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At least 289 records · Page 16
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NASA NTRS1984

Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites

Ion and electron microprobes were used to examine Mg-26 excesses from Al-26 decay in four Al-rich objects from the type 3 ordinary hibonite clast in the Dhajala chondrite. The initial Al-26/Al-27 ratio was actually significantly lower than Al-rich inclusions in carbonaceous chondrites. Also, no Mg-26 excesses were found in three plagioclase-bearing chondrules that were also examined. The Mg-26 excesses in the hibonite chondrites indicated a common origin for chondrites with the excesses. The implied Al-26 content in a proposed parent body could not, however, be confirmed as a widespread heat source in the early solar system.

Hinton, R. W.↗
NASA NTRS1984

The Colony meteorite and the possible existence of a new chemical subgroup of CO3 chondrites

The Colony meteorite, found in Oklahoma around 1975, has an unrecrystallized texture and contains heterogeneous olivine and low Ca pyroxene, kamacite with low Ni and Co and high Cr, amoeboid inclusions with low FeO and MnO, and numerous small chondrules with clear pink glass. These characteristics are shared by members of the least metamorphosed subgroup of CO3 chondrites. Colony contains a fine grained matrix that has higher FeO and K2O and lower MgO and Na2O than normal CO3 matrices. Allan Hills A77307 is another unmetamorphosed meteorite that has many petrologic similarities to normal CO chondrites, including matrix abundance, mineral compositions and chondrule size. However, it differs from them in its abundance of magnetite and presence of iron carbides. The olivine and low Ca pyroxene compositional distributions of Colony and A77307 are very similar. The shapes of the thermoluminescence glow curves of Colony and A77307 are very similar, but differ significantly from those of normal CO chondrites. It is suggested that Colony and A77307 represent a distinct chemical subgroup of CO3 chondrites, characterized by low Ni, Co, S, Ca, Mg, Mn and, possibly, high Cd.

Rubin, A. E.↗
NASA NTRS1984

Coarse-grained chondrule rims in type 3 chondrites

Coarse-grained rims composed of olivine, with or without low-Ca pyroxene, occur around all types of chondrules and compound chondrules in type 3 carbonaceous chondrites and around ordinary chondrites. The dark zoned chondrules and coarse grained rims were formed by the heating of clumps of opaque matrix material to subsolidus-subliquidus temperatures in the solar nebula. The most likely source of the heat that formed the coarse grained rims is the mechanism responsible for chondrule formation. CV chondrites may have formed in a region where the chondrule formation mechanism was less efficient, probably at greater solar distances than the ordinary chondrites. Alternatively, CV chondrules may have suffered fewer particle collisions prior to agglomeration.

Rubin, A. E.↗
NASA NTRS1985

The Colony meteorite and variations in CO3 chondrite properties

The Colony meteorite is one of the least equilibrated CO3 chondrites, yet differs from normal CO chondrites in that, while Al, Sc, V, Cr, Ir, Fe, Au, and Ga abundances are consistent with a CO chondrite classification, certain lithophile, siderophile, and chalcophile contents are depleted by factors of 10-40 percent. Colony is badly weathered, and its Fe, Ni abundance of about 19 wt pct is similar to that of the Kainsaz CO3 unweathered fall but higher than all other CO3 chondrites.

Rubin, A. E.↗
NASA NTRS1985

Impact melt products of chondritic material

Experimental data concerning impact melting processes in chondritic material are reviewed. It is shown that a large variety of objects in chondritic meteorites could have formed as a result of impact melting, including: shock veins; metal-troilite mixtures; metal and sulfide nodules; melt pockets and vugs. The type of object produced in an impact melt is related to the interaction of the shock waves with the particular target rock. It is suggested that various iron meteorites (including groups IAB, IIICD, and IIE, as well as several ungrouped irons) were formed from individual melt pools in chondritic regoliths. The small-scale structure of impact-melted metallic Fe,Ni and troilite in Weston (H chondrite regolith breccia) is illustrated in a photograph.

Rubin, A. E.↗
NASA NTRS1986

Enstatite chondrites and enstatite achondrites (aubrites) were not derived from the same parent body

Enstatite achondrites (aubrites) were not derived from known enstatite chondrites by melting and fractionation on one and the same parent body, for these and other reasons: (1) There is no satisfactory mechanism for fractionating metal plus troilite in enstatite chondrites to form these phases in different proportions and with different Ti contents in aubrites. (2) Many enstatite chondrites and aubrites are regolith or fragmental breccias, but clasts of one within the other have not been found. (3) Cosmic ray exposure ages of the two groups are difficult to explain if they are from the same parent body, but are easy to explain if they are from different parent bodies. Siderophile element abundances in metal from the Mt. Egerton meteorite, which consists of enstatite and metallic Fe, Ni, preclude it from being a complementary differentiate of the aubrites. Rather, it appears that Mt. Egerton was formed from the same source material as enstatite chondrites, but the components were mixed in different proportions.

Brett, Robin↗
NASA NTRS1987

Ubiquitous high-FeO silicates in enstatite chondrites

SEM and EMPA were used to determine the mineral contents of four EH3 chondrites. All four showed the dominant enstatite peak, Fs 0-5, with 4-8 percent of FeO-rich pyroxene with Fs 5-20. Among the 542 objects found to contain high-FeO silicates, 18 were chondrules, 381 were rimmed or unrimmed grains, and 143 were aggregates. The high-FeO silicates in these objects are very largely pyroxene with Fs 5-23. Large grains of both FeO-rich and FeO-poor silicates were found to be present in the FeO-rich chondrules. This fact, together with the absence of clasts of FeO-rich chondritic material in the EH3 chondrites, suggests that FeO-rich grains were introduced before or during chondrule formation. It is concluded that FeO-rich and FeO-poor silicates were both present in the nebular region where E chondrites originated.

Lusby, David↗
NASA NTRS1988

I-Xe systematics in LL chondrites

A stepwise heating analysis of Ar and Xe data from five neutron-irradiated whole rock LL chondrites (Soko Banja, Alta Ameen, Tuxtuac, Guidder, and Olivenza) is presented, emphasizing the complicated thermal history of ordinary chondrites. None of the present meteorites show a well-defined (Ar-40)-(Ar-39) apparent age plateau comprised of more than two release fractions. Most of the samples are found to yield well-defined high-temperature correlations between Xe-129/Xe-130 and Xe-128/Xe-130, and thus determinations of I-129/I-127 and Xe-129/Xe-130 at the time of isotopic closure for Xe. As in the case of other ordinary chondrites, the I-Xe systematics for LL chondrites correlate neither with a metamorphic grade nor with chronologies based opon other methods.

Bernatowicz, T. J.↗
NASA NTRS1988

Cumberland Falls chondritic inclusions. III - Consortium study of relationship to inclusions in Allan Hills 78113 aubrite

The contents of Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U, and Zn in large chondritic clasts from the Cumbersand Falls aubrite were determined by radiochemical neutron activation analysis, and the results, together with the results of a mineralogical investigation, were compared with respective data obtained for three primitive inclusions from the ALH A78113 aubrite. The results indicated that the clasts from both aubrite sources constitute a single chondritic suite. The analyses data, together with the results of thermoluminescence data for Cumberland Falls chondritic inclusions and achondritic host, indicate that inclusions in Cumberland Falls and in ALH A78113 aubrite represent a primitive chondrite sample suite whose properties were established during primary nebular accretion and condensation over a broad redox range.

Lipschutz, Michael E.↗
NASA NTRS1989

Ultrafine-grained mineralogy and matrix chemistry of olivine-rich chondritic interplanetary dust particles

Olivine-rich chondritic interplanetary dust particles (IDPs) are an important subset of fluffy chondritic IDPs collected in the earth's stratosphere. Particles in this subset are characterized by a matrix of nonporous, ultrafine-grained granular units. Euhedral single crystals, crystals fragments, and platey single crystals occur dispersed in the matrix. Analytical electron microscopy of granular units reveals predominant magnesium-rich olivines and FeNi-sulfides embedded in amorphous carbonaceous matrix material. The variable ratio of ultrafine-grained minerals vs. carbonaceous matrix material in granular units support variable C/Si ratios, and some fraction of sulfur is associated with carbonaceous matrix material. The high Mg/(Mg+Fe) ratios in granular units is similar to this distribution in P/Comet Halley dust. The chondritic composition of fine-grained, polycrystalline IDPs gradually breaks down into nonchondritic, and ultimately, single mineral compositions as a function of decreased particle mass. The relationship between particle mass and composition in the matrix of olivine-rich chondritic IDPs is comparable with the relationship inferred for P/Comet Halley dust.

Rietmeijer, F. J. M.↗
NASA NTRS1989

The Tsarev meteorite - Petrology and bidirectional reflectance spectra of a shock-blackened L chondrite

The Tsarev meteorite is a highly shocked black L5 ordinary chondrite. Meteorites of this class may have been subjected to regolith processes on or near the surface of ordinary chondrite bodies. The study of their optical alteration can provide valuable support in the spectral search for ordinary chondrite parent bodies. Although Tsarev is very dark in a cut-surface hand sample, it is characterized by subtle variations in apparent darkness, called in this work grey and black areas. Both areas are substantially optically altered from normal ordinary chondrites and are characterized by a much lower albedo and strongly suppressed absorption features. Although both areas are altered, they show differences in albedo and the strength of absorption features. Particulate samples from the black area have a slightly higher albedo and stronger absorption features, while the particulate samples from the grey area show a systematic suppression of spectral features and albedo. Chemical, mineralogical, and spectral analyses suggest that the spectral differences can be the result of one or a combination of several factors, including shock effects on the crystallographic structure of the minerals and differences in the size, distribution, and amount of metal and troilite grains.

Britt, D. T.↗
NASA NTRS1989

The early solar system abundance of Pu-244 as inferred from the St. Severin chondrite

The isotopic composition of Xe released in stepwise heating of neutron-irradiated samples of the St. Severin chondrite was measured. This analysis shows that at the time of formation of most chondritic meteorites, approximately 4.56 x 10 to the 9th yr ago, the atomic ratio of Pu-244/U-238 was 0.0068 + or - 0.0010 in chondritic meteorites. This value is believed to be more reliable that inferred from earlier analyses of St. Severin and is the best estimate for the early solar system abundance of Pu-244. The comparison of Xe compositions between irradiated and unirradiated samples shows that the composition of trapped (ambient) Xe in St. Severin has a significantly lower value of Xe-136/Xe-130 than average carbonaceous chondrites.

Hudson, G. B.↗
NASA NTRS1989

Tin in a chondritic interplanetary dust particle

Submicron platey Sn-rich grains are present in chondritic porous interplanetary dust particle (IDP) W7029 A and it is the second occurrence of a tin mineral in a stratospheric micrometeorite. Selected Area Electron Diffraction data for the Sn-rich grains match with Sn2O3 and Sn3O4. The oxide(s) may have formed in the solar nebula when tin metal catalytically supported reduction of CO or during flash heating on atmospheric entry of the IDP. The presence of tin is consistent with enrichments for other volatile trace elements in chondritic IDPs and may signal an emerging trend toward nonchondritic volatile element abundances in chondritic IDPs. The observation confirms small-scale mineralogical heterogeneity in fine-grained chondritic porous interplanetary dust.

Rietmeijer, Frans J. M.↗
NASA NTRS1989

Evolution of carbonaceous chondrite parent bodies: Insights into cometary nuclei

It is thought that cometary samples will comprise the most primitive materials that are able to be sampled. Although parent body alteration of such samples would not necessarily detract from scientists' interest in them, the possibility exists that modification processes may have affected cometary nuclei. Inferences about the kinds of modifications that might be encountered can be drawn from data on the evolution of carbonaceous chondrite parent bodies. Observations suggest that, of all the classes of chondrites, these meteorites are most applicable to the study of comets. If the proportion of possible internal heat sources such as Al-26 in cometary materials are similar to those in chondrites, and if the time scale of comet accretion was fast enough to permit incorporation of live radionuclides, comets might have had early thermal histories somewhat like those of carbonaceous chondrite parent bodies.

Mcsween, Harry Y., Jr.↗
NASA NTRS1989

Pyroxene thermobarometry in LL-group chondrites and implications for parent body metamorphism

Geothermometry based on the compositions of clinopyroxenes in type 6 and 7 LL chondrites gives coherent results, but the estimated temperatures from coexisting orthopyroxenes are consistently lower than for clinopyroxenes. Orthopyroxene thermometry is suspect because of compositional effects of polymorphic inversions and/or unknown kinetic factors. Lack of clinopyroxene equilibration precludes accurate estimation of peak metamorphic temperatures for type 4 and 5 chondrites. There is no apparent correlation between Al content (a pressure-dependent variable) and equilibration temperature in chondritic pyroxenes. This finding, which is at variance with a previously published conclusion that temperature and pressure are correlated in metamorphosed chondrites, may have important implications for asteroid thermal models.

Mcsween, Harry Y., Jr.↗
NASA NTRS1990

Carbon-rich aggregates in type 3 ordinary chondrites - Characterization, origins, and thermal history

Carbon-rich aggregates from three type 3.4-3.6 ordinary chondrites and two chondritic clasts have been characterized in detail, using TEM techniques. The aggregates in all the meteorites studied range in size from 5-1000 microns and consist of a fine scale intergrowth of poorly graphitized carbon, amorphous carbon, Fe,Ni metal, and minor chromite. Contrary to previous reports, well-crystallized graphite and magnetite are absent. The association of Fe,Ni metal and carbonaceous material suggests that the original carbonaceous material may have formed by Fischer-Tropsch (FT) type reactions at low temperatures (less than 400 K), possibly in the solar nebula. This carbonaceous material probably consisted of a complex mixture of hydrocarbons, kerogen-like material, and other complex organic molecules. The aggregates were subsequently accreted onto the ordinary chondrite parent bodies and underwent planetary thermal processing which resulted in the catalytic graphitization of hydrocarbons, in the presence of Fe,Ni metal, to produce poorly graphitized carbon. None of the meteorites studied experienced temperatures sufficiently high to produce crystalline, ordered graphite. Using the empirical geothermometer of Rietmeijer and Mackinnon (1985), the measured d(002) spacings of poorly graphitized carbon show that graphitization occurred at temperatures between 300 and 450 C. This range of temperatures is significantly lower than the generally quoted metamorphic temperatures for type 3.4-3.6 ordinary chondrites (about 450-500 C).

Brearley, Adrian J.↗
NASA NTRS1990

Titanium oxide Magneli phases in four chondritic porous interplanetary dust particles

Detailed analytical electron microscope analyses of four fine-grained chondritic porous interplanetary dust particles (IDPs) reveal the presence of titanium oxide Magneli phases, Ti(n)O(2n-1) (n = 4, 5, 6), and rare Ti-metal. The association of Magneli phases, Ti-metal, and carbonaceous material in chondritic IDPs, along with the grain size distributions of Ti-metal and Magneli phases and equilibrium dissociation pressures for these oxygen-deficient Ti-oxides, support in situ solid carbon gasification in these extraterrestrial particles. The active catalyst in this process is titanium metal that may be of interstellar origin. A model is proposed that explains the higher solid carbon content of chondritic IDPs relative to bulk carbon abundances typical for carbonaceous chondrite matrices that represent another type of more metamorphosed undifferentiated solar system bodies.

Rietmeijer, E. J. M.↗
NASA NTRS1990

Origin and history of chondrite regolith, fragmental and impact-melt breccias from Spain

Six ordinary chondrite breccias from the Museo Nacional de Ciencias Naturales, Madrid (Spain), are described and classified as follows: the solar gas-rich regolith breccia Oviedo (H5); the premetamorphic fragmental breccias Cabezo de Mayo (type 6, L-LL), and Sevilla (LL4); the fragmental breccias Canellas (H4) and Gerona (H5); and the impact melt breccia, Madrid (L6). It is confirmed that chondrites with typical light-dark structures and petrographic properties typical of regolith breccias may (Oviedo) or may not (Canellas) be solar gas-rich. Cabezo de Mayo and Sevilla show convincing evidence that they were assembled prior to peak metamorphism and were equilibrated during subsequent reheating. Compositions of olivine and low-Ca pyroxene in host chondrite and breccia clasts in Cabezo de Mayo are transitional between groups L and LL. It is suggested, based on mineralogic and oxygen isotopic compositions of host and clasts, that the rock formed on the L parent body by mixing, prior to peak metamorphism. This was followed by partial equilibrium of two different materials: the indigenous L chondrite host and exotic LL melt rock clasts.

Casanova, I.↗