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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Chemistry and petrology of the Apennine Front, Apollo 15. I - KREEP basalts and plutonic rocks. II - Impact melt rocks

The mineralogy, petrology, and chemistry of rock fragments for the Apennine Front coarse fines (10-4 and 4-2 mm) have been determined. The data are consistent with a single eruptive event that produced several flows. It is found that most of the plutonic rocks are ferroan in nature, with a few belonging to the Mg-suite. The mineral and bulk chemistry of KREEP basalts and the composition of ferroan anorthosites are discussed. Petrographic studies of 21 impact melts are also presented, showing a variety of textures. It is found that the Apollo 15 impact melts are mixtures of low-K Fra Mauro, KREEP, and plutonic components. The Ni/Ir ratios of the melt rocks are shown to be greater than chondritic values, indicating ancient and/or iron meteorite components.

Simon, S. B.↗
NASA NTRS1988

Metal with anomalously low Ni and Ge concentrations in the Allan Hills A77081 winonaite

The Ge content of metal in the Allan Hills A77081 winonaite was determined by high-sensitivity electron microprobe analysis. By optimizing analytical conditions for Ge determination, a detection limit of about 75 ppm could be achieved. In A77081 some small kamacite grains contain less Ni and Ge and more Co than coarse-grained metal. These small grains are always associated with sulfide, raising the possibility that anomalous metal is related to eutectic melting. However, when published partition coefficients for Ni and Ge in the Fe-Ni-S system are used to model fractionation of these elements during eutectic melting, one finds that secondary metal should be enriched in Ni and depleted in Ge. Thus, the positive Ni-Ge correlation found in this study is the opposite of the expected trend. No explanation for this discrepancy has yet been found. Nonetheless, the existence of anomalous metal is an indication that A77081, and probably other winonaites as well, have undergone some fractionation. This supports the notion that the high-temperature history of winonaites is related to the formation of IAB iron meteorites, whose silicate inclusions are very similar to winonaites.

Kracher, Alfred↗
NASA NTRS1990

Kamacite and olivine in ordinary chondrites - Intergroup and intragroup relationships

Results are presented from high-precision electron microprobe analyses of olivine and kamacite in a suite of 134 ordinary chondrites (OCs). The compositional ranges of these phases are defined for each OC group (high total Fe, low total Fe, and low total Fe/low metallic Fe). Anomalous OCs that have olivine and/or kamacite compositions that lie outside the established ranges are identified. The phases in the chondritic clasts of the Netschaevo iron meteorite are characterized to determine the relationship between Netschaevo and OCs. Intragroup variations of olivine and kamacite compositions with petrologic type are examined and OCs that contain olivine and/or kamacite grains with aberrant compositions are identified as fragmental breccias. Also, a search for new metallic Fe-Ni phases with extreme compositions is conducted. As a result of these analyses, several meteorites are reclassified.

Rubin, Alan E.↗
NASA NTRS1990

Metal silicate mixtures - Spectral properties and applications to asteroid taxonomy

The reflectance spectra of combinations of olivine, orthopyroxene, and iron meteorite metal are experimentally studied, and the obtained variations in spectral properties are used to constrain the physical and chemical properties of the assemblages. The presence of metal most noticeably affects band area ratios, peak-to-peak and peak-to-minimum reflectance ratios, and band widths. Band width and band areas are useful for determining metal abundance in olivine and metal and orthopyroxene and metal assemblages, respectively. Mafic silicate grain size variations are best determined using band depth criteria. Band centers are most useful for determining mafic silicate composition. An application of these parameters to the S-class asteroid Flora is presented.

Cloutis, Edward A.↗
NASA NTRS1991

Asteroid 1986 DA - Radar evidence for a metallic composition

Echoes from the near-earth object 1986 DA show it to be significantly more reflective than other radar-detected asteroids. This result supports the hypothesis that 1986 DA is a piece of NiFe metal derived from the interior of a much larger object that melted, differentiated, cooled, and subsequently was disrupted in a catastrophic collision. This 2-kilometer asteroid, which appears smooth at centimeter to meter scales but extremely irregular at 10- to 100-meter scales, might be (or have been a part of) the parent body of some iron meteorites.

Ostro, S. J.↗
NASA NTRS1991

First spacecraft encounter with an asteroid approaches

During the course of the Galileo spacecraft's journey to Jupiter it will make two excursions through the steroid belt situated between Mars and Jupiter. The first excursion involves an encounter with the asteroid 951 Gaspra, which will take place on October 29, 1991. Gaspra is a small (about 15 km diameter) asteroid near the outer edge of the main asteroid belt. It's spectral classification is S, suggesting a composition similar to those of stony-iron meteorites. A figure is given showing the brightness of this asteroid as a function of time.

Tholen, David J.↗
NASA NTRS1991

Cosmogenic Ar-36 from neutron capture by Cl-35 in the Chico L6 chondrite: Additional evidence for large shielding

The cosmic ray produced Ar-36/Ar-38 ratio measured in iron meteorites is about 0.65, but is not well determined for stone meteorites due to the common presence of trapped Ar or absorbed atmospheric Ar in bulk analysis. Almost all single-extraction measurements of stones give Ar-36/Ar-38 ratios intermediate between the trapped and air values of 5.3 and the expected cosmogenic value of about 0.65. The isotopic composition of Ar was measured for stepwise temperature release of both chondritic and melt portions of Chico. The Chico data suggest that for large chondrites, the cosmogenic Ar-36/Ar-38 ratio may well be higher than 0.65, and therefore the procedure of correcting bulk analysis results may underestimate the concentration of cosmogenic Ar-38. In this context we note that in analysis of many Antarctic chondrites observed that determined amounts of cosmogenic Ar-38 averaged about 13 percent too low in comparison to that expected from measurements of other cosmogenic species.

Garrison, D. H.↗
NASA NTRS1992

Mesosiderite clasts with the most extreme positive europium anomalies among solar system rocks

Pigeonite-plagioclase gabbros that occur as clasts in mesosiderites (brecciated stony-iron meteorites) show extreme fractionations of the rare-earth elements (REEs) with larger positive europium anomalies than any previously known for igneous rocks from the earth, moon, or meteorite parent bodies and greater depletions of light REEs relative to heavy REEs than known for comparable cumulate gabbros. The REE pattern for merrillite in one of these clasts is depleted in light REEs and has a large positive europium anomaly as a result of metamorphic equilibration with the silicates. The extreme REE ratios exhibited by the mesosiderite clasts demonstrate that multistage igneous processes must have occurred on some asteroids in the early solar system. Melting of the crust by large-scale impacts or electrical induction from an early T-Tauri-phase sun may be responsible for these processes.

Mittlefehldt, David W.↗
NASA NTRS1992

Processing of metal and oxygen from lunar deposits

On the moon, some whole rocks may be ores for abundant elements, such as oxygen, but beneficiation will be important if metallic elements are sought from raw lunar dirt. In the extraction process, a beneficiated metallic ore, such as an oxide, sulfide, carbonate, or silicate mineral, is converted to reduced metal. A variety of plausible processing technologies, which includes recovery of meteoritic iron, and processing of lunar ilmenite, are described in this report.

Acton, Constance F.↗
NASA NTRS1992

Asteroid core crystallization by inward dendritic growth

The physics of the asteroid core crystallization process in metallic asteroids is investigated, with special attention given to the initial conditions for core crystallization, the manner of crystallization, the mechanisms acting in the stirring of the liquid, and the effects of elements such as sulfur on crystallization of Fe-Ni. On the basis of theoretical considerations and the published data on iron meteorites, it is suggested that the mode of crystallization in asteroid core was different from the apparent outward concentric crystallization of the earth core, in that the crystallization of asteroidal cores commenced at the base of the mantle and proceeded inward. The inward crystallization resulted in complex dendritic growth. These dendrites may have grown to lengths of hundreds of meters or perhaps even as large as the core radius, thereby dividing the core into separate magma chambers.

Haack, Henning↗
NASA NTRS1993

Explosive volcanism and the compositions of cores of differentiated asteroids

Eleven iron meteorite groups show correlations between Ni and siderophile trace elements that are predictable by distribution coefficients between liquid and solid metal in fractionally crystallizing metal magmas. These meteorites are interpreted to be fragments of the fractionally crystallized cores of eleven differentiated asteroids. Many of these groups crystallized from S-depleted magmas which we propose resulted from removal of the first partial melt (the Fe,Ni-FeS cotectic melt) by explosive pyroclastic volcanism of the type envisaged by Wilson and Keil (1991). We show that these dense, negatively buoyant melts can be driven to asteroidal surfaces due to the presence of excess pressure in the melt and the presence of buoyant bubbles of gas which decrease the density of the melt. We also show that, in typical asteroidal materials, veins will form which grow into dikes and serve as pathways for migration of melt and gas to asteroidal surfaces. Since cotectic Fe, Ni-FeS melt consists of about 85 wt pct FeS and 15 wt pct Fe, Ni, removal of small volumes of eutectic melts results in major loss of S but only minor loss of Fe,Ni, thus leaving sufficient Fe,Ni to form sizeable asteroidal cores.

Keil, Klaus↗
NASA NTRS1993

Magnetic field signatures near Galileo's closest approach to Gaspra

Two large magnetic field rotations were recorded by the spacecraft Galileo 1 minute before and 2 minutes after its closest approach to the asteroid Gaspra. The timing and the geometry of the field changes suggest a connection with Gaspra, and the events can be interpreted as the result of the draping of the solar wind field around a magnetospheric obstacle. Gaspra's surface field is inferred to be within an order of magnitude of Earth's surface field, and its magnetic moment per unit mass is in the range observed for iron meteorites and highly magnetized chondrites. The location of the magnetic signatures suggests that perturbations are carried by waves in the magnetosonic-whistler mode with wavelengths between electron and ion gyro radii.

Kivelson, M. G.↗
NASA NTRS1993

Explosive volcanism and the compositions of the cores of differentiated asteroids

Eleven iron meteorite groups showing correlations between Ni and siderophile trace elements that are predictable by distribution coefficients between liquid and solid metal of fractionally crystallizing metal magmas, are interpreted to be fragments of the fractionally crystallized cores of 11 differentiated asteroids. Many of these groups crystallized from S-depleted magmas which we propose resulted from removal of the first partial melt (a Fe,Ni-FeS cotectic) by explosive pyroclastic volcanism. It is shown that these dense, negatively buoyant melts can be driven to asteroidal surfaces by the combination of an excess pressure in the melt and the presence of buoyant bubbles of gas which decrease the bulk density of the melt. It is also shown that in typical asteroidal materials, veins will form which grow into dikes and serve as pathways for migration of melt and gas to asteroidal surfaces. Since cotectic Fe,Ni-FeS melt consists of about 85 wt. percent FeS and 15 wt. percent Fe,Ni, removal of small volumes of eutectic melts results in major loss of S but only minor loss of Fe,Ni, thus leaving sufficient Fe,Ni to form sizeable asteroidal cores.

Keil, Klaus↗
NASA NTRS1994

More shock recovery experiments on mesosiderite analogs

Mesosiderites, a small but unique group of stony-iron meteorites with affinities to howardites, eucrites, and pallasites, remain enigmatic in terms of their petrogenesis. They are composed of approximately equal weight proportions of Fe-Ni metal plus troilite and gabbroic, basaltic, and orthopyroxenitic materials. The metal and silicates, which display variable grain sizes and shapes, are delicately intermingled, forming irregular grain boundaries that have been attributed to a wide range of origins from subsolidus metamorphism to supersolidus igneous processes. Perhaps the most relevant question regarding the petrogenesis of mesosiderites is: what is the source and duration of heating that could produce the unequilibrated textures and chemistry of these meteorites? A leading candidate appears to be impacts of metallic core fragments with a differentiated asteroidal surface. This provides not only a suitable source of heat, but also the metal component uniquely required by mesosiderites. A series of shock recovery experiments on mesosiderite analogs has been continued. Textural and chemical similarities have been found that support an impact-derived origin for these unusual meteorites.

Rowan, L. R.↗
NASA NTRS1995

Silica-rich orthopyroxenite in the Bovedy chondrite

A large (greater than 4.5 x 7 x 4 mm), igneous-textured clast in the Bovedy (L3) chondrite is notable for its high bulk SiO2 content (is approximately equal to 57.5 wt%). The clast consists of normally zoned orthopyroxene (83.8 vol%), tridymite (6.2%), an intergrowth of feldspar (5.8%) and sodic glass (3.1%), pigeonite (1.0%), and small amounts of chromite (0.2%), augite, and Fe,Ni-metal; it is best described as a silica-rich orthopyroxenite. The oxygen-isotopic composition of the clast is similar, but not identical, to Bovedy and other ordinary chondrites. The clast has a superchondritic Si/Mg ratio, but has Mg/(Mg + Fe) and Fe/Mn ratios that are similar to ordinary chondrite silicate. The closest chemical analogues to the clast are radial-pyroxene chondrules, diogenites, pyroxene-silica objects in ordinary chondrites, and silicates in the IIE iron meteorite Weekeroo Station. The clast crystallized from a siliceous melt that cooled fast enough to prevent complete attainment of equilibrium but slow enough to allow nearly complete crystallization. The texture, form, size and composition of the clast suggestion that it is an igneous differentiate from an asteroid or planetesimal that formed in the vicinity of ordinary chondrites. The melt probably cooled in the near-surface region of the parent object. It appears that in the source region of the clast, metallic and silicate partial melt were largely-to-completely lost during a relatively low degree of melting, and that during a higher degree of melting, olivine and low-Ca pyroxene separated from the remaining liquid, which ultimately solidified to form the clast. While these fractionation steps could not have all occurred at the same temperature, they could have been accomplished in a single melting episode, possibly as a result of heating by radionuclides or by electromagnetic induction. Fractionated magmas can also account for other Si-rich objects in chondrites.

Ruzicka, Alex↗
NASA NTRS1992

Black Carbon (Soot) Aerosol in the Lower Stratosphere and Upper Troposphere

We have used two approaches to measure BCA in the stratosphere. The first method consists of collecting aerosols by impaction on quartz rods, upon which are mounted strips of polystyrene filter material. Each 25 mm by 0.5 mm filter strip is alternately masked and exposed in 5 mm long sections. After sampling, the optical density of the exposed sections is measured and compared with the optical density of the masked (unexposed) sections. The filter strip is a Lambertian collector, the scattering property of which is not affected by the deposition of liquid sulfuric acid droplets which dominate the stratospheric aerosol. Hence, all attenuation greater in the exposed than in the non-exposed sections is due to absorption by BCA or rare opaque materials such as meteoritic iron. In a second approach we expose carbon-coated gold or palladium wires to air outside the aircraft boundary layer [Farlow et al., 1979; Pueschel et al., 1989]. With a field emission scanning electron microscope we are able to characterize BCA particles in addition to the dominant sulfuric acid droplets. Typically, soot is comprised of 20 nm diameter spheres linked together as branching chains or loosely packed aggregates (Figure 1a). The microscope has a lateral image resolution of 1.5 nm and can clearly resolve individual spheres making up BCA aggregates. Their morphology is uniquely different from that of the liquid sulfuric acid-water aerosol (Figure 1b). We approximate the size of each BCA particle by that of a sphere of equivalent volume and fit a lognormal size distribution to both the BCA and H2SO4/H2O aerosols. Both sampler types have been mounted on NASA's ER-2 high-altitude research aircraft (altitude ceiling of 70,000 feet), and on NASA's DC-8 (40,000 feet ceiling) aircraft to sample aerosols from a significant portion of the atmosphere.

Pueschel, R. F.↗
NASA NTRS2000

Mineralogy, Petrology, Chemistry, and Ar-39 - Ar-40 and Ages of the Caddo County IAB Iron: Evidence for Early Partial Melt Segregation of a Gabbro Area Rich in Plagioclase-Diopside

We found coarse-grained gabbroic material rich in plagioclase and diopside in the Caddo County IAB iron meteorite. The polished thin sections studied were made from areas rich in Al and Ca detected by a micro-focus X-ray fluorescence (XRF) mapping technique. The gabbro is not a clast within a breccia, but rather this area is located mainly at silicate-metal boundaries only a few cm away from an area with fine-grained, ultramafic silicate similar to winonaites. Medium-grained orthopyroxene and olivine are found in transitional areas showing no disturbance of their crystalline textures. A vein-like region, starting at the area rich in fine-grained mafic silicate, extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside. This texture and our accumulated knowledge of the formation mechanism of IAB/winonaltes meteorites, suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts of chondritic source materials. Compositional data on two mineralogically distinct samples of the gabbro-rich portion of the inclusion were obtained by INAA. Compared to an average of LAB silicate inclusions or winonaites, the Caddo County gabbro is enriched in the incompatible lithophile elements Na, Ca, Sc, REE and Hf, which is consistent with a melt origin for the gabbro. The cosmogenic space exposure age of Caddo County (511 Ma) is significantly younger than exposure ages of some other IAB meteorites, An 39Ar-40Ar age determination of the gabbroic material indicates a series of upward steps in age from 4.516 Ga to 4.523 Ga, with a few high temperature ages up to 4.54 Ga. The older age could approximate the primary recrystallization age of silicates. The stepped Ar age spectrum may indicate differences in Ar closure temperatures during slow cooling of -2-20'C/Myr in the parent body. Alternatively, the younger Ar-Ar ages may date a shock event which occurred while Caddo County was hot and which also created textures such as a linear metal vein with rounded zigzag walls in a diopside crystal and slightly miss-oriented rounded plagioclase domain.

Takeda, Hiroshi↗