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S B Simon - One of the best experts on this subject based on the ideXlab platform.

  • petrographic and isotopic investigations of two unusual ca al rich inclusions from primitive co3 chondrites
    Geochimica et Cosmochimica Acta, 2021
    Co-Authors: A T Hertwig, Mingchang Liu, Adrian J Brearley, S B Simon
    Abstract:

    Abstract We performed high-precision SIMS (secondary ion mass spectrometry) 26Al-26Mg and oxygen isotope analyses of two unique CAIs, “Mesquite” and “Y24”, found in the CO3.05 chondrites Northwest Africa 7892 and Yamato-81020, respectively. Mesquite is unusually large (∼5 × 3 mm) for a CAI from any CO chondrite and exhibits a layered texture comprising a melilite-rich core surrounded by Hibonite- and spinel-rich mantle layers and a semi-continuous spinel-dominated rim. The CAI Y24 stands out because of its distinct mineralogy: grossite, Hibonite, and spinel are accompanied by abundant ultra-refractory-element-rich phases such as warkite, kangite, and perovskite. Silicates are absent in Y24. Negatively fractionated δ25Mg values of phases in the core and mantle layers of Mesquite suggest that the inclusion as a whole was never molten and, hence, represents an aggregate of condensates. The relatively large grain sizes of melilite in the core (up to ∼300 µm) most likely are the result of solid-state recrystallization and coarsening of melilite in the course of a heating event occurring in the solar nebula. This heating event, however, did not disturb the Al-Mg systematics of Mesquite. Regardless of their position within Mesquite and the phases analyzed, spots analyzed for Al-Mg plot on a single isochron characterized by an initial 26Al/27Al of (4.95 ± 0.08) × 10–5 and a δ26Mg*0 of –0.14 ± 0.05‰. We suggest that this initial 26Al/27Al ratio corresponds to the formation of Mesquite in the solar nebula that was slightly heterogeneous with respect to Mg isotopes. Spinel in the rim is uniform in Δ17O (∼–25‰); in contrast, Hibonite in the core and mantle layers, albeit also 16O-rich, show variable oxygen isotope ratios (Δ17O ∼ –15‰ to –23‰), which would be consistent with Hibonite condensation in a gas with quickly-changing oxygen isotope compositions. The 16O-poor composition of melilite (Δ17O ∼ –1‰ to 0‰) in the core could be the result of isotope exchange with an 16O-poor gas, perhaps during the heating event that caused the solid-state recrystallization and coarsening of melilite or the result of oxygen isotope exchange with a fluid on the parent body. Abundant calcite, phyllosilicates, and sodalite are witnesses to late-stage and low-temperature alteration of the Mesquite CAI; calcite and phyllosilicates most likely are of terrestrial origin, but sodalite could have formed in the parent body. Inclusion Y24 is irregularly-shaped, indicating a condensation origin. Completely enclosing other phases, warkite forms the matrix of Y24, which could be the result of simultaneous condensation and growth of warkite, grossite, and Hibonite. Possibly, spinel formed by replacing grossite or Hibonite or both minerals in a gradually cooling gas before any silicates condensed. SIMS analyses indicate that condensation occurred in an 16O-rich gas when 26Al/27Al was (5.4 ± 1.0) × 10–5. Oxygen isotope exchange with an 16O-poor fluid in the parent body or with an 16O-poor gas in a nebular setting caused the 16O-poor compositions in grossite and kangite.

  • refractory inclusions in the unique carbonaceous chondrite acfer 094
    Meteoritics & Planetary Science, 2011
    Co-Authors: S B Simon, L Grossman
    Abstract:

    Abstract– Acfer 094 is an unshocked, nearly unaltered carbonaceous chondrite with an unusual suite of refractory inclusions. The refractory inclusions in a newly prepared thin section and a small aliquot of disaggregated material were studied to compare the population with previous work, and to report new or unusual inclusion types. A total of 289 Ca-, Al-rich inclusions in the thin section and 67 among the disaggregated material, having a total of 31 different mineral assemblages, were found. Inclusions are largely free of secondary alteration products, and are typically ≤200 μm across. The most common are gehlenitic melilite+spinel±perovskite, spinel+perovskite, and spinel with a thin, silicate rim, typically melilite±diopside. Such rims and (thicker) mantles are very common among Acfer 094 inclusions, and they exhibit a variety of zoning patterns with respect to akermanite and FeO contents. In the thin section, about 13% of the inclusions contain Hibonite and approximately 5% are grossite-bearing; in the disaggregated material, the percentages are 14 and 9, respectively, comparable to previous work. Among the unusual inclusions are a fine-grained, porous, Ti-rich Hibonite+spinel+perovskite+melilite inclusion with a compact, coarse, Ti-poor Hibonite+spinel+melilite clast; two inclusions in which Hibonite has reacted to form grossite; two inclusions with FeO-rich spinel; and a small object consisting of fassaite enclosing euhedral spinel, the first fragment of a Type B inclusion reported from Acfer 094. Inclusions similar to those found in CM or CV chondrites are rare; Acfer 094 contains a distinctive population of inclusions. The population, dominated by small, melilite-bearing inclusions, is most similar to that of CO chondrites. A distinguishing feature is that in Acfer 094, almost every phase in almost every refractory inclusion contains 0.5–1.5 wt% FeO. A lack of diffusion gradients and the pristinity of the matrix imply that the inclusions experienced prolonged exposure to FeO-bearing fluid prior to accretion into the Acfer 094 parent body. There are no known nebular conditions under which the refractory phases found in the present samples could acquire FeO enrichments to the observed levels. The most likely setting is therefore in an earlier, FeO-rich parent body. The inclusions were ejected from this parent body, mixed with typical CAIs, chondrules, amoeboid olivine aggregates, and amorphous material, and incorporated into the Acfer 094 parent body.

  • formation of spinel Hibonite rich inclusions found in cm2 carbonaceous chondrites
    American Mineralogist, 2006
    Co-Authors: S B Simon, L Grossman, Ian D Hutcheon, Douglas L Phinney, Peter K Weber, Stewart Fallon
    Abstract:

    We report petrography, mineral chemistry, bulk chemistry, and bulk isotopic compositions of a suite of 40 spinel-rich inclusions from the Murchison (CM2) carbonaceous chondrite. Seven types of inclusions have been identified based on mineral assemblage: spinel-Hibonite-perovskite; spinel-perovskite-pyroxene; spinel-perovskite-melilite; spinel-Hibonite-perovskite-melilite; spinel-Hibonite; spinel-pyroxene; and spinel-melilite-anorthite. Hibonite-bearing inclusions have Ti-poor spinel compared to the Hibonite-free ones, and spinel-Hibonite-perovskite inclusions have the highest average bulk TiO 2 contents (7.8 wt%). The bulk CaO/Al 2 O 3 ratios of the inclusions range from 0.005 to 0.21, well below the solar value of 0.79. Hibonite-, spinel-rich inclusions consist of phases that are not predicted by condensation calculations to coexist; in the equilibrium sequence, Hibonite is followed by melilite, which is followed by spinel. Therefore, Hibonite-melilite or melilite-spinel inclusions should be dominant instead. One explanation for the “missing melilite” is that it condensed as expected, but was lost due to evaporation of Mg and Ca during heating and melting of spherule precursors. If this theory were correct, melilite-poor spherules would have isotopically heavy Mg and Ca, assuming Rayleigh fractionation accompanied evaporation. Except for one inclusion with F Mg = 4.3 ± 2.6‰/amu and another with isotopically light Ca ( F Ca = −3.4 ± 2.0‰/amu), however, all the inclusions we analyzed have normal isotopic compositions within their 2σ uncertainties. Thus, we found no evidence for significant mass-dependent fractionation. Conditions necessary for non-Rayleigh evaporation are unlikely if not unrealistic, and our preferred explanation for the general lack of melilite among Hibonite-, spinel-bearing inclusions is kinetic inhibition of melilite condensation relative to spinel. Because of similarities between the crystal structures of Hibonite and spinel, it should be easier for spinel than for melilite to form from Hibonite.

  • a Hibonite corundum inclusion from murchison a first generation condensate from the solar nebula
    Meteoritics & Planetary Science, 2002
    Co-Authors: S B Simon, L Grossman, Andrew M Davis, K D Mckeegan
    Abstract:

    Through freeze-thaw disaggregation of the Murchison (CM) carbonaceous chondrite, we have recovered a ~90 x 75 μm refractory inclusion that consists of corundum and Hibonite with minor perovskite. Corundum occurs as small (~10 μm), rounded grains enclosed in Hibonite laths (~10 μm wide and 30-40 μm long) throughout the inclusion. Perovskite predominantly occurs near the edge of the inclusion. The crystallization sequence inferred petrographically - corundum followed by Hibonite followed by perovskite - is that predicted for the first phases to form by equilibrium condensation from a solar gas for Ptot ≤ 5 x 10^(-3) atm. In addition, the texture of the inclusion, with angular voids between subhedral Hibonite laths and plates, is also consistent with formation of the inclusion by condensation. Hibonite has heavy rare earth element (REE) abundances of ~40 x CI chondrites, light REE abundances ~20 x CI chondrites, and negative Eu anomalies. The chondrite-normalized abundance patterns, especially one for a Hibonite-perovskite spot, are quite similar to the patterns of calculated solid/gas partition coefficients for Hibonite and perovskite at 10^(-3) atm and are not consistent with formation of the inclusion by closed-system fractional crystallization. In contrast with the features that are consistent with a condensation origin, there are problems with any model for the formation of this inclusion that includes a molten stage, relic grains, or volatilization. If thermodynamic models of equilibrium condensation are correct, then this inclusion formed at pressures <5 x 10^(-3) atm, possibly with enrichments (<1000x) in CI dust relative to gas at low pressures (below 10^(-4) atm). Both Hibonite and corundum have δ17O ≈ δ18O ≈ -50‰, indicating formation from an 16O-rich source. The inclusion does not contain radiogenic 26Mg and apparently did not contain live 26Al when it formed. If the short-lived radionuclides were formed in a supernova and injected into the early solar nebula, models of this process suggest that 26Al-free refractory inclusions such as this one formed within the first ~6 x 10^5 years of nebular collapse.

  • formation of orange Hibonite as inferred from some allende inclusions
    Meteoritics & Planetary Science, 2001
    Co-Authors: S B Simon, Andrew M Davis, L Grossman
    Abstract:

    — We studied three fluffy Type A refractory inclusions from Allende that contain orange Hibonite. The melilite in the present samples is very Al-rich, averaging Ak6, Ak14, and Ak12 in the three samples studied. Hibonite in two inclusions, unlike that in Murchison, has low rare earth element abundances of <10 × CI; in the other inclusion, the Hibonite, melilite and perovskite have Group II-like patterns. The Hibonite and melilite in all three inclusions studied have excess 26Mg consistent with (26Al/27Al)I = 5 × 10−5. Much of the Hibonite and some of the spinel in these inclusions is corroded. These phases are found enclosed in melilite, but based on bulk compositions and phase equilibria, Hibonite should not be an early-crystallizing phase in these inclusions. We conclude that the Hibonite and probably some of the spinel is relic. Reversely zoned melilite, rounded spinel and isotopically heavy Mg in the inclusions probably reflect reheating events that involved melting and evaporation. Alteration of the gehlenitic melilite gave rise to some rare phases, including corundum and nearly pure CaTs pyroxene. Studies have shown that blue Hibonite contains Ti3+ while orange Hibonite does not (Ihinger and Stolper, 1986; Beckett et al., 1988). Orange Hibonite formed either under oxidizing conditions (such as at oxygen fugacities at least seven orders of magnitude greater than that of a solar gas at 1700 K), or under conditions reducing enough (e.g., solar) that it contained Ti3+, which was later oxidized in situ. Although V and Ce oxides are volatile at the temperature and range of oxygen fugacities at which orange Hibonite is known to be stable, we find that (a) the Hibonite is V-rich (∼1 wt% V2O3) and (b) there are no negative Ce anomalies in Allende Hibonite. This indicates that the Hibonite did not form by condensation under oxidizing conditions. In addition, there are slight excesses of Ti + Si cations relative to Mg + Fe cations (up to 0.1 of 0.8 cations per 19 oxygen anions), probably reflecting the original presence of Ti3+. The results of this study strongly support the suggestion (Ihinger and Stolper, 1986) that Allende Hibonite originally formed under reducing conditions and was later oxidized. Oxygen fugacities within ∼2–3 orders of magnitude of that of a solar gas are implied; otherwise, strong Ce and V depletions would be observed.

Ernst Zinner - One of the best experts on this subject based on the ideXlab platform.

  • aluminum calcium and titanium rich oxide stardust in ordinary chondrite meteorites
    The Astrophysical Journal, 2008
    Co-Authors: Larry R Nittler, Conel Od M Alexander, R Gallino, P Hoppe, Ann N Nguyen, Frank J Stadermann, Ernst Zinner
    Abstract:

    We report O-, Al-Mg-, K-, Ca-, and Ti-isotopic data for a total of 96 presolar oxide grains found in residues of several unequilibrated ordinary chondrite meteorites. Identified grain types include Al2O3 ,M gAl2O4, Hibonite (CaAl12O19), and Ti oxide. This work greatly increases the presolar Hibonite database, and is the first report of presolarTioxide.O-isotopiccompositionsof thegrainsspanpreviouslyobservedrangesandindicateanoriginin red giant and asymptotic giant branch (AGB) stars of low mass (<2.5 M� ) for most grains. Cool bottom processing in the parent AGB stars is required to explain isotopic compositions of many grains. Potassium-41 enrichments in Hibonite grains are attributable to in situ decay of now-extinct 41 Ca. Inferred initial 41 Ca/ 40 Ca ratios are in good agreement with model predictionsfor low-mass AGB star envelopes,provided that ionizationsuppresses 41 Cadecay. Stable Mg and Ca isotopic ratios of most of the Hibonite grains reflect primarily the initial compositions of the parent starsandaregenerallyconsistentwithexpectationsforGalacticchemicalevolution,butrequiresomelocalinterstellar chemical inhomogeneity. Very high 17 O/ 16 Oo r 25 Mg/ 24 Mg ratios suggest an origin for some grains in binary star systemswheremasstransferfromanevolvedcompanionhasalteredtheparentstarcompositions.Asupernovaorigin for the hitherto enigmatic 18 O-rich Group 4 grains is strongly supported by multielement isotopic data for two grains. The Group 4 data are consistent with an origin in a single supernova in which variable amounts of material from the deep 16 O-rich interior mixed with a unique end-member mixture of the outer layers. The Ti oxide grains primarily formed in low-mass AGB stars. They are smaller and rarer than presolar Al2O3, reflecting the lower abundance of Ti than Al in AGB envelopes. Subject headingg dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — supernovae: general

  • aluminum calcium and titanium rich oxide stardust in ordinary chondrite meteorites
    arXiv: Astrophysics, 2008
    Co-Authors: Larry R Nittler, Conel Od M Alexander, R Gallino, P Hoppe, Ann N Nguyen, Frank J Stadermann, Ernst Zinner
    Abstract:

    We report isotopic data for a total of 96 presolar oxide grains found in residues of several unequilibrated ordinary chondrite meteorites. Identified grain types include Al2O3, MgAl2O4, Hibonite (CaAl12O19) and Ti oxide. This work greatly increases the presolar Hibonite database, and is the first report of presolar Ti oxide. O-isotopic compositions of the grains span previously observed ranges and indicate an origin in red giant and asymptotic giant branch (AGB) stars of low mass (<2.5 MSun) for most grains. Cool bottom processing in the parent AGB stars is required to explain isotopic compositions of many grains. Potassium-41 enrichments in Hibonite grains are attributable to in situ decay of now-extinct 41Ca. Inferred initial 41Ca/40Ca ratios are in good agreement with model predictions for low-mass AGB star envelopes, provided that ionization suppresses 41Ca decay. Stable Mg and Ca isotopic ratios of most of the Hibonite grains reflect primarily the initial compositions of the parent stars and are generally consistent with expectations for Galactic chemical evolution, but require some local interstellar chemical inhomogeneity. Very high 17O/16O or 25Mg/24Mg ratios suggest an origin for some grains in binary star systems where mass transfer from an evolved companion has altered the parent star compositions. A supernova origin for the hitherto enigmatic 18O-rich Group 4 grains is strongly supported by multi-element isotopic data for two grains. The Group 4 data are consistent with an origin in a single supernova in which variable amounts of material from the deep 16O-rich interior mixed with a unique end-member mixture of the outer layers. The Ti oxide grains primarily formed in low-mass AGB stars. They are smaller and rarer than presolar Al2O3, reflecting the lower abundance of Ti than Al in AGB envelopes.

  • origin of Hibonite pyroxene spherules found in carbonaceous chondrites
    Meteoritics & Planetary Science, 1998
    Co-Authors: S B Simon, L Grossman, Andrew M Davis, Ernst Zinner
    Abstract:

    — We have studied both of the known glass-free, Hibonite-pyroxene spherules: MYSM3, from Murray (CM2), and Y17–6, from Yamato 791717 (CO3). They consist of Hibonite plates (∼2 wt% TiOtot2) enclosed in Al-rich pyroxene that has such high amounts of CaTs (CaAl2SiO6) component, up to ∼80 mol%, that it must have crystallized metastably. Within the pyroxene, abundances of MgO and SiO2 are strongly correlated with each other and are anticorrelated with those of Al2O3, reflecting an anticorrelation between the diopside and CaTs components of the pyroxene. In contrast with previous results for Type B fassaite, however, we do not observe an anticorrelation between MgO and TiOtot2, possibly reflecting different relative distribution coefficients for Ti3+ and Ti4+ in the aluminous pyroxene of the spherules from those found for fassaite in Type B inclusions. Previously described Hibonite-silicate spherules have 26Mg deficits but the present samples do not. Furthermore, the pyroxene in Y17-6 has excess 26Mg, while the Hibonite it encloses does not, indicating that the two phases either had different initial 26Al/27Al ratios or different initial 26Mg/24Mg ratios. The Ti isotopic compositions of the present samples are highly unusual: δ50Ti = 103.4 ± 5.2%o in MYSM3 and -61.4 ± 4.1%0 in Y17-6, which are among the largest 50Ti anomalies reported for any refractory inclusion. The textures suggest that Hibonite crystallized first; but based on the calculated bulk compositions of both spherules, it is not the liquidus phase in either sample, which suggests that the Hibonite in both samples is relict. The presence of ragged Hibonite grains in MYSM3 and rounded Hibonite grains in Y17-6 and a lack of isotopic equilibrium between pyroxene and Hibonite support this conclusion. The spherules crystallized from liquid droplets that probably formed as a result of the melting of solid precursor grains that included Hibonite. The heating events were too short and/or not hot enough to melt all the Hibonite. The droplets cooled quickly enough that CaTs-rich pyroxene crystallized instead of anorthite. Based on the observed differences in isotopic composition, it is unlikely that the precursors of the present samples formed in the same reservoir as each other or as the previously described Hibonite-silicate spherules, providing further evidence of the isotopic heterogeneity of the early solar nebula.

  • corundum bearing residues produced through the evaporation of natural and synthetic Hibonite
    Meteoritics & Planetary Science, 1998
    Co-Authors: Christine Floss, Ernst Zinner, Ahmed El Goresy, H Palme, G Weckwerth, Werner Rammensee
    Abstract:

    — We have produced corundum-bearing residues through the evaporation of natural and synthetic Hibonite samples. The sequence of major element losses as well as volatility related trace element fractionations in these residues are similar to those previously observed in residues from the evaporation of chondritic starting material, which suggests that the processes by which these fractionations occur may be largely independent of the starting material used. However, the mineralogy of the residues does depend on the composition of the starting material and, to some extent, on the conditions under which evaporation took place. Similarly, the degree of isotopic mass fractionation observed in the residues is composition-dependent. This observation means that it may be possible to use isotopic data for several elements to constrain the compositions of precursor materials of Ca-Al-rich inclusions, which have an evaporation origin. Although corundum-bearing inclusions are known, their origins are complex and variable, and the scarcity of such inclusions indicates that melting of Hibonite, with or without concomitant evaporation, must have been a rare process in the solar nebula. By evaporating mixtures of synthetic oxides of the rare earth elements, we have reproduced the patterns of Group III inclusions and some of the characteristics of ultrarefractory patterns. However, the extreme conditions required to do so indicate that refractory inclusions with these patterns probably have a condensation rather than evaporation origin.

  • elemental and isotopic fractionations produced through evaporation of the allende cv chondrilte implications for the origin of hal type Hibonite inclusions
    Geochimica et Cosmochimica Acta, 1996
    Co-Authors: Christine Floss, Ernst Zinner, Ahmed El Goresy, Gerrit Kransel, Werner Rammensee, H Palme
    Abstract:

    Abstract Through evaporation of samples from the Allende carbonaceous chondrite we have produced a series of residues that show correlated variations in mineralogy, chemistry, and isotopic compositions. Major and minor elements are evaporated in the order (Fe, Mn, Cr) → (Mg, Si) → (Ca, Ti) → (Al) and their loss is reflected in the mineralogy of the remaining samples. Residues of low to moderate degrees of evaporation consist of increasingly Mg-rich olivine and silicate glass. After complete evaporation of Fe, Mg, and Si at approximately 96% mass loss, the residues consist of very fine-grained Ca aluminates. Evaporation at higher temperatures produced three residues that contain Hibonite and a less refractory CaAl glass. Magnesium, Si, Ca, Ti, and O isotopes show mass-dependent fractionations that are consistent with Rayleigh-type distillation. The rare earth elements and other refractory trace elements are enriched in the residues up to ∼100 × CI, although several elements (V, Ba, Ce) are depleted due to their increased volatilities under oxidizing conditions. The most refractory residues also exhibit depletions in Eu, an element that is volatile under reducing conditions, but is as refractory as the other light rare earth elements under oxidizing conditions. The apparently contradictory presence of both Ce and Eu depletions in the residues is a result of changing evaporation dynamics in the course of the experiments: the release of large amounts of O during evaporation of the major element oxides creates “locally oxidizing” conditions in the samples; later, after most major elements have been vaporized, the local sample environments become more “reducing.” The three Hibonite-bearing residues share many chemical and isotopic characteristics with five HAL-type Hibonite inclusions for which an origin as distillation residues has been proposed; our data show that many of the unique features of these inclusions can be produced in a single evaporation event. Strong similarities between the Hibonite-bearing residues and the Hibonite inclusions HAL and DH-H1 suggest that the evaporation histories of these inclusions may be roughly comparable to those of our residues.

L P Keller - One of the best experts on this subject based on the ideXlab platform.

  • complex intergrowths of non stoichiometric defect structured Hibonite and al rich spinel in an allende ca al rich inclusion
    Annual Meeting of the Meteoritical Society (MetSoc 2019), 2019
    Co-Authors: J Han, I Ohnishi, L P Keller
    Abstract:

    Hibonite is a primary ultra-refractory mineral occurring in many Ca-Al-rich inclusions (CAIs) and is predicted to condense as the second major phase from a cooling gas of solar composition. Our previous microstructural studies of Hibonite in carbonaceous chondrites revealed its unique microstructures consisting of numerous defects that contain Mg-enriched, wider spinel blocks in stoichiometric Hibonite. Han, J. et al. [2 items] demonstrated experimentally that defect-structured Hibonites can grow easily in the presence of minor Mg and are kinetically more stable than equilibrium assemblages predicted by thermodynamic calculations. However, a thermo-dynamic vs. structural stability of defect-structured Hibonite relative to other early-condensed Al-rich phases such as corundum, grossite, and spinel remains poorly constrained. Here, we present the results of atomic resolution TEM (Transmission Electron Microscopy) imaging of Hibonite in a compact Type A CAI in the Allende CV3 chondrite in order to better understand the crystal structure and chemistry of defect-structured Hibonite and its associated Al-rich phases, especially non-stoichiometric, Al-rich spinel, in the context of the formation of first refractory solids in the early solar nebula.

  • correlations among microstructure morphology chemistry and isotopic systematics of Hibonite in cm chondrites
    80th Annual Meeting of the Meteoritical Society, 2017
    Co-Authors: J Han, M C Liu, L P Keller, A M Davis
    Abstract:

    Introduction: Hibonite is a primary refractory phase occurring in many CAIs, typically with spinel and perovskite. Our microstructural studies of CAIs from carbonaceous chondrites reveal a range of stacking defect densities and correlated non-stoichiometry in Hibonite. We also conducted a series of annealing experiments, demonstrating that the Mg-Al substitution stabilized the formation of defect-structured Hibonite. Here, we continue a detailed TEM analysis of Hibonite-bearing inclusions from CM chondrites that have been well-characterized isotopically. We examine possible correlations of microstructure, morphology, mineralogy, and chemical and isotopic systematics of CM Hibonites in order to better understand the formation history of Hibonite in the early solar nebula. Methods: Fifteen Hibonite-bearing inclusions from the Paris CM chondrite were analyzed using a JEOL 7600F SEM and a JEOL 8530F electron microprobe. In addition to three Hibonite-bearing inclusions from the Murchison CM chondrite previously reported, we selected three inclusions from Paris, Pmt1-6, 1-9, and 1-10, representing a range of 26Al/27Al ratios and minor element concentrations for a detailed TEM study. We extracted TEM sections from Hibonite grains using a FEI Quanta 3D field emission gun SEM/FIB. The sections were then examined using a JEOL 2500SE field-emission scanning TEM equipped with a Thermo-Noran thin window EDX spectrometer. Results and Discussion: A total of six Hibonite-bearing inclusions, including two platy Hibonite crystals (PLACs) and four spinel-Hibonite inclusions (SHIBs), were studied. There are notable differences in chemical and isotopic compositions between the inclusions (Table 1), indicative of their different formation environment or timing. Our TEM observations show perfectly-ordered, stoichiometric Hibonite crystals without stacking defects in two PLACs, 2-7-1 and 2-8-2, and in three SHIBs, Pmt1-6, 1-9, and 1-10. In contrast, SHIB 1-9-5 Hibonite grains contain a low density of stacking defects linked to an increase in MgO contents, indicating complex, disordered intergrowths of stoichiometric and MgO-enriched Hibonites. From the data collected to date, we find no clear correlation between the microstructures of Hibonite and its morphological and mineralogical types that reflect distinct chemical and isotopic systematics [6-8,10]. Interestingly, the presence of no or few stacking defects in Hibonite from the PLACs and SHIBs are in contrast to our experimental studies that produced very high densities of stacking defects in Hibonite [3-5]. Unlike our experi-ments, electron microprobe data from the PLACs and SHIBs Hibonite grains show a strong correlation between (Ti4++Si4+) and Mg2+ cations, suggesting that coupled substitutions of (Ti4++Mg2+) and (Si4++Mg2+) for 2Al3+ inhibit the formation of defect-structured Hibonite. However, our experimental studies suggest that kinetics (e.g., cooling rate) or other thermal effects also exert a strong control on the microstructures and chemical compositions of Hibonite. In Pmt1-6, elongated perovskite grains present at the Hibonite grain boundaries display (121) twinning, indicative of a fast cooling (>50degC/min) after high-temperature events. Therefore, the nebular microstructural characteristics of Hibonite, at least in this inclusion, would not have destroyed by subsequent high-temperature annealing. Conclusions: Our TEM observations thus far show no clear correlation in microstructures, morphological and mineralogical characteristics, and chemical and isotopic systematics of Hibonites from CM chondrites. The observed variation in stacking defect densities in the Hibonites may be controlled by thermal processes in the early solar nebula. A detailed TEM analysis of additional CM Hibonite samples is underway to evaluate this hypothesis.

  • microstructural constraints on the formation history of Hibonite in refractory inclusions
    Lunar and Planetary Science Conference, 2017
    Co-Authors: J Han, L P Keller, L Koop, A M Davis
    Abstract:

    Hibonite is a primary refractory phase occurring in many Ca-Al-rich inclusions (CAIs), typically with spinel and perovskite [1]. Previous mi-crostructural studies of Hibonite in CAIs revealed the presence of numerous stacking defects along the (001) plane and correlated non-stoichiometry in Hibonite [2,3]. These features are interpreted as complex inter-growths of stoichiometric and Ca-deficient Hibonites, as shown by experimental studies of reaction-sintered CaO-Al2O3 compounds [4]. Here, we extend our transmission electron microscope (TEM) studies to Hibonite-bearing CAIs in CM chondrites that have been well characterized isotopically [5-7]. In addition, we have undertaken a series of anneal-ing experiments to explore the effect of minor elements (Mg and Ti) on the microstucure of Hibonite [8,9,this study]. The results of these experiments are being applied to Hibonite in CAIs in order to better understand its formation conditions.

  • corundum Hibonite inclusions and the environments of high temperature processing in the early solar system
    Geochimica et Cosmochimica Acta, 2017
    Co-Authors: A W Needham, J Han, S Messenger, L P Keller
    Abstract:

    Abstract Corundum-bearing Ca–Al-rich inclusions (CAIs) are a rare class of high-temperature condensates from the inner regions of the protoplanetary disk. Their mineralogy is intermediate between isolated corundum grains and CAIs where corundum has been replaced by lower-temperature phases. These inclusions sample a critical transitional period of the inner nebula where both the Sun and protoplanetary disk were rapidly evolving. We conducted O isotopic, Al–Mg chronological, petrographic, and crystallographic studies of four corundum-bearing inclusions in the Murchison CM2 and ALHA 77307 CO3.0 carbonaceous chondrites. Within each inclusion, corundum, Hibonite, and spinel have indistinguishable 16 O-rich compositions. The O isotopic compositions from all inclusions fall within a narrow range of Δ 17 O = −22.8 ± 3.6‰ that matches values of most previously studied micrometer-sized corundum grains and mineralogically pristine CAIs. These data indicate that, with few exceptions, the most refractory inclusions in carbonaceous chondrites formed from the same O isotopic reservoir. One CAI from ALHA 77307, ALH-61, exhibits a continuous corundum mantle overlying a Hibonite core, opposite the equilibrium condensation sequence at typical nebular pressures and dust/gas ratios. Transmission electron microscopy examination of the Hibonite–corundum interface suggests that the corundum condensed on the Hibonite and was itself then partially overlain with spinel. Additionally, high dust/gas ratios are interpreted from the W- and Mo-depleted composition of a refractory metal nugget within a second corundum-bearing CAI, ALH-160. Together, these observations show that the primary formation conditions of some corundum-bearing CAIs involved non-equilibrium condensation in environments with elevated dust-gas ratios. The corundum-bearing CAIs studied here have inferred initial 26 Al/ 27 Al ratios that fall within the roughly bimodal distribution of values observed in most CAIs. ALH-160 retains no resolvable excess 26 Mg while ALH-61 has a well-resolved initial 26 Al/ 27 Al ratio of 4.2 ± 0.4 × 10 −5 . The presence or absence of live 26 Al at the time of CAI formation may record distinct chronology if 26 Al was initially homogeneously distributed in the early Solar System. Alternatively, variations in 26 Al/ 27 Al ratios may reflect late injection and/or heterogeneous distribution of 26 Al. Regardless of which model for 26 Al distribution is correct, the data presented here indicate that formation of corundum-bearing CAIs was repeated during multiple heating and non-equilibrium condensation events throughout early Solar System history and within a single oxygen isotopic reservoir.

  • magnesium and titanium isotopic compositions of an unusual Hibonite perovskite refractory inclusion from allende it is fun
    LPICo, 2016
    Co-Authors: M C Liu, L P Keller, K D Mckeegan
    Abstract:

    Introduction: Hibonite-rich refractory inclusions are among the first solids that formed in the solar nebula, and thus provide constraints on the earliest environment in the Solar System. An unusual Hibonite-perovskite inclusion from Allende, SHAL, consists of a large (approximately 500 by 200 microns) single Hibonite crystal and coexisting blocky perovskite (approximately 200 microns in size). The Hibonite is characterized by chemical and oxygen isotopic compositions similar to those in the FUN (Fractionated and Unknown Nuclear anomalies) inclusion HAL. However, the rare earth element (REE) patterns measured at different spots of SHAL Hibonite are highly variable, ranging from Group II-like (light REEs enriched relative to heavy REEs) to Group III-like (relatively flat with slight Eu depletions), but overall contrast largely with that of HAL, especially in the Ce and Yb abundances. This implies that SHAL Hibonite formed and underwent distillation processes under more reducing conditions. Interestingly, the accompanying perovskite has uniform, unfractionated oxygen isotopic compositions (averaging delta (sup 17) O equals delta (sup 18) O equals −7 per mille) and REE abundances that are completely different from those of SHAL Hibonite. This has been interpreted that perovskite and Hibonite may not be co-genetic. Here we performed Al-Mg and Ti isotopic measurements of SHAL Hibonite and perovskite to determine if the FUN characteristics are observed in these two isotope systems, and to further constrain the origin and evolution of SHAL. Results: Isotopic measurements of Al-Mg and Ti in SHAL were performed on the UCLA CAMECA ims-1290 ion microprobe by following the analytical protocols described in [1]. The Al-Mg and Ti data obtained in both terrestrial standards and SHAL Hibonite and perovskite are shown below. Both SHAL Hibonite and perovskite, despite very high (sup 27) Al to (sup 24) Mg ratios, are devoid of (sup 26) Mg excesses that can be attributed to the decay of (sup 26) Al. Delta (sup 25) Mg (mass-dependent fractionation) in Hibonite is approximately −5 per mille per atomic mass unit relative to Madagascar Hibonite, but is not well constrained for perovskite due to very large uncertainties owing to extremely low Mg contents. Similar to Mg isotopes, SHAL Hibonite and perovskite show essentially the same Ti isotopic compositions, with anomalies in (sup 50) Ti of approximately 14 per mille, but the former shows greater Ti isotope fractionation than the latter (2.5 per mille per atomic mass unit versus 0 per mille). Discussion and Conclusions: The Al-Mg and Ti isotopic compositions of SHAL Hibonite are consistent with those of HAL, suggesting that SHAL Hibonite is a FUN inclusion and likely formed prior to homogenization of (sup 26) Al and Ti isotope variations in the solar nebula. However, the formation mechanisms for SHAL and HAL differ, given the differences in the REE patterns and degrees of oxygen mass-dependent fractionation. The Group-II to Group-III like REE patterns, the Yb depletions, and negative delta (sup 25) Mg observed in SHAL Hibonite are all consistent with condensation of the Hibonite precursor in a reducing environment.. The lack of large Ce depletions in SHAL Hibonite implies that distillation processes that fractionated Hibonite's oxygen isotopes must have taken place under a reducing condition, but the extent to which SHAL Hibonite was distilled appears to be less than HAL because of the smaller degree of oxygen mass-dependent fractionation. The perovskite shares essentially the same Ti and Mg isotopic compositions as Hibonite and probably formed in the same reservoir.. The ultrarefractory REE pattern seen in perovskite likely resulted from gas-solid fractionation which depleted HREEs in this reservoir. This process also explains why SHAL Hibonite is generally depleted in HREEs relative to LREEs.

Andrew J. Berry - One of the best experts on this subject based on the ideXlab platform.

  • the limitations of Hibonite as a single mineral oxybarometer for early solar system processes
    Chemical Geology, 2017
    Co-Authors: A. N. Kravtsova, Andrew Walker, Andrew J. Berry, P F Schofield, Laura A Miller, N R Stephen, A V Soldatov, Trevor Ireland
    Abstract:

    The relationships between the composition of Hibonite with the general formula CaAl12-2x-yMgxTi4 +xTi3 +yO19, the oxidation state of Ti (Ti3 +/ΣTi, where ΣTi = Ti3 + + Ti4 +), and oxygen fugacity (fO2) were investigated experimentally. It was found that Hibonite can be synthesised with a range of Ti3 +/ΣTi values at constant fO2 and with a constant Ti3 +/ΣTi value for a range of fO2s. It was also found that if Hibonite with the formula CaAl12-yTi3 +yO19 (Ti3 +/ΣTi = 1) is equilibrated with a melt of CAI composition at fO2s below the iron-wustite buffer then the resulting Hibonite contained Mg, with Mg per formula unit (pfu) ~ 0.8 Ti pfu, and Ti3 +/ΣTi ~ 0.2, irrespective of the fO2. These results suggest that the availability of Mg, rather than fO2, is the key factor that determines Ti3 +/ΣTi of Hibonite. The structures of synthetic samples of Hibonite with the general formula CaAl12-2xMgxTi4 +xO19, where 0 ≤ X < 1, were determined by Rietveld refinement of X-ray powder diffraction data. The predominant site occupied by Ti4 + was found to change from M2 to M4 with increasing Ti content. The range of Ti concentrations over which the site occupancy changed corresponds to that observed in meteoritic Hibonite. This change in the Ti4 + site produces changes in the Ti K-edge XANES spectra, particularly in the intensity of the pre-edge feature, for constant Ti3 +/ΣTi. The observed dependence of the pre-edge on the Ti4 + site was reproduced by ab initio simulations of the XANES spectra. The XANES spectra of natural Hibonite with variable Ti content from the Murchison carbonaceous chondrite closely match the spectra of the synthetic samples with similar Ti contents. These differences in the spectra of meteoritic Hibonite could be misinterpreted as being due to changes in Ti3 +/ΣTi, but are instead due to differences in ΣTi, which relate to the petrogenetic history. Crystal chemistry exerts a first order control on the Ti site occupancy and Ti3 +/ΣTi value of Hibonite. As a result, no simple relationship between Ti3 +/ΣTi and fO2 should be expected. It is unlikely that Hibonite will be useful as an oxybarometer for solar processes without Ti3 +/ΣTi standards that are compositionally matched to the unknown.

  • the effect of site geometry ti content and ti oxidation state on the ti k edge xanes spectrum of synthetic Hibonite
    Geochimica et Cosmochimica Acta, 2016
    Co-Authors: Andrew J. Berry, P M Doyle, P F Schofield, J F W Mosselmans
    Abstract:

    Abstract The Al-rich oxide Hibonite (CaAl 12 O 19 ) is modeled to be the second mineral to condense from a gas of solar composition and is found within calcium–aluminum-rich inclusions and the matrix of chondritic meteorites. Both Ti 3+ and Ti 4+ are reported in meteoritic Hibonite, so Hibonite has been proposed as a single mineral oxybarometer that could be used to elucidate conditions within the first 0.2 Myrs of the Solar System. Synthetic Hibonites with Ti 3+ /(Ti 3+  + Ti 4+ ) (hereafter Ti 3+ /ΣTi) ranging between 0 and 1 were prepared as matrix-matched standards for meteoritic Hibonite. The largest yield of both Ti-free and Ti-bearing Hibonite at ∼1300 and ∼1400 °C was obtained by a single sinter under reducing conditions. In situ micro-beam Ti K-edge X-ray absorption near edge structure (XANES) spectra were recorded from the synthetic Hibonites, as well as from terrestrial Hibonite. Spectral features in the post-crest region were shown to correlate with the Ti 4+ content. Furthermore, Ti 4+ on the M2 trigonal bipyramidal and the adjoining M4 octahedral sites appears to cause variability in the post-crest region as a function of orientation. For this suite of synthetic Hibonites it was observed that the pre-edge peak region is not influenced by orientation, but is controlled by Ti 3+ /ΣTi, site geometry and/or Ti concentration. In particular, the pre-edge peak intensities reflect Ti coordination environment and distortion of the M4 octahedral site. Therefore, although pre-edge peak intensities have previously been used to determine Ti 3+ /ΣTi in meteoritic minerals, we excluded use of the pre-edge peak intensities for quantifying Ti valence states in Hibonite. The energy of the absorption edge at a normalized intensity of 0.8 ( E 0.8 ) and the energy of the minimum between the pre-edge region and the absorption edge ( E m1 ) were found to vary systematically with Ti 3+ /ΣTi. Ti 3+ /ΣTi in Hibonite as a function of E m1 was modeled by a quadratic function that may be used to quantify Ti 3+ /ΣTi in meteoritic Hibonite when the synthetic Hibonite standards are crystal-chemically matched to the natural samples and are measured during the same analytical session as the meteoritic Hibonites.

  • The Ti environment in natural Hibonite: XANES spectroscopy and computer modelling
    Journal of Physics: Conference Series, 2016
    Co-Authors: A. N. Kravtsova, Alexander V. Soldatov, Andrew Walker, Andrew J. Berry
    Abstract:

    The local atomic structure around Ti in Ti-bearing Hibonite (CaAl12O19) was studied using X-ray absorption near-edge structure (XANES) spectroscopy and computer modelling. Structural models of the direct substitution of Al by Ti3+, Al by Ti4+ charge balanced by the coupled substitution of Mg2+ for Al, and small Ti clusters were considered. The Ti K-XANES spectra of natural Hibonite with different Ti concentration were recorded. Theoretical Ti K- XANES spectra for structural models of Hibonite were calculated. It was shown that the theoretical Ti K-XANES spectra for a model with Ti at the five-coordinated M2 site are in agreement with the experimental XANES spectra of Hibonite with low concentrations of Ti, while the theoretical spectra for a structural model of clustered Ti are in agreement with the experimental spectra of Hibonite with higher Ti contents.

  • substitution of ti3 and ti4 in Hibonite caal12o19
    American Mineralogist, 2014
    Co-Authors: Andrew Walker, Andrew J. Berry, P F Schofield, Patricia M Doyle, K S Knight
    Abstract:

    The structures of eight synthetic samples of Hibonite, with variable Ti oxidation state and Ti concentration (2.4–15.9 wt% TiO2) that span the range reported for natural Hibonite found in meteorites, were determined by Rietveld refinements of neutron powder diffraction data. Ti3+ was found to exclusively occupy the octahedral face-sharing M4 site irrespective of the presence or absence of Ti4+. Ti4+ partitions between the trigonal bipyramidal M2 site and the M4 site. The ratio (Ti4+ on M2):(Ti4+ on M4) appears to be constant for all the samples, with an average of 0.18(2) irrespective of the concentrations of Ti3+ and Ti4+. These substitutional sites were shown to be the most stable configurations for Ti in Hibonite from calculations using density functional theory, although the predicted preference of Ti4+ for M4 over M2 is not as strong as is observed. This is attributed to the different Ti contents of the experimental and calculated structures and suggests that the Ti site occupancies might change between these concentrations. Furthermore, it is shown that Ti has a preference to occupy neighboring M4 sites such that Ti-Ti interactions occur with stabilization energies of 83 kJ/mol for Ti3+-Ti3+ and at least 15 kJ/mol for Ti4+-Ti4+. Features in optical spectroscopy and electron spin resonance data from meteoritic and synthetic Hibonites that have been used to infer Ti3+/Ti4+ are shown to actually derive from these Ti-Ti interactions. The amount of Ti4+ in Hibonite can be determined from the unit-cell parameters if ∑Ti is determined independently. Ti3+/Ti4+ in Hibonite may record the oxygen fugacity ( f O2) of the early solar nebula, however, the existence of Ti3+-Ti3+ and Ti4+-Ti4+ interactions and the potential for Ti4+-Ti3+ interactions need to be considered when interpreting spectroscopic data in terms of Ti valence state and f O2. Hibonite as a single-mineral oxybarometer must be used with caution due to the potential role of crystal chemistry (including Ti-Ti interactions) to stabilize Ti oxidation states independently of f O2.

  • the oxidation state of ti in synthetic and meteoritic Hibonite with application to early solar nebula processes
    Workshop on Formation of the First Solids in the Solar System, 2011
    Co-Authors: P M Doyle, Andrew J. Berry, P F Schofield, J F W Mosselmans, A D Smith, Andreas Scholl, A T Young
    Abstract:

    Introduction: Hibonite (CaAl12O19) is a Ti-bearing mineral found in calcium aluminium inclusions (CAIs) and is thought to be the second mineral to condense from a solar composition gas [1]. As such, the crystal chemistry of Hibonite could provide insight into the conditions of the early Solar System. Ti may occur as Ti 3+ under reducing conditions, with up to 23% of the Ti in meteoritic Hibonite previously reported as Ti 3+

K D Mckeegan - One of the best experts on this subject based on the ideXlab platform.

  • magnesium and titanium isotopic compositions of an unusual Hibonite perovskite refractory inclusion from allende it is fun
    LPICo, 2016
    Co-Authors: M C Liu, L P Keller, K D Mckeegan
    Abstract:

    Introduction: Hibonite-rich refractory inclusions are among the first solids that formed in the solar nebula, and thus provide constraints on the earliest environment in the Solar System. An unusual Hibonite-perovskite inclusion from Allende, SHAL, consists of a large (approximately 500 by 200 microns) single Hibonite crystal and coexisting blocky perovskite (approximately 200 microns in size). The Hibonite is characterized by chemical and oxygen isotopic compositions similar to those in the FUN (Fractionated and Unknown Nuclear anomalies) inclusion HAL. However, the rare earth element (REE) patterns measured at different spots of SHAL Hibonite are highly variable, ranging from Group II-like (light REEs enriched relative to heavy REEs) to Group III-like (relatively flat with slight Eu depletions), but overall contrast largely with that of HAL, especially in the Ce and Yb abundances. This implies that SHAL Hibonite formed and underwent distillation processes under more reducing conditions. Interestingly, the accompanying perovskite has uniform, unfractionated oxygen isotopic compositions (averaging delta (sup 17) O equals delta (sup 18) O equals −7 per mille) and REE abundances that are completely different from those of SHAL Hibonite. This has been interpreted that perovskite and Hibonite may not be co-genetic. Here we performed Al-Mg and Ti isotopic measurements of SHAL Hibonite and perovskite to determine if the FUN characteristics are observed in these two isotope systems, and to further constrain the origin and evolution of SHAL. Results: Isotopic measurements of Al-Mg and Ti in SHAL were performed on the UCLA CAMECA ims-1290 ion microprobe by following the analytical protocols described in [1]. The Al-Mg and Ti data obtained in both terrestrial standards and SHAL Hibonite and perovskite are shown below. Both SHAL Hibonite and perovskite, despite very high (sup 27) Al to (sup 24) Mg ratios, are devoid of (sup 26) Mg excesses that can be attributed to the decay of (sup 26) Al. Delta (sup 25) Mg (mass-dependent fractionation) in Hibonite is approximately −5 per mille per atomic mass unit relative to Madagascar Hibonite, but is not well constrained for perovskite due to very large uncertainties owing to extremely low Mg contents. Similar to Mg isotopes, SHAL Hibonite and perovskite show essentially the same Ti isotopic compositions, with anomalies in (sup 50) Ti of approximately 14 per mille, but the former shows greater Ti isotope fractionation than the latter (2.5 per mille per atomic mass unit versus 0 per mille). Discussion and Conclusions: The Al-Mg and Ti isotopic compositions of SHAL Hibonite are consistent with those of HAL, suggesting that SHAL Hibonite is a FUN inclusion and likely formed prior to homogenization of (sup 26) Al and Ti isotope variations in the solar nebula. However, the formation mechanisms for SHAL and HAL differ, given the differences in the REE patterns and degrees of oxygen mass-dependent fractionation. The Group-II to Group-III like REE patterns, the Yb depletions, and negative delta (sup 25) Mg observed in SHAL Hibonite are all consistent with condensation of the Hibonite precursor in a reducing environment.. The lack of large Ce depletions in SHAL Hibonite implies that distillation processes that fractionated Hibonite's oxygen isotopes must have taken place under a reducing condition, but the extent to which SHAL Hibonite was distilled appears to be less than HAL because of the smaller degree of oxygen mass-dependent fractionation. The perovskite shares essentially the same Ti and Mg isotopic compositions as Hibonite and probably formed in the same reservoir.. The ultrarefractory REE pattern seen in perovskite likely resulted from gas-solid fractionation which depleted HREEs in this reservoir. This process also explains why SHAL Hibonite is generally depleted in HREEs relative to LREEs.

  • mineralogy and oxygen isotope compositions of an unusual Hibonite perovskite refractory inclusion from allende
    75th Annual Meeting of the Meteoritical Society, 2012
    Co-Authors: L P Keller, C Snead, Z Rahman, K D Mckeegan
    Abstract:

    Hibonite-rich Ca- and Al-rich inclusions (CAIs) are among the earliest formed solids that condensed in the early nebula. We discovered an unusual refractory inclusion from the Allende CV3 chondrite (SHAL) containing an approx 500 micron long single crystal of Hibonite and co-existing coarse-grained perovskite. The mineralogy and petrography of SHAL show strong similarities to some FUN inclusions, especially HAL. Here we report on the mineralogy, petrography, mineral chemistry and oxygen isotopic compositions in SHAL.

  • isotopic records in cm Hibonites implications for timescales of mixing of isotope reservoirs in the solar nebula
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: Mingchang Liu, K D Mckeegan, J N Goswami, K K Marhas, S Sahijpal, T R Ireland, Andrew M Davis
    Abstract:

    Abstract The magnesium isotopic compositions of 26 Hibonite-bearing inclusions from the CM chondrite Murchison, as well as isotopic measurements on a subset of these samples for oxygen, titanium, and lithium–beryllium–boron are reported along with oxygen isotopic data for an additional 13 Hibonites that were previously investigated for other isotope systems (magnesium, potassium, calcium, and titanium) and rare earth element concentrations. Magnesium isotopic compositions divide CM Hibonites into two distinct populations which correlate perfectly with their mineralogy and morphology, as previously discovered by Ireland [Ireland T. R. (1988) Correlated morphological, chemical, and isotopic characteristics of Hibonites from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta 52, 2827–2839]: Spinel-Hibonite spherules (SHIBs) bear evidence of in situ 26Al decay, whereas PLAty-Crystals (PLACs) and Blue AGgregates (BAGs) either lack resolvable 26Mg-excesses or exhibit 26Mg deficits by up to ∼4‰. High precision, multiple collector SIMS analyses show that 6 of 7 SHIBs investigated fall on a single correlation line implying 26Al/27Al = (4.5 ± 0.2) × 10−5 at the time of isotopic closure, consistent with the “canonical” 26Al abundance characteristic of internal isochrons in many calcium–aluminum-rich inclusions (CAIs). One SHIB sample exhibits Δ26Mg∗ consistent with a “supracanonical” 26Al/27Al ratio of (6.4 ± 0.5) × 10−5. The PLAC Hibonites contain highly anomalous titanium isotopic compositions, with δ50Ti values ranging from −80‰ to almost +200‰, whereas SHIBs generally lack large Ti isotopic anomalies. Eight out of 11 26Al-free PLAC Hibonite grains record 10B/11B excesses that correlate with Be/B; the inferred initial 10Be/9Be ratio of (5.1 ± 1.4) × 10−4 is lower than the best-constrained 10Be/9Be of (8.8 ± 0.6) × 10−4 in a CV CAI. The data demonstrate that 10Be cannot be used as a relative chronometer for these objects and that most of the 10Be observed in CAIs must be produced by irradiation of precursor solids in the early solar system. The lack of 26Al in PLAC Hibonites indicates that significant amounts of 26Al were not formed in the same spallogenic processes that made 10Be in PLAC precursors. This is most easily understood as indicating very early formation of the PLAC Hibonites, prior to the incorporation and mixing of 26Al into the solar nebula, although an alternative scenario, which invokes irradiation under different solar flare conditions, cannot be ruled out. Lithium isotopes are normal within uncertainties, probably reflecting contamination and/or postcrystallization exchange. The oxygen isotopic compositions of SHIBs and PLACs are all highly 16O-enriched, but are not derived from a homogeneous reservoir: Δ17O values span a range of ∼−28‰ to −15‰. The ranges of 16O-enrichment in SHIBs and PLACs overlap and are less “anomalous” than the most 16O-enriched compositions found in meteorites [Kobayashi S., Imai H. and Yurimoto H. (2003) New extreme 16O-rich chondrule in the early solar system. Geochem. J. 37, 663–669]. Both PLACs and SHIBs formed in 16O-enriched reservoirs characterized by small-scale heterogeneities in the gas phase. If such heterogeneities were generated by an admixture of relatively 16O-poor gas created by self-shielding during CO photolysis and transported to the hot inner regions of the accretion disk, then this process must have been initiated very early on, prior to the arrival of fresh radioactivity into the inner solar system. Oxygen isotope heterogeneities persisted throughout the formation interval of PLACs, CAI precursors, and SHIBs which could be as long as 3 × 105 years based on 26Al records. One SHIB and one BAG exhibit mass fractionated oxygen isotopic compositions similar to those seen in FUN inclusions and in several platy Hibonite crystals [Lee T., Mayeda T. K. and Clayton R. N. (1980) Oxygen isotopic anomalies in Allende inclusion HAL. Geophys. Res. Lett. 7, 493–496; Ireland T. R., Zinner E. K., Fahey A. J. and Esat T. M. (1992) Evidence for distillation in the formation of HAL and related Hibonite inclusions. Geochim. Cosmochim. Acta 56, 2503–2520; Ushikubo T., Hiyagon H. and Sugiura N. (2007) A FUN-like Hibonite inclusion with a large 26Mg-excess. Earth Planet. Sci. Lett. 254, 115–126]. The suite of mass-fractionated Hibonites exhibit a range of isotopic properties, including 26Al/27Al ratios from below detection to a “canonical” level and oxygen and titanium anomalies that are not exceptional by PLAC standards. This suggests that F (fractionation) processes—evaporation under (oxidizing) conditions—are not necessarily associated with sampling a special isotopic reservoir.

  • on an irradiation origin for magnesium isotope anomalies in meteoritic Hibonite
    The Astrophysical Journal, 2009
    Co-Authors: Mingchang Liu, K D Mckeegan
    Abstract:

    We investigate spallogenic effects on magnesium isotopic compositions in solids with Hibonite-like (CaAl12O19) chemistry under an irradiation setting in the early solar system. We consider a series of nuclear reactions triggered by gradual flare irradiation, with an energy spectrum dN/dE E –2.7 and a proton flux Fp (E≥ 10 MeV) = 1.9 × 1010 cm–2 s–1, on isotopically normal Hibonite-like solids. The proton fluence is constrained by matching the 10Be/9Be measured in meteoritic Hibonite platy crystals. The net result, accounting for both production and destruction reactions of Mg isotopes, shows small deviations from the terrestrial isotopic composition with the sign and magnitude of Δ26Mg effects dependent on both low energy cutoff and total fluence. The total span of predicted spallogenic deviations can explain small apparent 26Mg excesses observed in some Hibonite grains, but does not account for the magnitude of apparent 26Mg deficits found in other grains. As previously indicated, irradiation by gradual flares decouples Δ26Mg variations and 26Al/27Al from 10Be/9Be.

  • a Hibonite corundum inclusion from murchison a first generation condensate from the solar nebula
    Meteoritics & Planetary Science, 2002
    Co-Authors: S B Simon, L Grossman, Andrew M Davis, K D Mckeegan
    Abstract:

    Through freeze-thaw disaggregation of the Murchison (CM) carbonaceous chondrite, we have recovered a ~90 x 75 μm refractory inclusion that consists of corundum and Hibonite with minor perovskite. Corundum occurs as small (~10 μm), rounded grains enclosed in Hibonite laths (~10 μm wide and 30-40 μm long) throughout the inclusion. Perovskite predominantly occurs near the edge of the inclusion. The crystallization sequence inferred petrographically - corundum followed by Hibonite followed by perovskite - is that predicted for the first phases to form by equilibrium condensation from a solar gas for Ptot ≤ 5 x 10^(-3) atm. In addition, the texture of the inclusion, with angular voids between subhedral Hibonite laths and plates, is also consistent with formation of the inclusion by condensation. Hibonite has heavy rare earth element (REE) abundances of ~40 x CI chondrites, light REE abundances ~20 x CI chondrites, and negative Eu anomalies. The chondrite-normalized abundance patterns, especially one for a Hibonite-perovskite spot, are quite similar to the patterns of calculated solid/gas partition coefficients for Hibonite and perovskite at 10^(-3) atm and are not consistent with formation of the inclusion by closed-system fractional crystallization. In contrast with the features that are consistent with a condensation origin, there are problems with any model for the formation of this inclusion that includes a molten stage, relic grains, or volatilization. If thermodynamic models of equilibrium condensation are correct, then this inclusion formed at pressures <5 x 10^(-3) atm, possibly with enrichments (<1000x) in CI dust relative to gas at low pressures (below 10^(-4) atm). Both Hibonite and corundum have δ17O ≈ δ18O ≈ -50‰, indicating formation from an 16O-rich source. The inclusion does not contain radiogenic 26Mg and apparently did not contain live 26Al when it formed. If the short-lived radionuclides were formed in a supernova and injected into the early solar nebula, models of this process suggest that 26Al-free refractory inclusions such as this one formed within the first ~6 x 10^5 years of nebular collapse.