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At least 1,207 records · Page 67
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NASA NTRS1984

Mapping convection in the mantle

Long-period (100-250 sec.) Love and Rayleigh waves are used to map heterogeneity and azimuthal anisotropy in the upper mantle. Spherical harmonic descriptions of anisotropy up to l = m = 3 and 2 theta are derived. Azimuthal anisotropy obtains values as high as 1.5 percent. There is good correlation of fast Rayleigh wave directions with upper mantle return flow directions derived from kinematic considerations. This is consistent with a-axis of olivine being aligned in the flow direction. The main differences between the flow models and the Rayleigh wave azimuthal variation maps occur in the vicinity of hotspots. The correlation of azimuthal anisotropy with upper mantle return flow directions, rather than with plate directions, suggests that part of the return flow is in the upper mantle and this, in turn, implies a low viscosity channel.

Tanimoto, T.↗
NASA NTRS1984

Two forsterite-bearing FUN inclusions in the Allende meteorite

Two forsterite-, fassaite-, spinel-rich inclusions in Allende which share common mineralogy and texture with three previously described inclusions are described. These inclusions were at least partially molten at temperatures over 1400 C, and their crystallization sequence was spinel, olivine, fassaite, and Mg-rich melilite. At least some of them experienced partial volatilization of MgO and SiO2 from their outer margins. At least one of the inclusions is highly enriched in MgO relative to CaO and Al2O3 compared to Allende coarse-grained inclusions, although it is just as strongly enriched in refractory trace elements as the latter, relative to C1 chondrites. Two of the objects are FUN inclusions on the basis of their oxygen, magnesium, and silicon isotopic compositions.

Clayton, R. N.↗
NASA NTRS1984

Repeated shock and thermal metamorphism of the Abernathy meteorite

Based on the example of Abernathy (L6 chondrite), it is shown how petrographic investigation can be used to unravel the nature, chronology and conditions of superposed metamorphic events in chondrites. Features considered include the texture of the rock, optical characteristics of olivine, pyroxene and plagioclase, refractive index of plagioclase, metallographical characteristics and microhardness of Fe-Ni alloys. It is deduced that Abernathy has been involved in at least six metamorphic events since the formation of the chondrite. Four distinct shock events and two separate reheating events have been identified. The chronology of these events is established. The conditions for the last four events are reasonably well constrained. These include severe reheating (T greater than 1200 C); severe shock causing complete melting of plagioclase and local melting of the rock (P between 90 and 110 GPa, T between 1250 and 1350 C); mild shock (P between 10 and 25 GPa, T less than 500 C); and reheating below 800 C.

Lambert, P.↗
NASA NTRS1984

Mineralogical-petrological characterization of near-earth asteroids

The mineralogy and petrology of 17 near-earth asteroids are characterized using reflectance spectroscopy with ground-based telescopes. Assuming the surfaces are composed of cosmically abundant materials, the presence of certain mineralogical species can be determined from diagnostic absorption features and spectral characteristics which have been studied under known laboratory conditions and understood in terms of crystal-field theory. With one possible exception, the surface composition of near-earth asteroids consists of common rock-forming minerals such as olivine, pyroxene, and phyllosilicates. Opaque components are present but cannot be mineralogically identified with existing experimental data. The spectrum of 2201 Oljato cannot be interpreted in terms of common rock-forming minerals and is examined for cometary features because its high orbital eccentricity suggests a possible relation to comets, but no common cometary features are identified. The predominance of mafic silicate absorption features in spectra of near-earth asteroids (compared to the majority of main-belt asteroids) may be a primary compositional feature or may be the signature of relatively fresher asteroid material.

Mcfadden, L. A.↗
NASA NTRS1984

The mineral chemistry and origin of inclusion matrix and meteorite matrix in the Allende CV3 chondrite

The mineralogy and mineral chemistry of the inclusion and meteorite matrices in the Allende CV3 chondrite are described, and the physical and chemical parameters of the conventional equilibrium condensation model of the origin of chondrite meteorites are evaluated. An alternative model of the origin of the mafic constituent of Allende inclusions is presented, on the basis of a new model of chondrule petrogenesis and the physical evolution of the primitive solar nebula. The model shows that the mineral chemistry of the olivine matrix in Allende CV3 seems to preserve a good record of nebular and planetary processes, including: (1) vapor-to-solid condensation under relatively oxidizing nonequilibrium conditions; (2) Fe/Mg equilibration in the meteorite parent body; and (3) recrystallization and incipient melting in the solar nebula.

Kornacki, A. S.↗
NASA NTRS1984

Transmission electron microscopy of the 'LOW-CA' hydrated interplanetary dust particle

Transmission electron microscopy of a hydrated interplanetary dust particle indicates that it consists largely of a poorly crystalline phyllosilicate containing Fe, Mg and Al with an interlayer spacing of 10 to 12 A and so is distinct from the major phyllosilicate in CI and CM carbonaceous chondrites. The silicate is probably an Fe- and Mg-rich smectite or mica. Submicron, spherical to euhedral pyrrhotite and pentlandite are prominent. Unusual, low-Ni pentlandite is also common and typically occurs as rectangular platelets. Unlike many chondritic interplanetary dust particles, olivine is rare and pyroxene was not observed. Other less abundant phases are magnetite, chromite, and an unidentified phase containing Fe, Mg, Si, Ca, and Mn. This particle differs from a hydrated micrometeorite described previously by Brownlee (1978), indicating there are mineralogically different subsets of hydrated interplanetary dust particles. Despite gross similarities in mineralogy between the particle and the carbonaceous chondrites, they show appreciable differences in detail.

Tomeoka, K.↗
NASA NTRS1984

Is phosphorus predictably incompatible in igneous processes?

Siderophile element abundances are central to recent models for core formation in the Earth and Moon and the origin of the Moon. It is important to identify siderophile elements whose behavior in igneous processes is predictable, so that primary mantle abundances can be deduced by subtracting out the effects of igneous processes. Newsom's model for core formation in the Moon requires subchondritic P, and suggests that P was depleted due to volatility. Experiments were conducted to determine P olivine/liquid distribution coefficients. Preliminary results indicate that P can be compatible with olivine during rapid cooling, but is not during isothermal crystallization with long growth times, and tends to be expelled during annealing. It is therefore not likely that P is compatible under any widespread igneous conditions, and the incompatible behavior of P in lunar crustal rocks can be safety assumed. In addition, low fO2 is insufficient to cause P compatibility, so it is unlikely that P-rich silicates formed during the early evolution of the Earth or Moon. These results indicate that P is depleted in the Moon.

Goodrich, C. A.↗
NASA NTRS1984

Major element chemical compositions of chondrules in unequilibrated chondrites

The chemical compositions (except for metals and sulfides in chondrules) of more than 500 chondrules from unequilibrated E, H, L, LL, and C chondrites were measured using a broad beam of an electron-probe microanalyzer. The compositions of chondrules can be represented by various mixtures of normative compositions of olivine, low-Ca pyroxene, plagioclase, and high-Ca pyroxene with minor amounts of spinel, feldspathoid, SiO2-minerals, etc., indicating that the chondrule precursor materials consisted of aggregates of these minerals. The Al, Na, and K contents of most chondrules reflect the compositions of the ternary feldspar (An-Ab-Kf) of the chondrule precursor materials, and chemical types of chondrules (KF, SP, IP, and CP) are defined on the basis of the atomic proportion of Al, Na, and K.

Ikeda, Y.↗
NASA NTRS1984

Fine-grained millimeter-sized objects in type 3 ordinary chondrites and their relation to chondrules and matrix

Fine-grained, opaque, silicate-rich matrix material, occurring as rims on chondrules and clasts, discrete clasts, and between chondrule and mineral fragments is a primitive component of chondrites and may resemble the material from which chondrules formed. To elucidate the relationship between chondrules and matrix and to help identify chondrule precursor material, the nimeralogy and bulk compositions of 30 mm-sized, fine-grained objects in Tieschitz (h3.6), Sharps (H3.4), Allan Hills A77299 (H3.7), Yamato 74191 (L3.6), Semarkona (LL3.0), and St. Mary's Co. (LL3.3) were studied. All are composed of submicron to micron sized material and may contain larger mineral fragments. Three texturally and mineralogically distinct types of objects were identified; all are minor components of the chondrites studied ( 1% except Sharps where melt-breccia-textured objects are approximately minus 1%). Bulk compositions of network and melt-breccia-textured objects define a rather narrow trend on a plot of Si/Al vs. Mg/Al, emphasizing a possible genetic relationship. Data show that Si/Mg is higher for network and melt-breccia-textured objects than for porphyritic and barred olivine chondrules. Melting in vaccuo of network-textured objects would cause a loss of Si relative to Mg (3), suggesting that material compositionally similar to fine grained objects could have been precursors to some chondrules.

Recca, S. I.↗
NASA NTRS1984

Chondrule rims and interchondrule matrix in UOC

Opaque rims around chondrules and clasts were distinguished from opaque, interchondrule matrix apparently unrelated spatially to chondrules and clasts. Microprobe and electron microscope techniques were used. The mean chemical composition for dark rim and matrix in Bishunpur and Tieschitz are similar to the opaque matrix of Huss, et al. However, the mean dark rim compositions in Bishunpur have significantly higher Fe, and lower Na, K, Al and Si than opaque interchondrule matrix. The opaque matrix of Huss, et al, essentially lies between these compositions. In Tieschitz only rim material was observed. In Tieschitz the rims are Si-poor and dominated by normative olivine (Fo50). Again there is an Na, K, Al component but is often nepheline normative rather than albitic. It too is probably present as glass, Ashworth (pers.comm.). In Bishunpur rims as well as the silicate-FeS, FeNi layering described by Allen, et al, discontinuous layering was observed within the silicate portion. This is apparently due to variations in the proportions of the components, particularly in the glassy phase. In Bishunpur there is a strong genetic link between matrix and rims, although rims seem to have formed under different, possibly more oxidizing, conditions. Also the presence of the same component in rims, matrix chondrules, and clasts suggests a common source.

Alexander, C.↗
NASA NTRS1985

Basalts of Mars: Constraints on Volatile Evolution

The mineralogy of relatively unweathered rocks on Mars place important constraints on models of planetary oxidation state, volcanism and associated outgassing, accretion and associated volatile inventory, and geochemical changes over time. Reflectance spectra (300 to 2500 nm) provide direct mineralogic information about the surface materials. Spectra measured using Earth-based telescopes were analyzed by a multiple high-order derivative spectroscopy technique. Nine areas, measured during the 1978 opposition, were analyzed by the technique. Each contained suites of well-defined absorption bands. These bands can be interpreted in terms of constituent mineralogy by the technique of Adams. Among the features extracted from the spectra were bands attributed to iron oxides. Other bands can be attributed to silicates within chemically unweathered rocks/fines. Absorption features suggest the presence of Mg-olivine and plagioclase feldspar.

Huguenin, R. L.↗
NASA NTRS1985

Origin of mesosiderites as a natural consequence of planet formation

The mineral composition of mesosiderites is described and a theory of the origin and evolution of these meteorites is presented. It is suggested that the asteroid parent body of the mesosiderites also formed in the inner solar system, perhaps just within the orbit of Mars. As a result of close planetary encounters, some bodies that formed near Earth or Venus were gravitationally perturbed into non-circular orbits; a few such bodies passed through the mesosiderite region at high relative velocities, colliding with and destroying a few of the native asteroids. Olivine-rich silicate mantles shattered into small pieces, but the stronger metal cores remained as large fragments. Much of the debris remained in circular orbits and accreted to the basaltic regoliths of intact native asteroids at low relative velocities. The large core fragments that collided with the crust greatly enriched restricted regions of the surface in metal. These localized regions were the mesosiderite progenitors; they accounted for only about 1% of the surface area of the parent bodies.

Wasson, J. T.↗
NASA NTRS1985

Mineralogy and chemistry of the Earth

The Earth is the prototype if not typical terrestrial planet. Ideas about the origin, evolution, structure and chemistry of the planets can be tested most thoroughly on the Earth. Similarly, the study of the other planets has generated new ideas which may be applicable to the Earth. For example the concepts of magma oceans, large polar wander, global stress fields, buoyant lithosphere, deep cumulate reservoirs, multiple tectonic styles and crust generation may also apply to the Earth, present or past. It is no longer valid to think of the mantle as an essentially homogeneous undifferentiated shell of olivine with pockes of basalt providing melts to midocean ridges and oceanic islands. It appears to be a well differentiated, outgassed body with both radial and lateral chemical variations. The lower mantle is close to chondritic in its major element chemistry. The transition region is garnet and clinopyroxene rich and may be a major basalt reservoir. This would explain the thin crust paradox. Chemical stratification of the Earth probably occurred during accretion.

Anderson, D. L.↗
NASA NTRS1985

Ultraheavy cosmic ray tracks in meteorites: A reappraisal, based on calibrations with relativistic ions

Experiments were carried out on tracks of high energy U ions in olivine, a common meteoritic mineral. The results offer an explanation for the lack of success of previous attempts to derive the Ultraheavy Cosmic Ray composition from the study of tracks in meteorites. They also suggest how such experiments should be performed. The methods tested are described and illustrated.

Perron, C.↗
NASA NTRS1985

Depth and size effects on cosmogenic nuclide production in meteorites

The galactic cosmic particle radiation (GCR) can cause changes in condensed extraterrestrial matter in different ways. It can lose energy via ionization processes of induced nuclear reactions which lead to a wide variety of stable and radioactive cosmogenic nuclides. Heavy particles incur radiation damage in minerals such as olivine and pyroxene. Light particles predominantly tend to induce nuclear reactions, causing the development of a secondary particle cascade of neutrons, protons, pions and gamma-rays and the production of cosmogenic nuclides. Such processes are described by various models, which predict the depth and size dependent production of cosmogenic nuclides.

Englert, P.↗
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 NTRS1984

Size-distributions of chondrule types in the Inman and Allan Hills A77011 L3 chondrites

The size distributions of barred-olivine (BO) and radial-pyroxene/cryptocrystalline (RPC) chondrules are investigated in a petrographic study of nine and 18 thin sections (respectively) of the L3 chondrites Inman and ALHA77011. It is found that the Inman chondrules are significantly larger than the ALHA77011 chondrules and that RPC chondrules in Inman and BO chondrules in ALHA77011 are relatively more numerous, but no significant difference in the BO and RPC size distributions is observed. A formation mechanism involving size sorting of dustball chondrule precursors by aerodynamic particle-gas interactions in a large region of the solar nebula, melting of sorted dustballs in relatively small regions, and chondrule mixing and agglomeration (from subreservoirs containing uniformly large or uniformly small chondrules of a particular compositional type) to form chondrites is proposed.

Rubin, A. E.↗
NASA NTRS1984

The Brownell and Ness County (1894) L6 chondrites - Further sorting-out of Ness County meteorites

Brownell is a new moderately shocked L6 chondrite from Ness County, KS, that is petrologically distinct from the other L6 chondrites from Ness County. These latter meteorites, Wellmanville, Franklinville, and Ness County (1894), are very similar in their olivine and low-Ca pyroxene compositional distributions, kamacite Co contents, modal abundances of metallic Fe, Ni, and presence of martensitic metallic Fe, Ni (about 14 wt pct Ni). However, their silicates indicate that they have been shocked to different extents. It is suggested that all three probably represent a single heterogeneously-shocked L6 fragmental breccia that fell over a large area over 30 km in length.

Rubin, A. E.↗