Now showing items 1-20 of 26

    • NanoSIMS studies of Ba isotopic compositions in single presolar silicon carbide grains from AGB stars and supernovae

      Marhas, K. K.; Hoppe, P.; Ott, U. (The Meteoritical Society, 2007-01-01)
      We have studied 74 single presolar silicon carbide grains with sizes between 0.2 and 2.6 micrometers from the Murchison and Murray meteorites for Ba isotopic compositions using NanoSIMS. We also analyzed 7 SiC particles either consisting of sub-micron-size SiC grains or representing a morphologically and isotopically distinct subgroup. Of the 55 (likely) mainstream grains, originating from asymptotic giant branch (AGB) stars, 32 had high enough Ba contents for isotopic analysis. For 26 of them, CsHx interferences were either negligible or could be corrected with confidence. They exhibit typical s-process Ba isotopic patterns with slightly higher than solar 134Ba/136Ba and lower than solar 135,137,138Ba/136Ba ratios. Results are generally well explained in the context of neutron capture nucleosynthesis in low mass (1-3 M(sun)) AGB stars and provide constraints on AGB models, by reducing the needed 13C spread from factor of ~20 down to 2. Out of the 19 supernova X grains, three had sufficient concentrations for isotopic analysis. They tend to exhibit higher than solar 134Ba/136Ba and 138Ba/136Ba ratios, close to solar 137Ba/136Ba, and 135Ba/136Ba lower than solar but higher than in mainstream grains. This signature could indicate a mixture of n-burst type Ba with either normal Ba more s-process-rich than solar, or normal Ba plus weak s-process Ba. In the n-burst component Cs may have to be separated from Ba at ~10 years after the SN explosion. Depending on predictions for its composition, another possibility is early separation (at ~1 year) coupled with addition of some unfractionated n-burst matter. Abundances of trace elements (Sr, Zr, Cs, La, and Ce) analyzed along with Ba signify that implantation may have been an important process for their introduction.
    • Titanium isotopic compositions of well-characterized silicon carbide grains from Orgueil (CI): Implications for s-process nucleosynthesis

      Huss, G. R.; Smith, J. B. (The Meteoritical Society, 2007-01-01)
      We have measured the titanium isotopic compositions of 23 silicon carbide grains from the Orgueil (CI) carbonaceous chondrites for which isotopic compositions of silicon, carbon, and nitrogen and aluminum-magnesium systematics had been measured previously. Using the 16 mostprecise measurements, we estimate the relative contributions of stellar nucleosynthesis during the asymptotic giant branch (AGB) phase and the initial compositions of the parent stars to the compositions of the grains. To do this, we compare our data to the results of several published stellar models that employ different values for some important parameters. Our analysis confirms that s-process synthesis during the AGB phase only slightly modified the titanium compositions in the envelopes of the stars where mainstream silicon carbide grains formed, as it did for silicon. Our analysis suggests that the parent stars of the >1 micrometer silicon carbide grains that we measured were generally somewhat more massive than the Sun (2-3 M(sun)) and had metallicities similar to or slightly higher than solar. Here we differ slightly from results of previous studies, which indicated masses at the lower end of the range 1.5-3 M(sun) and metallicities near solar. We also conclude that models using a standard 13C pocket, which produces a good match for the main component of s-process elements in the solar system, overestimate the contribution of the 13C pocket to s-process nucleosynthesis of titanium found in silicon carbide grains. Although previous studies have suggested that the solar system has a significantly different titanium isotopic composition than the parent stars of silicon carbide grains, we find no compelling evidence that the Sun falls off of the array defined by those stars. We also conclude that the Sun does lie on the low-metallicity end of the silicon and titanium arrays defined by mainstream silicon carbide grains.
    • Non-mass-dependent oxygen isotopic fractionation in smokes produced in an electrical discharge

      Kimura, Y.; Nuth, J. A.; Chakraborty, S.; Thiemens, M. H. (The Meteoritical Society, 2007-01-01)
      We report the first production of non-mass-dependently fractionated silicate smokes from the gas phase at room temperature from a stream of silane and/or pentacarbonyl iron in a molecular hydrogen (or helium) flow mixed with molecular oxygen (or nitrous oxide). The smokes were formed at the Goddard Space Flight Center (GSFC) at total pressures of just under 100 Torr in an electrical discharge powered by a Tesla coil, were collected from the surfaces of the copper electrodes after each experiment and sent to the University of California at San Diego (UCSD) for oxygen isotopic analysis. Transmission electron microscopy studies of the smokes show that they grew in the gas phase rather than on the surfaces of the electrodes. We hypothesize at least two types of fractionation processes occurred during formation of the solids: a mass-dependent process that made isotopically lighter oxides compared to our initial oxygen gas composition followed by a mass-independent process that produced oxides enriched in 17O and 18O. The maximum Delta-17O observed is +4.7 per mil for an iron oxide produced in flowing hydrogen, using O2 as the oxidant. More typical displacements are 1-2 per mil above the equilibrium fractionation line. The chemical reaction mechanisms that yield these smokes are still under investigation.
    • Trace element studies of silicate-rich inclusions in the Guin (UNGR) and Kodaikanal (IIE) iron meteorites

      Kurat, G.; Zinner, E.; Varela, M. E. (The Meteoritical Society, 2007-01-01)
      A devitrified glass inclusion from the Guin (UNGR) iron consists of cryptocrystalline feldspars, pyroxenes, and silica and is rich in SiO2, Al2O3, and Na2O. It contains a rutile grain and is in contact with a large Cl apatite. The latter is very rich in rare earth elements (REEs) (~80 x CI), which display a flat abundance pattern, except for Eu and Yb, which are underabundant. The devitrified glass is very poor in REEs (<0.1 x CI), except for Eu and Yb, which have positive abundance anomalies. Devitrified glass and Cl apatite are out of chemical equilibrium and their complementary REE patterns indicate a genesis via condensation under reducing conditions. Inclusion 1 in the Kodaikanal (IIE) iron consists of glass only, whereas inclusion 2 consists of clinopyroxene, which is partly overgrown by low-Ca pyroxene, and apatite embedded in devitrified glass. All minerals are euhedral or have skeletal habits indicating crystallization from the liquid precursor of the glass. Pyroxenes and the apatite are rich in trace elements, indicating crystallization from a liquid that had 1050 CI abundances of REEs and refractory lithophile elements (RLEs). The co-existing glass is poor in REEs (~0.1-1 x CI) and, consequently, a liquid of such chemical composition cannot have crystallized the phenocrysts. Glasses have variable chemical compositions but are rich in SiO2, Al2O3, Na2O, and K2O as well as in HFSEs, Be, B, and Rb. The REE abundance patterns are mostly flat, except for the glass-only inclusion, which has heavy rare earth elements (HREEs) light rare earth elements (LREEs) and deficits in Eu and Yb—an ultrarefractory pattern. The genetic models suggested so far cannot explain what is observed and, consequently, we offer a new model for silicate inclusion formation in IIE and related irons. Nebular processes and a relationship with E meteorites (Guin) or Ca-Al-rich inclusions (CAIs) (Kodaikanal) are indicated. A sequence of condensation (CaS, TiN or refractory pyroxene-rich liquids) and vapor-solid elemental exchange can be identified that took place beginning under reducing and ending at oxidizing conditions (phosphate, rutile formation, alkali and Fe^2+ metasomatism, metasomatic loss of REEs from glasses).
    • Characterization of insoluble organic matter in primitive meteorites by microRaman spectroscopy

      Busemann, H.; Alexander, C. M. O'D.; Nittler, L. R. (The Meteoritical Society, 2007-01-01)
      We have analyzed the chemically and isotopically well-characterized insoluble organic matter (IOM) extracted from 51 unequilibrated chondrites (8 CR, 9 CM, 1 CI, 3 ungrouped degrees C, 9 CO, 9 CV, 10 ordinary, 1 CB and 1 E chondrites) using confocal imaging Raman spectroscopy. The average Raman properties of the IOM, as parameterized by the peak characteristics of the so-called D and G bands, which originate from aromatic degrees C rings, show systematic trends that are correlated with meteorite (sub-) classification and IOM chemical compositions. Processes that affect the Raman and chemical properties of the IOM, such as thermal metamorphism experienced on the parent bodies, terrestrial weathering and amorphization due to irradiation in space, have been identified. We established separate sequences of metamorphism for ordinary, CO, oxidized, and reduced CV chondrites. Several spectra from the most primitive chondrites reveal the presence of organic matter that has been amorphized. This amorphization, usually the result of sputtering processes or UV or particle irradiation, could have occurred during the formation of the organic material in interstellar or protoplanetary ices or, less likely, on the surface of the parent bodies or during the transport of the meteorites to Earth. D band widths and peak metamorphic temperatures are strongly correlated, allowing for a straightforward estimation of these temperatures.
    • A cornucopia of presolar and early solar system materials at the micrometer size range in primitive chondrite matrix

      Bland, P. A.; Stadermann, F. J.; Floss, C.; Rost, D.; Vicenzi, E. P.; Kearsley, A. T.; Benedix, G. K. (The Meteoritical Society, 2007-01-01)
      We have used a variety of complementary microanalytical techniques to constrain the mineralogy, trace-element distributions, and oxygen-isotopic compositions in a 50 x 50 micrometer area of Acfer 094 matrix. The results reveal the exceptional mineralogical and compositional heterogeneity of this material at the sub-m level. We observe micrometer-scale and sub-micrometer grains with elemental associations suggesting feldspar, metal with widely varying Ni contents, and a Cr-Fe alloy (in addition to forsterite, pyroxene, sulfide, ferrihydrite, and amorphous groundmass previously described). A new class of micrometer-scale CAI (micro-CAI) is also observed, which show sub-m compositional zoning, and a range of oxygen isotopic compositions. Unlike the larger CAIs in Acfer 094, which are uniformly 16O-enriched, two of the three CAIs we analyzed are isotopically normal. We also observed a Li-rich hotspot that detailed analysis by ToF-SIMS suggests may be a LiCr-oxide grain. Within the resolution of the NanoSIMS, this grain has isotopically normal Li. Finally, in our 50 x 50 micrometer area, we positively identified a presolar grain that is the most 18O-rich silicate found so far in meteorites. The grain may originate from an asymptotic giant branch (AGB) star, or more likely, a supernova. In line with previous TEM studies (Greshake 1997), we find no evidence for clastic material (e.g., fragmental chondrules) in the matrix of Acfer 094: although the matrix is volatile-depleted, this depletion does not appear to result from dilution of a primordial starting material with (depleted) chondrule fragments. Assuming that matrix experienced the depletion event, our data on the detailed mineralogy of Acfer 094 are currently equivocal in constraining the nature of that event. We observe carrier phases for several elements consistent with conditions approaching equilibrium condensation; however, the presence of an amorphous groundmass is suggestive of more rapid cooling.
    • High spatial resolution ion microprobe measurements refine chronology of carbonate formation in Orgueil

      Hoppe, P.; MacDougall, D.; Lugmair, G. W. (The Meteoritical Society, 2007-01-01)
      Aqueous activity on meteorite parent bodies is indicated by the presence of carbonates. High spatial resolution ion microprobe analyses of nine individual carbonate grains (four dolomites, five breunnerites) from the Orgueil meteorite reveal linear correlations between 53Cr excesses and Mn/Cr ratios in all grains, indicative of in situ decay of radioactive 53Mn (half-life 3.7 million years). The well-defined isochrons appear to have chronological significance. If this is the case, then 53Mn/ 55Mn ratios between 2.1 and 4.7 x 10^(-6) are inferred for the time of carbonate formation and absolute ages of between 4561 and 4565 Ma are calculated (systematic uncertainty of +/- 2 Ma). Dolomites tend to have formed slightly earlier than the breunnerites. Our data imply extensive aqueous activity on the Orgueil parent body over a period of several million years, starting ~3-4 Myr after formation of the solar system, that most likely was the result of impact heating and latent heat from the decay of radioactive 26Al and 60Fe.
    • Comparison of cosmic-ray exposure ages and trapped noble gases in chondrule and matrix samples of ordinary, enstatite, and carbonaceous chondrites

      Eugster, O.; Lorenzetti, S.; Krähenbühl, U.; Marti, K. (The Meteoritical Society, 2007-01-01)
      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 degrees 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 Baths CRE age falls on the 7 Ma double-peak of H chondrites, while Ogis fits the 22 Ma peak. The studied chondrules contain trapped 20Ne and 36Ar concentrations in the range of 10^(-6)-10^(-9) cm^3 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.
    • Constraints on the cooling history of the H-chondrite parent body from phosphate and chondrule Pb-isotopic dates from Estacado

      Blinova, A.; Amelin, Y.; Samson, C. (The Meteoritical Society, 2007-01-01)
      To constrain the metamorphic history of the H-chondrite parent body, we dated phosphates and chondrules from four H6 chondritic meteorites using U-Pb systematics. Reconnaissance analyses revealed that only Estacado had a sufficiently high 206Pb/204Pb ratio suitable for our purposes. The Pb- Pb isochron date for Estacado phosphates is measured to be 4492 +/- 15 Ma. The internal residue second leachate isochron for Estacado chondrules yielded the chondrule date of 4546 +/- 18 Ma. An alternative age estimate for Estacado chondrules of 4527.6 +/- 6.3 Ma is obtained from an isochron including two chondrules, two magnetically separated fractions, and four bulk chondrite analyses. This isochron date might represent the age of termination of Pb diffusion from the chondrules to the matrix. From these dates and previously established closure temperatures for Pb diffusion in phosphates and chondrules, we estimate an average cooling rate for Estacado between 5.5 +/- 3.2 Myr/degrees C and 8.3 +/- 5.0 Myr/degrees C. Using previously published results for Ste. Marguerite (H4) and Richardton (H5), our data reveal that the cooling rates of H chondrites decrease markedly with increasing metamorphic grade, in agreement with the predictions of the onion-shell asteroid model. Several issues, however, need to be addressed before confirming this model for the H-chondrite parent body: the discrepancies between peak metamorphic temperatures established by various mineral thermometers need to be resolved, diffusion and other mechanisms of element migration in polycrystalline solids must be better understood, and dating techniques should be further improved.
    • Coordinated isotopic and mineralogic analyses of planetary materials enabled by in situ lift-out with a focused ion beam scanning electron microscope

      Zega, T. J.; Nittler, L. R.; Busemann, H.; Hoppe, P.; Stroud, R. M. (The Meteoritical Society, 2007-01-01)
      We describe a focused ion beam scanning electron microscope (FIB-SEM) technique that enables coordinated isotopic and mineralogic analysis of planetary materials. We show that site specific electron-transparent sections can be created and extracted in situ using a microtweezer and demonstrate that they are amenable to analysis by secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). These methods greatly advance the ability to address several fundamental questions in meteoritics, such as accretion and alteration histories of chondrules and the origin and history of preserved nebular and presolar materials.
    • Pb isotopic age of the Allende chondrules

      Amelin, Y.; Krot, A. (The Meteoritical Society, 2007-01-01)
      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.
    • 36Cl, 26Al, and O isotopes in an Allende type B2 CAI: Implications for multiple secondary alteration events in the early solar system

      Ushikubo, T.; Guan, Y.; Hiyagon, H.; Sugiura, N.; Leshin, L. A. (The Meteoritical Society, 2007-01-01)
      We measured 36Cl-36S and 26Al-26Mg systematics and O isotopes of secondary phases in a moderately altered type B2 CAI (CAI#2) from the Allende CV3 chondrite. CAI#2 has two distinct alteration domains: the anorthite-grossular (An-Grs) domain that mostly consists of anorthite and grossular, and the Na-rich domain that mostly consists of sodalite, anorthite, and Fe-bearing phases. Large 36S excesses (up to ~400) corresponding to an initial 36Cl/35Cl ratio of (1.4 +/- 0.3) x 10^(-6) were observed in sodalite of the Na-rich domain, but no resolvable 26Mg excesses were observed in anorthite and sodalite of the Na-rich domain (the initial 26Al/27Al ratio 4.4 x 10^(-7)). If we assume that the 36Cl-36S and the 26Al-26Mg systematics were closed simultaneously, the 36Cl/35Cl ratio would have to be on the order of ~10^(-2) when CAIs were formed. In contrast to sodalite in Na-rich domain, significant 26Mg excesses (up to ~35 ppm) corresponding to an initial 26Al/27Al ratio of (1.2 +/- 0.2) x 10^(-5) were identified in anorthite of the An-Grs domain. The 26Al-26Mg systematics of secondary phases in CAI#2 suggest that CAIs experienced multiple alteration events. Some of the alteration processes occurred while 36Cl (half-life is 0.3 Myr) and 26Al (half-life is 0.72 Myr) were still alive, whereas others took place much later. Assuming that 26Al was homogeneously distributed in the solar nebula, our study implies that alteration of CAIs occurred as early as within 1.5 Myr of CAI formation and as late as 5.7 Myr after.
    • Late accretion and lithification of chondritic parent bodies: Mg isotope studies on fragments from primitive chondrites and chondritic breccias

      Sokol, A. K.; Bischoff, A.; Marhas, K. K.; Mezger, K.; Zinner, E. (The Meteoritical Society, 2007-01-01)
      Recent results of isotopic dating studies (182Hf-182W, 26Al-26Mg) and the increasing number of observed igneous and metamorphosed fragments in (primitive) chondrites provide strong evidence that accretion of differentiated planetesimals predates that of primitive chondrite parent bodies. The primitive chondrites Adrar 003 and Acfer 094 contain some unusual fragments that seem to have undergone recrystallization. Magnesium isotope analyses reveal no detectable radiogenic 26Mg in any of the studied fragments. The possibility that evidence for 26Al was destroyed by parent body metamorphism after formation is not likely because several other constituents of these chondrites do not show any metamorphic features. Since final accretion of a planetesimal must have occurred after formation of its youngest components, formation of these parent bodies must thus have been relatively late (i.e., after most 26Al had decayed). Al-Mg isotope data for some igneous-textured clasts (granitoids and andesitic fragments) within the two chondrite regolith breccias Adzhi-Bogdo and Study Butte reveal also no evidence for radiogenic 26Mg. As calculated from the upper limits, the formation of these igneous clasts, the incorporation into the parent body regolith, and the lithification must have occurred at least 3.8 Myr (andesite in Study Butte) and 4.7 Myr (granitoids in Adzhi-Bogdo) after calciumaluminum-rich inclusions (CAI) formation. The absence of 26Mg excess in the igneous inclusions does not exclude 26Al from being a heat source for planetary melting. In large, early formed planetesimals, cooling below the closure temperature of the Al-Mg system may be too late for any evidence for live 26Al (in the form of 26Mg excess) to be preserved. Thus, growing evidence exists that chondritic meteorites represent the products of a complex, multi-stage history of accretion, parent body modification, disruption and re-accretion.
    • Bulk Mg isotopic compositions of Ca-Al-rich inclusions and amoeboid olivine aggregates

      Wang, G.; Lin, Y.; Dai, D. (The Meteoritical Society, 2007-01-01)
      High-precision Mg isotopic compositions of Ca-Al-rich inclusions (CAIs) from both Ningqiang (ungrouped) and Allende (CV3) carbonaceous chondrites and amoeboid olivine aggregations (AOAs) from Allende were analyzed by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). The CAIs from Allende plot on a line, with an inferred initial 26Al/ 27Al ratio of (4.77 +/- 0.39) 10^(-5) close to the canonical value. This indicates a relatively closed Al-Mg system in the CAIs and no significant Mg isotope exchange with ambient materials, although two of the CAIs are severely altered. The AOAs contain excess 26Mg and plot close to the CAI regression line, which is suggestive of their contemporary formation. The CAIs from Ningqiang define a different line with a lower inferred (26Al/27Al)0 ratio of (3.56 +/- 0.08) 10^(-5). None of the CAIs and AOAs studied in this work shows significant mass fractionation with enrichment of the heavier Mg isotopes, arguing against an evaporation origin.
    • Oxygen and magnesium isotopic compositions of amoeboid olivine aggregates from the Semarkona LL3.0 chondrite

      Itoh, S.; Russell, S. S.; Yurimoto, H. (The Meteoritical Society, 2007-01-01)
      Amoeboid olivine aggregates (AOAs) in the LL3.0 Semarkona chondrite have been studied by secondary ion mass spectrometry. The AOAs mainly consist of aggregates of olivine grains with interstitial Al-Ti-rich diopside and anorthite. Oxygen-isotopic compositions of all phases are consistently enriched in 16O, with delta-17,18O = ~-50 ppm. The initial 26Al/27Al ratios are calculated to be 5.6 +/- 0.9 (2-sigma) x 10^(-5). These values are equivalent to those of AOAs and fine-grained calciumaluminum- rich inclusions (FGIs) from pristine carbonaceous chondrites. This suggests that AOAs in ordinary chondrites formed in the same 16O-rich calcium-aluminum-rich inclusion (CAI)-forming region of the solar nebula as AOAs and FGIs in carbonaceous chondrites, and subsequently moved to the accretion region of the ordinary chondrite parent body in the solar nebula.
    • Condensation and aggregation of solar corundum and corundum-hibonite grains

      Nakamura, T. M.; Sugiura, N.; Kimura, M.; Miyazaki, A.; Krot, A. N. (The Meteoritical Society, 2007-01-01)
      Forty-three corundum grains (1-11 micrometers in size) and 5 corundum-hibonite grains with corundum overgrown by hibonite (4-7 micrometers in size), were found in the matrix of the mineralogically pristine, ungrouped carbonaceous chondrite Acfer 094 by using cathodoluminescence imaging. Some of the corundum and corundum-hibonite grains occur as aggregates of 2 to 6 grains having similar sizes. The oxygen isotopic compositions of some of the corundum-bearing grains suggest their solar nebula origin. 26Al-26Mg systematics of one corundum grain showed the canonical initial 26Al/27Al ratio, also suggesting a solar nebula origin. Quantitative evaluation of condensation and accretion processes made based on the homogeneous nucleation of corundum, diffusion-controlled hibonite formation, collisions of grains in the nebula, and critical velocity for sticking, indicates that, in contrast to the hibonite-bearing aggregates of corundum grains, the hibonite-free corundum aggregates could not have formed in the slowly cooling nebular region with solar composition. We suggest instead that such aggregates formed near the protosun, either in a region that stayed above the condensation temperature of hibonite for a long time or in a chemically fractionated, Ca-depleted region, and were subsequently physically removed from this hot region, e.g., by disk wind.
    • Remelting of refractory inclusions in the chondrule-forming regions: Evidence from chondrule-bearing type C calcium-aluminum-rich inclusions from Allende

      Krot, A. N.; Yurimoto, H.; Hutcheon, I. D.; Chaussidon, M.; MacPherson, G. J.; Paque, J. (The Meteoritical Society, 2007-01-01)
      We describe the mineralogy, petrology, oxygen, and magnesium isotope compositions of three coarse-grained, igneous, anorthite-rich (type degrees C) Ca-Al-rich inclusions (CAIs) (ABC, TS26, and 93) that are associated with ferromagnesian chondrule-like silicate materials from the CV carbonaceous chondrite Allende. The CAIs consist of lath-shaped anorthite (An99), Cr-bearing Al-Ti-diopside (Al and Ti contents are highly variable), spinel, and highly åkermanitic and Na-rich melilite (Åk63-74, 0.4-0.6 wt% Na2O). TS26 and 93 lack Wark-Lovering rim layers; ABC is a CAI fragment missing the outermost part. The peripheral portions of TS26 and ABC are enriched in SiO2 and depleted in TiO2 and Al2O3 compared to their cores and contain relict ferromagnesian chondrule fragments composed of forsteritic olivine (Fa6-8) and low-Ca pyroxene/pigeonite (Fs1 Wo1-9). The relict grains are corroded by Al-Ti-diopside of the host CAIs and is surrounded by haloes of augite (Fs0.5 Wo30-42), augite (Fs0.5 Wo38-42), and anorthitic plagioclase (An84). Relict olivine and low-Ca pyroxene/pigeonite in ABC and TS26, and the pigeonite-augite rim around 93 are 16O-poor (Delta-17O ~ -1 ppm to -8 ppm). Spinel and Al-Ti-diopside in cores of CAIs ABC, TS26, and 93 are 16O-enriched (Delta-17O down to -20 ppm), whereas Al-Ti-diopside iin the outer zones, as well as melilite and anorthite, are 16O-depleted to various degrees (Delta-17O = -11 ppm to 2 ppm). In contrast to typical Allende CAIs that have the canonical initial 26Al/27Al ratio of ~5 x 10^(-5), ABC, 93, and TS26 are 26Al-poor with (26Al/27Al)0 ratios of (4.7 +/- 1.4) x 10^(-6), (1.5 +/- 1.8) x 10^(-6), and <1.2 x 10^(-6), respectively. We conclude that ABC, TS26, and 93 experienced remelting with addition of ferromagnesian chondrule silicates and incomplete oxygen isotopic exchange in an 16O-poor gaseous reservoir, probably in the chondrule-forming region. This melting episode could have reset the 26Al-26Mg systematics of the host CAIs, suggesting it occured ~Myr after fomation of most CAIs. These observations and the common presence of relict CAIs inside chondrules suggest that CAIs predated formation of chondrules.
    • Al-Mg isotopic evidence for episodic alteration of Ca-Al-rich inclusions from Allende

      Fagan, T. J.; Guan, Y.; MacPherson, G. J. (The Meteoritical Society, 2007-01-01)
      Textures, mineral assemblages, and Al-Mg isotope systematics indicate a protracted, episodic secondary mineralization history for Allende Ca-Al-rich inclusions (CAIs). Detailed observations from one type B1 CAI, one B2, one compact type A (CTA), and one fluffy type A (FTA) indicate that these diverse types of CAIs are characterized by two distinct textural and mineralogic types of secondary mineralization: (1) grossular-rich domains, concentrated along melilite grain boundaries in CAI interiors, and (2) feldspathoid-bearing domains, confined mostly to CAI margins just interior to the Wark-Lovering rim sequence. The Al-Mg isotopic compositions of most secondary minerals in the type B1 CAI, and some secondary minerals in the other CAIs, show no resolvable excesses of 26Mg, whereas the primary CAI phases mostly yield correlated excesses of 26Mg with increasing Al/Mg corresponding to canonical initial 26Al/27Al ~ 4.55 10^(-5). These secondary minerals formed at least 3 Ma after the primary CAI minerals. All but two analyses of secondary minerals from the fluffy type-A CAI define a correlated increase in 26Mg/24Mg with increasing Al/Mg, yielding (26Al/27Al)0 = (4.9 +/- 2.8) 10^(-6). The secondary minerals in this CAI formed 1.8-3.2 Ma after the primary CAI minerals. In both cases, the timing of secondary alteration is consistent with, but does not necessarily require, alteration in an asteroidal setting. One grossular from the type B2 CAI, and several grossular and secondary feldspar analyses from the compact type A CAI, have excesses of 26Mg consistent with initial 26Al/27Al ~ 4.5 10^(-5). Especially in the compact type A CAI, where 26Mg/24Mg in grossular correlates with increasing Al/Mg, these 26Mg excesses are almost certainly due to in situ decay of 26Al. They indicate a nebular setting for formation of the grossular. The preservation of these diverse isotopic patterns indicates that heating on the Allende parent body was not pervasive enough to reset isotopic systematics of fine-grained secondary minerals. Secondary mineralization clearly was not restricted to a short time interval, and at least some alteration occurred coincident with CAI formation and melting events (chondrule formation) in the nebula. This observation supports the possibility that alteration followed by melting affected the compositional evolution of CAIs.
    • 26Al-26Mg systematics of Ca-Al-rich inclusions, amoeboid olivine aggregates, and chondrules from the ungrouped carbonaceous chondrite Acfer 094

      Sugiura, N.; Krot, A. N. (The Meteoritical Society, 2007-01-01)
      We report in situ magnesium isotope measurements of 7 porphyritic magnesium-rich (type I) chondrules, 1 aluminum-rich chondrule, and 16 refractory inclusions (14 Ca-Al-rich inclusions [CAIs] and 2 amoeboid olivine aggregates [AOAs]) from the ungrouped carbonaceous chondrite Acfer 094 using a Cameca IMS 6f ion microprobe. Both AOAs and 9 CAIs show radiogenic 26Mg excesses corresponding to initial 26Al/27Al ratios between ~5 x 10^(5) and ~7 x 10^(5), suggesting that formation of the Acfer 094 CAIs may have lasted for ~300,000 years. Four CAIs show no evidence for radiogenic 26Mg; three of these inclusions (a corundum-rich, a grossite-rich, and a pyroxene-hibonite spherule CAI) are very refractory objects and show deficits in 26Mg, suggesting that they probably never contained 26Al. The fourth object without evidence for radiogenic 26Mg is an anorthite-rich, igneous (type degrees C) CAI that could have experienced late-stage melting that reset its Al-Mg systematics. Significant excesses in 26Mg were observed in two chondrules. The inferred 26Al/27Al ratios in these two chondrules are (10.3 +/- 7.4) 10^(6) and (6.0 +/- 3.8) 10^(6) (errors are 2-sigma), suggesting formation 1.6^(1.2 over 0.6) and 2.2^(-0.4 over 0.3) Myr after CAIs with the canonical 26Al/27Al ratio of 5 10^(5). These age differences are consistent with the inferred age differences between CAIs and chondrules in primitive ordinary (LL3.0-LL3.1) and carbonaceous (CO3.0) chondrites.
    • Gamma rays from cosmic radioactivities

      Diehl, R.; Hartmann, D. H.; Prantzos, N. (The Meteoritical Society, 2007-01-01)
      Gamma rays from radioactive byproducts of cosmic nucleosynthesis are direct messengers from nuclear processes taking place in various cosmic sites, and can be measured with telescopes operated in space. Due to low detector sensitivity, up until now, only a handful of sources have been detected in that electromagnetic window. Cobalt lines from SN1987A and 44Ti lines from the Cassiopeia A (Cas A) supernova remnant offer unique constraints on the properties of the innermost regions of core collapse supernovae. Diffuse gamma-ray lines from the decay of radioactive 26Al and the annihilation of positrons are bright enough for mapping the Milky Way in the MeV regime, and are both measured by recent spaceborne spectrometers with unprecedented precision. This constrains the sources of Al production and the state of interstellar gas in the vicinity of these sites: the total mass of 26Al produced by stellar sources throughout the Galaxy is estimated to be ~3 M(sun) per Myr, and the interstellar medium near those sources appears to be characterized by velocities of ~100 km s^(-1). Positron annihilation must occur in a modestly ionized, warm phase of the interstellar medium, but at present the major positron production site(s) remain unknown. The spatial distribution of the annihilation gamma-ray emission constrains positron production sites and positron propagation in the Galaxy. 60Fe radioactivity has been clearly detected recently; the flux ratio relative to 26Al of about 15% is on the lower side of predictions from massive star and supernova nucleosynthesis models. Those views at nuclear and astrophysical processes in and around cosmic sources by space-based gamma-ray telescopes offer invaluable information on cosmic nucleosynthesis.