models which predict large excesses of Zn-66 to accompany excesses in the neutron-rich isotopes of Ca, Ti, Cr, and Ni. Some of the CAIs show clearly resolved but small excesses for Zn-66 which are at least an order of magnitude smaller than predicted. This result may simply reflect the volatility and chemical behavior of Zn as compared to the other (more refractory) anomalous elements found in these samples. Alternatively, revision of parameters and assumptions used for the model calculations may be required.

A new look at differentiation of the Earth from melting experiments on the Allende meteorite

CARBONACEOUS chondrites contain approximately solar abundances of the non-volatile elements1, but are much more FeO-rich2 than upper mantle peridotites3 or hypothetical 'chondritic' mantle compositions4,5. If the Earth accreted from unfractionated, primitive meteoritic debris similar to the carbonaceous chondrites, how then did it become a fractionated, layered body with a crust, mantle and core? Here I report the results of high-pressure melting experiments on the Allende CV3 carbonaceous chondrite, which address this question and provide a new look at the Earth's earliest stage of differentiation. Multi-anvil experiments at 24, 26 and 26.5 GPa show that FeO-rich magnesiowustite is an abundant crystallizing phase at temperatures near the Allende silicate liquidus. If a chondritic Earth experienced a high-temperature molten stage, then during cooling and crystallization, FeO-rich magnesiowustite could be segregated to the deepest levels of the Earth's interior. Magnesiowustite fractionation may thus have depleted the initial FeO content of the primitive chondritic mantle and contributed to the formation and growth of the Earth's core.

Zinc isotope anomalies

The Zn isotope composition in refractory-element-rich inclusions of the Allende meteorite are determined. Typical inclusions contain normal Zn. A unique inclusion of the Allende meteorite shows an excess for Zn-66 of 16.7 + or - 3.7 eu (1 eu = 0.01 percent) and a deficit for Zn-70 of 21 + or - 13 eu. These results indicate the preservation of exotic components even for volatile elements in this inclusion. The observed excess Zn-66 correlates with excesses for the neutron-rich isotopes of Ca-48, Ti-50, Cr-54, and Fe-58 in the same inclusion.

FeO‐rich rims and veins in Allende forsterite: Evidence for high temperature condensation at oxidizing conditions

Abstract— Forsteritic olivine grains in the Allende meteorite are commonly rimmed by FeO‐rich olivine. New evidence is presented in this paper that the fayalitic rims formed by condensation from a gas and not by thermal equilibration of forsterite with FeO‐rich metal or FeO‐rich olivine in the interior of a parent body. A similar origin is inferred for fayalitic veins within forsterite crystals.A good correlation of FeO with MnO was observed along profiles from forsteritic cores to fayalite‐rich rims, excluding oxidation of metal as a source of rim FeO. The fayalite content of the rim is generally lower than that of the adjacent matrix olivines excluding formation of FeO‐rich rims by equilibration with present matrix. This is also supported by systematic differences in Cr between rim and matrix olivines.Chromites, associated with FeO‐rich olivine, were found at the boundary between forsterite and fayalite‐rich rim, within the rim itself and in fayalitic veins. Condensation of chromite from a gas phase is likely for the latter two occurrences since no other suitable source of Cr is available. Chromite at the forsterite‐fayalite interface was formed by diffusion of Cr from forsterite. Increasingly oxidizing conditions, apparently enabled formation of chromite.The steep compositional gradient between forsterite and fayalite provides severe constraints on the thermal history of forsterite grains. The width of oxidized zones (halos) around metal inclusions in the interior of forsterite grains are not compatible with this steep gradient. Their development requires longer diffusion times than would be allowed by the rims, indicating that these forsterite grains must have had an independent history of oxidation prior to formation of the rim.Condensation calculations indicate that FeO‐rich olivine can be formed by condensation from a gas with enhanced oxygen fugacity. Increasing the oxygen fugacity would provide conditions allowing the thermodynamic stability of chromite.High temperatures and oxidizing conditions must have prevailed for some time in that part of the solar nebula where the Allende meteorite formed.

Alkali (Rb/K) abundances in Allende barred‐olivine chondrules: Implications for the melting conditions of chondrules

Abstract— In order to study abundances of alkali metals in chondrules, 25 petrographically characterized chondrules, including 18 barred olivine (BO) chondrules from the Allende (CV3) meteorite, were analyzed for alkalis (K and Rb) and alkaline earths (Sr, Ba, Ca and Mg) by mass spectrometric isotope dilution.Most BO chondrules with higher alkalis (&gt;CI level) have nearly CI‐chondritic Rb/K ratios, while those with lower alkalis clearly show higher Rb/K ratios than the CI‐chondritic. In general, BO chondrules with higher Rb/K exhibit more depletion of alkalis relative to Ca. The mean olivine Fa for individual chondrules positively correlates with bulk alkali concentrations in BO type but not in porphyritic type chondrules. These observations suggest that some BO chondrules formed from more reducing assemblages of precursor minerals, which experienced more intensive vaporization losses of alkalis, accompanied by Rb/K fractionation, during the chondrule‐formation melting.

Heating and melting of small icy satellites by the decay of 26Al.

We study the effect of radiogenic heating due to 26Al on the thermal evolution of small icy satellites. Our object is to find the extent of internal melting as a function of the satellite radius and of the initial 26Al abundance. The implicit assumption, based on observations of young stars, is that planet and satellite accretion occurred on a time scale of approximately 10(6) yr (comparable with the lifetime of 26Al). The icy satellites are modeled as spheres of initially amorphous ice, with chondritic abundances of 40K, 232Th, 235U, 238U, corresponding to an ice/dust mass ratio of 1. Evolutionary calculations are carried out, spanning 4.5 x 10(9) yr, for different combinations of the two free parameters. Heat transfer by subsolidus convection is neglected for these small satellites. Our main conclusion is that the initial 26Al abundance capable of melting icy bodies of satellite size to a significant extent is more than 10 times lower than that prevailing in the interstellar medium (or that inferred from the Ca-Al rich inclusions of the Allende meteorite, approximately 7 x 10(-7) by mass). We find, for example, that an initial 26Al mass fraction of approximately 4 x 10(-8) is sufficient for melting almost completely icy spheres with radii of 800 km, typical of the larger icy planetary satellites. We also find that for any given 26Al abundance, there is a narrow range of radii below which only marginal melting occurs and above which most of the ice melts (and refreezes later). Since extensive melting may have important consequences, such as differentiation, gas release, and volcanic activity, the effect of 26Al should be included in future studies of satellite interiors.

Recondite interstellar carbon components in the Allende meteorite revealed by preparative precombustion

Prolonged combustion (preparative precombustion) at low temperatures (390–450°C and 450–510°C) removes large quantities of isotopically normal carbon from an acid (HF/HCl) residue of the Allende meteorite to reveal two isotopically heavy carbon components. One of them, called C ℵ (carbon aleph) because of its possible relationship to C α the host of Ne-E(L), has δ 13 C of+345ℵ. and a combustion temperature of 700–750°C. It is believed that the presence of C ℵ is masked in the bulk acid residue because of a “tailing effect” in the combustion of less stable forms of carbon. The second heavy carbon component burns over the temperature regime 900–1000°C and has a δ 13 C of >527ℵ. This component called C κ (carbon kappa) is distinguishable from the well known types of heavy carbon called C β and C ϵ the hosts of s-Xe and NeE(H), respectively, and now recognised as forms of SiC. C κ , however, could also be a form of very poorly crystalline SiC; it appears to be released from the Allende acid residue because the precombustion slowly degrades a secondary host mineral, which is possibly a variety of spinel. Likewise, a component of light carbon, possibly C θ , poorly crystalline graphite, is also liberated by the precombustion reaction after between 52 and 108 h of treatment. Presumably C θ too is located within a specific type of spinel. The discovery of C %, C κ , and C θ , together with C λ , a component very rich in 12C (ASH et al. 1988a), confirms that primitive meteorites are a prolific source of interstellar grains. Preparative precombustion demonstrates that the legacy to be found in Allende is far more complicated than at first envisaged from the noble gas record of this meteorite.

Oxygen isotopes in chondrules and coarse-grained chondrule rims from the Allende meteorite

The oxygen isotopic compositions of porphyritic chondrules and coarse-grained chondrule rims from the Allende CV chondrite form a 16O-mixing trend situated near the carbonaceous chondrite anhydrous minerals (CCAM) mixing line (lying ∼ 0.1–0.7‰ higher in δ 17O ). Coarse-grained rims are depleted in 16O relative to their enclosed chondrules. Chondrules and rims exhibit significant correlations between their oxygen isotopic and chemical compositions: objects rich in 16O tend to have higher contents of refractory lithophiles and alkalies; objects poor in 16O tend to have higher contents of FeO and volatile siderophiles. CV chondrite matrix material is chemically and isotopically similar to some coarse-grained chondrule rims. Previous work showed that O-isotope data for CV barred olivine (BO) chondrules plot just below the terrestrial fractionation (TF) line, near δ 18O ⋍ 4‰ , well off the CCAM line. The Allende data suggest that three solid precursor components are required to account for the O-isotopic compositions of CV chondrules and rims: (1) a high-temperature, refractory-, alkali- and 16O-rich component, perhaps near δ 18O = −40‰ , δ 17O = −42‰ (relative to SMOW), (2) a low-temperature, FeO-rich, refractory- and 16O-poor component lying near the CCAM line, perhaps near its intersection with the TF line at δ 18O ≈ 10‰ , and (3) an additional component required to explain the composition of BO chondrules.