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At least 289 records · Page 16

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NASA NTRS1981

The interstellar dust as a precursor of Ca,Al-rich inclusions in carbonaceous chondrites

Multiple dust components are considered as a source of the observed anomalies in meteoritic oxygen isotope compositions. It is pointed out that these anomalies are not due to an incomplete mixing of several dust or gas-plus-dust components in the solar nebula. If they were, other elements would display similar anomalies. The anomalies must therefore stem from differing degrees of incomplete exchange of oxygen isotopes between the primordial gas and dust components of the nebula. The dust is more likely to have been the component which was enriched with 0-16. Since the isotopic difference between dust and gas probably could not have been preserved if the dust was ever completely vaporized in the nebula, the Ca,Al-rich inclusions in carbonaceous chondrites are unlikely to be condensates, but instead are distillation residues.

Wood, J. A.↗

NASA NTRS1980

Refractory metal particles in refractory inclusions in the Allende meteorite

SEM and X-ray analysis were used to study refractory metal particles in five calcium-aluminum-rich inclusions in the Allende meteorite, and a complex variety of compositions and large departures from equilibrium were found. It is suggested that these particles could have been primordial condensates which were isolated from the nebula and from each other at different times by cocondensing oxides. Selective diffusion and/or oxidation of the more oxidizable metals (Mo, W, Fe, and Ni), phase segregations into different alloy phases (fcc, bcc, hcp, and, possibly, ordered phases), and the formation of metastable condensates could have been involved in the genesis of these materials

Fuchs, L. H.↗

NASA NTRS1982

Chemical composition of HAL, an isotopically-unusual Allende inclusion

Samples of hibonite, black rim, and portions of friable rim from an unusual Allende inclusion, named HAL, were analyzed by INAA and RNAA for 37 major, minor, and trace elements. An unusually low amount of Ce was found in HAL, although it otherwise was highly enriched in REE compared to C1 chondrites. HAL is also depleted in Sr, Ba, U, V, Ru, Os, and Ir relative to other refractory elements. It is concluded that the distribution of REE between hibonite and rims was established when hibonite and other refractory minerals were removed at slightly different temperatures from a hot, oxidizing gas in which they previously coexisted as separate grains. Possible locations for the chemical and mass dependent isotopic fractionation are considered to be in ejecta from the low temperature helium-burning zone of a supernova and in the locally oxidizing environment generated by evaporation of interstellar grains of near-chondritic chemical composition.

Davis, A. M.↗

NASA NTRS1984

The microstructure of minerals in coarse-grained Ca-Al-rich inclusions from the Allende meteorite

Transmission electron microscopy (TEM) is used to investigate the microstructure and microchemistry of minerals in Ca-Al-rich, coarse-grained inclusions (CAI) from the meteorite. The spinels contain only low to moderate dislocation densities and are characterized by a ubiquitous, fine black spotty texture that is thought to originate from a slightly nonstoichiometric composition. Whereas the Ti-Al-pyroxenes are relatively devoid of features, they contain veins of secondary phases apparently deposited in unhealed cracks. Chromite is identified in the veins, indicating transport of oxidized iron during alteration. The melilites show the greatest variety of microstructures and are the most heavily altered phase in CAI. High dislocation densities are common and the crystals exhibit considerable internal strain, suggesting that they have not been annealed.

Barber, D. J.↗

NASA NTRS1985

Inclusive angular distribution of alpha and Li fragments produced in the Fe-C and Fe-Pb collisions at 1.88 GeV/u

The LS (laboratory system) emission angles theta for 2188 and 298 Li fragments, produced inclusively in relativistic Fe-C and Fe-Pb collisions, have been measured in reference to incident Fe-ion beam tracks nearby in nuclear emulsion. An empirical differential frequency formula, dN(cot theta) = exp (a + b cot theta)d(cot theta) is obtained with the constant b approx. = -0.026 at 1.88 GeV/u, which seems to be independent on the kinds of target nucleus as well as on the kinds of projectile fragments.

Kim, C. O.↗

NASA NTRS1985

Inclusion of exact exchange in the noniterative partial-differential-equation method of electron-molecule scattering - Application to e-N2

The noniterative partial-differential-equation (PDE) approach to electron-molecule scattering of Onda and Temkin (1983) is modified to account for the effects of exchange explicitly. The exchange equation is reduced to a set of inhomogeneous equations containing no integral terms and solved noniteratively in a difference form; a method for propagating the solution to large values of r is described; the changes in the polarization potential of the original PDE method required by the inclusion of exact static exchange are indicated; and the results of computations for e-N2 scattering in the fixed-nuclei approximation are presented in tables and graphs and compared with previous calculations and experimental data. Better agreement is obtained using the modified PDE method.

Weatherford, C. A.↗

NASA NTRS1985

A refractory inclusion in the Kaba CV3 chondrite - Some implications for the origin of spinel-rich objects in chondrites

The first detailed petrographic and mineralogical study of a Ca, Al-rich inclusion (CAI) from the Kaba CV3 chondrite is reported. This 'fine-grained' CAI contains abundant small, rounded, rimmed, spinel-rich objects which have important features in common with the spinel-rich objects in other carbonaceous and ordinary chondrites. These nodules are interpreted as fractionated distillation residues of primitive dust. However, the available data do not unambiguously rule out a condensation origin for at least some of these objects. Finally, the preservation of distinct diopside-hedenbergite rims on the spinel-rich bodies and the small grain size of many minerals in the CAI matrix material both suggest that the CAI accreted cool and had a relatively cool thermal history in the Kaba parent body.

Fegley, B., Jr.↗

NASA NTRS1987

Carbon in dark inclusions of the Allende meteorite

Carbon contents of three dark inclusions (DI's) of the Allende meteorite, measured chemically, range from 0.56 to 1.17 wt pct. When one includes the data reported by Bunch, Chang, and Ott (1980), the lower limit is 0.44 pct. The C-concentration map of a 1.6 x 1.6 sq mm area straddling the boundary of a DI and matrix, or of a DI and its dark halo, obtained with a C-12(d,p)C-13 nuclear microprobe, shows that the C-content of the core of the DI is very uniform, and that the C-content of the rim is 2.9 + or - 0.3 times larger. Variability of the C-content of matrix and matrix-like areas of Allende appears to be the rule. DIs cannot be a reworked bulk Allende, or a precursor for bulk Allende without the addition or, respectively, removal of significant amounts of carbon. However, some type 1 DIs might be reworked Allende matrix or precursor matter for that matrix.

Heymann, D.↗

NASA NTRS1987

Trace element and petrologic clues to the formation of forsterite-bearing Ca-Al-rich inclusions in the Allende meteorite

New trace element and petrographic data are presented for the TE, 818a, 110-A forsterite-bearing Ca-Al-rich inclusions (CAIs) from the Allende meteorite; these form a continuum with type B1 and B2 CAIs, and are here accordingly designated 'type B3'. Evidence is adduced to the effect that 818a was strongly reheated and modified in the nebula after its initial crystallization. A three-stage formation process is required for 818a, involving crystallization of the primary CAI, reheating and partial volatilization of Mg and Si from the outer portion of the CAI, and a metasomatic alteration of the melilite-rich mantle.

Wark, D. A.↗

NASA NTRS1988

'Domestic' origin of opaque assemblages in refractory inclusions in meteorites

Experimental studies indicate that opaque assemblages rich in refractory siderophile elements were formed within host calcium- and aluminum-rich inclusions (CAIs) by exsolution, oxidation and sulphidization of homogeneous alloys, rather than by aggregation of materials in the solar nebula before the formation of CAIs. These opaque assemblages are thus not the oldest known solid materials, as was once thought, and they do not constrain processes in the early solar nebula before CAI formation. Instead, the assemblages record the changing oxygen fugacity experienced by CAIs during slow cooling in nebular and/or planetary environments.

Blum, Joel D.↗

NASA NTRS1988

Iodine-xenon studies of Allende inclusions - Eggs and the Pink Angel

The I-Xe systems of six Allende inclusions (five Eggs and the Pink Angel) appear to have been altered by nonnebular secondary processes. Evidence for this includes temperature-ordered variations in the initial I isotopic composition within several objects (with older apparent I-Xe ages associated with higher extraction temperatures) and the absence of primitive I-Xe ages. The span of apparent ages seen in Allende objects (10 Myr or more) is probably too long to reflect any nebular process, so at least some alteration probably occurred ont the parent body. The range in initial (Pu-244)/(U-238) ratios for the Eggs (0.003-0.014) includes the current best estimates of the bulk solar system value (0.004-0.007). For Egg 3, the Pu/U ratio varies by a factor of two between extractions, probably the result of fractionation of Pu from U among different phases.

Swindle, T. D.↗

NASA NTRS1990

Processing of refractory meteorite inclusions (CAIs) in parent-body atmospheres

Ca-Al-rich inclusions (CAIs) in refractory meteorites are shown to have been subject to partial melting during a suitably high gas density/small scale height regime arising during gasdynamic deceleration in a temporary atmosphere around an accreting parent body. The presence of dust in such an atmosphere would have increased the pressure gradient with height, lowering the boiloff rate, and permitting dust particles to become trapped in the partially melted material. CAIs may therefore be studied as probes of a primitive atmosphere.

Podolak, Morris↗

NASA NTRS1990

Crystal chemical effects on the partitioning of trace elements between mineral and melt - An experimental study of melilite with applications to refractory inclusions from carbonaceous chondrites

Results are presented on the partitioning experiments for trace elements Be, Sc, Ba, La, Ce, and Tm between melilite and melt for a bulk composition relevant to Type B inclusions, using a technique, described by Brody and Flemings (1966), by which distributon coeffecients can be obtained over the entire range of crystallization from trace element analysis of a single zoned crystal. Zoned crystals of melilite were grown during controlled cooling rate experiments, and trace element concentrations were determined by SIMS. At each step in the crystal growth, trace element concentrations in the coexisting melt were determined by mass balance, using stepwise integration of a Rayleigh fractionation equation.

Beckett, John R.↗

NASA NTRS1990

Chemical compositions of siderophile element-rich opaque assemblages in an Allende inclusion

Chemical compositions of ten opaque assemblages, or Fremdlinge, from an Allende Type B Ca-,Al-rich coarse-grained inclusion were determined. Attempts to model the abundances of refractory siderophiles assuming condensation from the solar nebula into a single phase failed to match the observed combination of subchondritic Re/Os and Ir/Pt ratios. However, virtually all refractory siderophile fractionations in these Fremdlinge could be matched by a different model, in which all metals condensed into three separate alloys according to their crystal structures.

Sylvester, Paul J.↗

NASA NTRS1992

On the inclusion of the hydrogen dimer in the analysis of Voyager IRIS spectra

Empirical formulas are fitted to existing theoretical absorption spectra of H2-H2 pairs in the far-infrared allowing the inclusion of dimer absorption, parameterized with the height dependence of the para-hydrogen profile, in the calculations. Comparison between synthetic and Voyager IRIS spectra shows that once the dimer absorption is included it is now possible to reproduce the hydrogen portion of the IRIS spectrum to within the precision of the measurements.

Carlson, Barbara E.↗

NASA NTRS1993

Tomographic Imaging Of Low-Density Inclusions

Computed neutron tomography reveals structure of volume containing material of lesser density surrounded by material of greater density. Technique used, for example, to examine and measure flaws or other inclusions containing equally dense or less-dense material hidden in complicated structure made of denser material.

Roy, Jagatjit↗

NASA NTRS1992

Chemical and isotopic constraints on the formation and crystallization of SA-1, a basaltic Allende plagioclase-olivine inclusion

Results are presented of a detailed petrologic, chemical, and Mg-isotope study of SA-1, a basaltic Allende plagioclase-olivine inclusion. The chemical and isotopic data were used to identify the signatures of nebular vs. planetary processes and to determine if SA-1 contains a component produced by planetary differentiation. It was found that the results of major-element and trace-element abundances in SA-1 are inconsistent with its formation as a mixture of nebular materials. It is suggested that SA-1 contains a chemically fractionated component produced by igneous differentiation.

Kennedy, Allen K.↗

NASA NTRS1991

The inclusion of L sup p(mu) in L sup q(nu)

A theorem (the so-called main theorem) for the validity of the inclusion L sup p(mu) in L sup q(nu) is given, and proved by an application of the closed graph theorem. The main theorem is then applied to some interesting special cases, and concrete results are obtained including different versions of the results of Subramanian (1978) and Romero (1983).

Miamee, A. G.↗