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At least 433 records · Page 24

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NASA NTRS1986

Electrical conductivity of carbonaceous chondrites and electric heating of meteorite parent bodies

The electrical conductivity of samples of the Murchison and Allende carbonaceous chondrites is 4 to 6 magnitudes greater than rock forming minerals such as Olivine up to 700 C. The remarkably high electrical conductivity of these meteorites is attributed to carbon at grain boundaries. The environment in the wake of the space station can be exploited to produce conditions which will allow pyrolysis of carbonaceous chondrites. An experimental package consisting of a one square meter shield attached to a 15 cm diameter by 40 cm long furnace and tied to a conductance bridge, furnace controller, and digital voltmeter inside the space station via umbilical cable could make the required measurements. Since heating rates as low as 0.1 C/hour are required to study kinetics of the pyrolysis reations which are the cause of the high conductivity of the carbonaceous chondrites, experimental times up to 3 months will be needed.

Duba, A.↗

NASA NTRS1985

Carbon, hydrogen and nitrogen in carbonaceous chondrites Abundances and isotopic compositions in bulk samples

Whole-rock samples of 25 carbonaceous chondrites were analyzed for contents of C, H and N and delta C-13, delta D and delta N-15. Inhomogeneous distribution of these isotopes within individual meteorites is pronounced in several cases. Few systematic intermeteorite trends were observed; N data are suggestive of isotopic inhomogeneity in the early solar system. Several chondrites revealed unusual compositions which would repay further, more detailed study. The data are also useful for classification of carbonaceous chondrites; N abundance and isotopic compositions can differentiate existing taxonomic groups with close to 100 percent reliability; Al Rais and Renazzo clearly constitute a discrete 'grouplet', and there are hints that both CI and CM groups may each be divisible into two subgroups.

Kerridge, J. F.↗

NASA NTRS1987

Analytical electron microscopy of fine-grained phases in primitive interplanetary dust particles and carbonaceous chondrites

In order to describe the total mineralogical diversity within primitive extraterrestrial materials, individual interplanetary dust particles (IDPs) collected from the stratosphere as part of the JSC Cosmic Dust Curatorial Program were analyzed using a variety of AEM techniques. Identification of over 250 individual grains within one chondritic porous (CP) IDP shows that most phases could be formed by low temperature processes and that heating of the IDP during atmospheric entry is minimal and less than 600 C. In a review of the mineralogy of IDPs, it was suggested that the occurrence of other silicates such as enstatite whiskers is consistent with the formation in an early turbulent period of the solar nebula. Experimental confirmation of fundamental chemical and physical processes in a stellar environment, such as vapor phase condensation, nucleation, and growth by annealing, is an important aspect of astrophysical models for the evolution of the Solar System. A detailed comparison of chondritic IDP and carbonaceous chondrite mineralogies shows significant differences between the types of silicate minerals as well as the predominant oxides.

Mackinnon, Ian D. R.↗

NASA NTRS1986

Composition and formation of metal nodules and veins in ordinary chondrites

Five large metal nodules, a composite sample of five small metal nodules, one troilite nodule, and two metal veins from five ordinary chondrites were analyzed by electron microprobe and neutron activation analysis. Metal nodules and veins in H chondrites generally consist of large single crystals of kamacite, whereas L nodules contain significant taenite. Most nodules and veins are depleted by large factors ranging up to 240 in refractory siderophiles (Re, Os, Ir, Pt). Tungsten (normally a refractory siderophile) and Au, As, and Ga (volatile siderophiles) have abundance ratios similar to those of the common siderophiles Fe, Co and Ni. It is proposed that the metal with extremely low refractory-element contents was produced by shock-induced vaporization of chondritic material. The refractory elements condensed near the point of vaporization and were not transported with the vapor. Because the shock-generated gas was mildly oxidizing, W formed volatile oxides.

Widom, E.↗

NASA NTRS1986

Accretion, metamorphism, and brecciation of ordinary chondrites - Evidence from petrologic studies of meteorites from Roosevelt County, New Mexico

The olivines and pyroxenes from twenty-nine ordinary chondrites from Roosevelt County, New Mexico are examined. The mineralogical properties of the chondrites studied are described. Correlations between mineral compositions and petrologic type and between petrologic type and bulk chemistry are analyzed. It is observed that mean CaO concentrations in olivine show significant variations among equilibrated chondrites, but these are not correlated with petrologic type; the degree of heterogeneity of FeO concentrations in olivines of types 4-6 is not correlated with the degree of metamorphism; and mean FeO concentrations of silicates show average increases of 3-5 percent from type 4 to type 6 in each group.

Scott, Edward R. D.↗

NASA NTRS1987

Plagioclase-rich inclusions in carbonaceous chondrite meteorites - Liquid condensates?

The characteristics and formation of coarse-grained, plagioclase-rich inclusions are investigated. The textures, mineralogical compositions, and initial Al-26/Al-27 ratios for the plagioclase-rich inclusions are described. It is observed that plagioclase-rich inclusions in carbonaceous chondrites are either Ca-Al-rich inclusions (CAIs) composed of 30-60 vol pct anorthite, and less than 35 vol pct Ti-Al-pyroxene and melilite, or CA chondrites composed of plagioclase, pyroxene, olivine, spinel, and melilite. It is observed that CA chondrules are chemically and mineralogically the most similar components shared by carbonaceous and ordinary chondrites. The textural changes observed in the inclusions are examined. The data reveal that the CAIs have three textural groups: coarse anorthite laths, equigranular anorthite and Ti-Al-pyroxene, and lacy Ti-Al-pyroxene and fine-grained anorthite.

Wark, D. A.↗

NASA NTRS1987

Analysis of laser extracted volatiles in carbonaceous chondrites

It is scientifically important to understand the composition of volatile compounds from interplanetary dust particles (IDPs) because they may be related to the primordial inventory of planetary materials which were necessary to provide environments conducive to the formation of life. The use of a laser microprobe to measure volatiles in IDPs was evaluated. Because primitive meteorites are thought to be closely related to IDPs, carbonaceous chondrites were used for the evaluation. Three sets of experiments were performed to determine the volatiles released from potential substrate materials, to analyze the volatiles released from matrices of bulk samples of carbonaceous chondrites, and to analyze volatiles released from approx. 100 to 200 micron meteorite particles to simulate IDPs. Aluminum appeared to be the best choice of substrate material. Mass ratios between carbonaceous chondrite matrices of Allende and Murchison show fair reproducibility with somewhat high uncertainties. Particles from the Orgueil, Murchison, and Allende meteorites produced measurable quantities of volatiles that appear to have mass spectra comparable to the bulk matrices.

Blanford, George E.↗

NASA NTRS1988

Nebular processes recorded in chondritic meteorites

The key fact concerning the chondritic meteorites is that they consist of grains that were present in the solar nebulae. The most primitive (least altered) of them have preserved in their chemical compositions and in their structures evidence concerning processes that occurred in the nebulae 4.5 Ga ago. The chondrites that were not metamorphosed, however, preserve much information about the earliest history of the nebula and are worthy of intense, long-term study. By studying them we can attempt to infer the times and places at which the various chondrite group formed and the conditions present at each of these places.

Wasson, John T.↗

NASA NTRS1987

Aqueous alteration in carbonaceous chondrites - Mass balance constraints on matrix mineralogy

Bulk chemical compositions of matrix material were determined in eleven Antarctic CM chondrites and five non-Antarctic CM and CI chondrites, using microprobe defocused-beam technique. The results, along with previously published data, are used to provide mass balance constraints on the relative proportions of intergrown and intermixed phyllosilicate phases in carbonaceous chondrite matrices. In terms of Fe, Si, and Mg, the CM matrix bulk compositions plot within a triangle defined by the compositions of PCP (a mixture of 25 percent tochilinite and 75 percent cronstedtite), Mg-rich serpentine, and Fe-rich serpentine. Results indicate differing amounts of PCP and serpentines in individual CM matrices. The discrepancies found between predicted and measured S and Ni values require additional sulfide phases. CI matrices were found to contain little, if any, PCP; they consist mostly of serpentine and montmorillonite.

Mcsween, Harry Y., Jr.↗

NASA NTRS1988

Chondritic meteorites and the solar nebula

Present understanding of the formation of chondritic meteorites in the solar nebula is reviewed, focusing on the nebular history of chondritic material. The stages in the history of chondritic material are summarized. The processing of chondrules and refractory inclusions is considered, and the relatedness of chondrules and refractory inclusions is discussed.

Wood, John A.↗

NASA NTRS1988

Spectral alteration effects in chondritic gas-rich breccias - Implications for S-Class and Q-Class asteroids

Reflection spectra were obtained from sawed or freshly broken surfaces of gas-rich matrix regions in a number of ordinary chondrites (Dubrovnik, Cangas de Onis, Olivenza, and Dimmitt) which are believed to represent lithified portions of asteroid regoliths that were once directly exposed to space, with the purpose of relating these chondrites to Class S asteroids. However, the observations did not detect curved red continuum characteristic for Class S asteroid spectra, suggesting that it is unlikely for any well-observed Class S asteroid to be a source of ordinary chondrites. It is suggested that the newly discovered spectral class 'Q', of which asteroid 1982 Apollo is the prototype, provides a viable alternative of a parent body.

Bell, Jeffrey F.↗

NASA NTRS1988

A new kind of primitive chondrite, Allan Hills 85085

Allan Hills (ALH) 85085, a chemically and mineralogically unique chondrite whose components have suffered little metamorphism or alteration, is discussed. It is found that ALH 85085 has 4 wt pct chondrules (mean diameter 16 microns), 36 wt pct Fe, Ni, 56 wt pct lithic and mineral silicate fragments, and 2 wt pct trolite. It is suggested that, with the exception of matrix lumps, the components of ALH 85085 formed and accreted in the solar nebula. It is shown that ALH 85085 does not belong to any of the nine chondrite groups and is very different from Kakangari. Similarities between ALH 85085 and Bencubbin and Weatherford suggest that the latter two primitive meteorites may be chondrites with high metal abundances and very large, partly fragmented chondrules.

Scott, Edward R. D.↗

NASA NTRS1988

ALH85085 - A unique volatile-poor carbonaceous chondrite with possible implications for nebular fractionation processes

The characteristics and possible origin of the chondrite ALH85085 are discussed. It is found that the metal silicate, and sulfide compositions of ALH85085 are similar to those in CM-CO and Al Rais-Renazzo chondrites. High siderophile abundances and volatile depletion are observed. It is found that ALH85085 agglomerated when temperatures in the nebula were near 1000 K, in the same region where Renazzo, Al Rais, and the CI chondrites formed. It is suggested that ALH85085 was annealed and lightly shocked after agglomeration. C1 and other clasts were subsequently incorporated during late-stage brecciation.

Grossman, Jeffrey N.↗

NASA NTRS1988

Primitive material surviving in chondrites - Matrix

A logical place to search for surviving pristine nebular material is in the fine-grained matrices of ordinary and carbonaceous chondrites of petrographic type 3. Unfortunately, many of these chondrites have experienced brecciation, thermal metamorphism, and aqueous alteration, so that interpreting individual features in terms of specific nebular conditions and/or processes is difficult. It follows that the origin and evolutionary history of such matrix phases are controversial, and a consensus is difficult to define. In this chapter, therefore, after summarizing the salient mineralogical, petrographic, chemical, and isotopic features of matrix in apparently primitive chondrites, an attempt is made to provide an overview both of areas of agreement and of topics that are currently in dispute.

Scott, E. R. D.↗

NASA NTRS1989

Nature and origin of matrix in the unique type 3 chondrite, Kakangari

The characteristics of the matrix of the Kakangari type-3 chondrite were investigated using high-resolution TEM and AEM. It was found that the textural and mineralogical characteristics of the fine-grained matrix of Kakangari are different from those of the other chondrite groups. Unlike the matrix phases present in the CV3 and CO3 chondrites, the matrix mineralogy of Kakangari contains clear information regarding its possible origin and thermal history. Data obtained are most consistent with an origin of Kakangari by annealing, without melting at high temperature (possibly during a chondrule-forming thermal event), of clusters or aggregates of ultrafine-grained amorphous or crystalline phases. The precursor materials were most likely to have been a mixture of nebula and presolar dust. Chondrules in Kakangari were probably formed from identical precursors to the matrix, but reached temperatures sufficiently high to produce melting.

Brearley, Adrian J.↗

NASA NTRS1989

Identification of solar nebula condensates in interplanetary dust particles and unequilibrated ordinary chondrites

Orthopyroxene and olivine grains, low in FeO, but containing MnO contents up to 5 wt percent were found in interplanetary dust particles (IDP) collected in the stratosphere. The majority of olivines and pyroxenes in meteorites contain less than 0.5 wt percent MnO. Orthopyroxenes and olivines high in Mn and low in FeO have only been reported from a single coarse grained chondrule rim in the Allende meteorite and from a Tieschitz matrix augite grain. The bulk MnO contents of the extraterrestrial dust particles with high MnO olivines and pyroxenes are close to CI chondrite abundances. High MnO, low FeO olivines and orthopyroxenes were also found in the matrix of Semarkona, an unequilibrated ordinary chondrite. This may indicate a related origin for minerals in extraterrestrial dust particles and in the matrix of unequilibrated ordinary chondrites.

Kloeck, W.↗

NASA NTRS1990

Matrix mineralogy of the Lance CO3 carbonaceous chondrite - A transmission electron microscope study

Results are presented on electron microprobe analyses of three CO chondrites, all of which are falls: Lance, Kainsaz, and Warrenton. The TEM mineralogy results of Lance chondrite show that Fe-rich matrix olivines have been altered to Fe-bearing serpentine and Fe(3+) oxide; matrix metal was also altered to produce Fe(3+) oxides, leaving the residual metal enriched in Ni. Olivine grains in Lance's matrix contain channels along their 100-line and 001-line directions; the formation and convergence of such channels resulted in a grain-size reduction of the olivine. A study of Kainsaz and Warrenton showed that these meteorites do not contain phyllosilicates in their matrices, although both contain Fe(3+) oxide between olivine grains. It is suggested that, prior to its alteration, Lance probably resembled Kainsaz, an unaltered CO3 chondrite.

Keller, Lindsay P.↗

NASA NTRS1990

Phyllosilicates in the Mokoia CV carbonaceous chondrite - Evidence for aqueous alteration in an oxidizing environment

Most CV chondrites contain little if any phyllosilicate mineralization. A petrographic and transmission electron microscopy study of the Mokoia CV carbonaceous chondrite shows that the matrix, chondrules, aggregates, and inclusions all contain considerable amounts of phyllosilicates. The mineralogy and occurrence of phyllosilicates in Mokoia differ from those in the CI and CM chondrites. The differences suggests that aqueous alteration of the three meteorite groups probably occurred under a variety of conditions.

Tomeoka, Kazushige↗