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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Regional variation in the Amitsoq gneisses related to crustal levels during late Archean granulite facies metamorphism: Southern west Greenland

The dominant lithology at Kangimut sangmissoq is described as nebulitic tonalitic gneiss containing highly distended plagioclase phyric amphibolites. The gneiss amphibolite complex was intruded by Nuk gneiss between 3.05 and 2.90 Ga and later (2.6 to 2.7 Ga) by post granulite facies granitoid sheets. The amphibolites are though to be Ameralik dikes and the older gray gneiss are then Amitsoq by definition. The problem arises when the isotopic data are considered, none of which indicate rocks older that about 3.0 Ga.

Nutman, A. P.↗
NASA NTRS1986

Shock metamorphism and petrography of the Shergotty achondrite

The shock history of the Shergotty meteorite has been evaluated by analyzing the observable shock effects. In addition, the results of petrographic and mineralogical analyses of new Shergotty subsamples are presented. The overall modal composition of these subsamples is similar to that of the other Shergotty samples, except for the rather high whitlockite content. The shock effects observed in the mineral constituents include mosaicism, deformation bands, planar fractures, and mechanical twin lamellae in clinopyroxene; isotropization of plagioclase with very rare remnants of birefringence; and planar deformation structures, mosaicism, and strongly reduced birefringence in quartz. Other shock effects include mechanical twinning of ilmenite, and localized in situ melting of neighboring minerals at the contact of low and high density phases. The observed shock effects could be explained by a single shock event.

Stoeffler, D.↗
NASA NTRS1988

Cl-rich minerals in Archean granulite facies ironstones from the Beartooth Mountains, Montana, USA: Implications for fluids involved in granulite metamorphism

The implications of Cl-rich minerals in granulite facies rocks are discussed. Results from ironstones of the Beartooth Mountains, Montana are discussed. It is suggested that CO2-brine immiscibility might be applicable to granulite facies conditions, and if so, then aqueous brines might be preferentially adsorbed onto mineral surfaces relative to CO2.

Henry, D. J.↗
NASA NTRS1988

Metamorphic conditions in the Nilgiri Granulite Terrane and the adjacent Moyar and Bhavani Shear Zones: A reevaluation

Data were presented on pressure and temperature determinations from the Nilgiri Hills. About 70 samples were analyzed by probe and several calibrations of garnet-pyroxene thermometry and barometry applied. Most calibrations gave considerable scatter; however, a new calibration by Bhattacharya, Raith, Lal, and others, accounting for nonideality in both garnet and orthopyroxene, gave consistent results of 754 + or - 52 C and 9.2 + or - 0.7 kbar. On the regional scale, a pressure increase of 6.5 to 7 kbar in the SW to 11 kbar in the NE was related to block tilting. A continuous pressure gradient into the Moyar shear zone suggests that the zone is not a suture juxtaposing unrelated blocks.

Raith, M.↗
NASA NTRS1988

Shock metamorphism of deformed quartz

The effect produced by shock loading (to peak pressures of 12 and 24) on deformed synthetic quartz containing a dislocation and abundant bubbles and small inclusions was investigated, and the relationships between preexisting dislocation density shock lamellae in the target material were examined. The resultant material was found to be inhomogeneously deformed and extremely fractured. Results of TEM examinations indicate that no change in dislocation density was caused by shock loading except in regions containing shock lamellae, where the dislocation density was lowered. The shock-induced defects tend to nucleate on and be controlled by preexisting stress concentrators; shock lamellae, glassy veins, and most curviplanar defects form in tension, presumably during release. An extremely mobile silica fluid is formed and injected into fractures during release, which forcibly removes crystalline fragments from vein walls. It is concluded that shock deformation in quartz is dominated by fracture and melting.

Gratz, Andrew J.↗
NASA NTRS1989

Intensive parameters of enstatite chondrite metamorphism

A geothermometer based on the assemblage kamacite-quartz-enstatite-oldhamite-troilite found in enstatite chondrites is described. Data obtained with the geothermometer reveal that the EL6 meteorites experienced temperatures exceeding 1000 C. These temperatures imply a metal-sulfide melting event that may have fractionated the melt from the source region.

Fogel, Robert A.↗
NASA NTRS1990

Quantitative lithologic mapping in spectral ratio feature space - Volcanic, sedimentary and metamorphic terrains

The nature of spectral lithologic mapping is studied utilizing ratios centered around the wavelength means of TM imagery. Laboratory-derived spectra are analyzed to determine the two-dimensional relationships and distributions visible in spectral ratio feature space. The spectral distributions of various rocks and minerals in ratio feature space are found to be controlled by several spectrally dominant molecules. Three study areas were examined: Rawhide Mining District, Nevada; Manzano Mountains, New Mexico; and the Sevilleta Long Term Ecological Research site in New Mexico. It is shown that, in the comparison of two ratio plots of laboratory reflectance spectra, i.e., 0.66/0.485 micron versus 1.65/2.22 microns with those derived from TM data, several molecules spectrally dominate the reflectance characteristic of surface lithologic units. Utilizing the above ratio combination, two areas are successfully mapped based on their distribution in spectral ratio feature space.

Campos-Marquetti, Raul, Jr.↗
NASA NTRS1991

Composition of chondrule silicates in LL3-5 chondrites and implications for their nebular history and parent body metamorphism

The composition of 75 type-IA and type-II porphyritic olivine chondrules from nine LL type 3 to type 5 chondrites was determined and was compared with that of chondrules from the Semarkona type 3.0 meteorite. Chemical data for silicates in the LL3.3-5 chondrites indicated that porphyritic olivine chondrules in these chondrites could be derived from chondrules similar to those from the Semarkona LL3.0, the least metamorphosed one of the known LL chondrites. It is shown that the chemical trends defined by the minerals of type-IA and type-II chondrules can be satisfactorily accounted for by the process of solid-state diffusive equilibration between minerals in chondrules and the opaque matrix rather than by changes in conditions during chondrule crystallization.

Mccoy, Timothy J.↗
NASA NTRS1992

Chemical and physical studies of chondrites. X - Cathodoluminescence and phase composition studies of metamorphism and nebular processes in chondrules of type 3 ordinary chondrites

The cathodoluminescence (CL) characteristics of eight type-3 ordinary chondrites and one L5 chondrite were investigated with particular emphasis on detailed compositions of the relevant phases in four of these chondrites: Semarkona (type-3.0); Krymka (3.1); Allan Hills A77214 (3.5); and Dhajala (3.8). By sorting the chondrules into eight groups according to the CL of mesostasis and to certain compositional criteria and by determining the number of chondrules in these groups as a function of petrological type, it was possible to deduce genetic/evolutionary sequences of the chondrules. It is shown that there are major compositional differences in chondrules, which account for their CL properties and the chondrule groups.

Dehart, John M.↗
NASA NTRS1993

Evidence of thermal metamorphism on the C, G, B, and F asteroids

Reflectance spectra (0.3 to 2.6 micrometers) of 14 C, G, B, and F asteroids and 21 carbonaceous chondrite powders are compared in detail. Only three thermally metamorphosed CM-CI chondrites that have a weak UV absorption are shown to have close counterparts among those asteroids. Reflectance spectra of heated Murchison CM2 chondrite are compared with the average C and G type asteroid spectra. Murchison heated at 600 to 1000 C exhibits a similar weak UV absorption and provides the best analog for those spectra. Comparison of UV absorption strengths between 160 C, G, B, and F asteroids and carbonaceous chondrites suggests that surface minerals of most of those asteroids are thermally metamorphosed at temperatures around 600 to 1000 C.

Hiroi, Takahiro↗
NASA NTRS1993

Chondrite thermal histories from Low-CA pyroxene microstructures: Autometamorphism versus prograde metamorphism revisited

In order to constrain the thermal histories of chondritic meteorites, a detailed study of the microstructures of low-Ca pyroxenes produced experimentally and in types 4 and 5 ordinary chondrites was carried out. Cooling experiments on synthetic MgSiO3 at cooling rates between 2 and 10000 C/hr from the protopyroxene stability field into that of orthopyroxene (OPX) were performed and the products of these experiments were annealed for a variety of annealing times. There are clear microstructural differences between samples which were cooled and those which were subsequently annealed. A comparison of the microstructures observed in the experimental samples with those in H4-5 ordinary chondrites shows that they cannot have experienced a single stage cooling history, as proposed for the autometamorphism model.

Brearley, Adrian J.↗
NASA NTRS2001

Field Ion Microscopy and Atom Probe Tomography of Metamorphic Magnetite Crystals

Magnetite has been analysed using Field Ion Microscopy (FIM) and Atom Probe Tomography (APT), highly attractive techniques for the nanoanalysis of geological materials despite the difficulties inherent in analyzing semiconducting and insulating materials. Additional information is contained in the original extended abstract.

Kuhlman, K.↗
NASA NTRS1991

Isotopic compositions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the effects of diagenesis and metamorphism

The carbon isotope geochemistry of carbonates and organic carbon in the late Proterozoic Damara Supergroup of Namibia, including the Nama, Witvlei, and Gariep groups on the Kalahari Craton and the Mulden and Otavi groups on the Congo Craton, has been investigated as an extension of previous studies of secular variations in the isotopic composition of late Proterozoic seawater. Subsamples of microspar and dolomicrospar were determined, through petrographic and cathodoluminescence examination, to represent the "least-altered" portions of the rock. Carbon-isotopic abundances in these phases are nearly equal to those in total carbonate, suggesting that 13C abundances of late Proterozoic fine-grained carbonates have not been significantly altered by meteoric diagenesis, although 18O abundances often differ significantly. Reduced and variable carbon-isotopic differences between carbonates and organic carbon in these sediments indicate that isotopic compositions of organic carbon have been altered significantly by thermal and deformational processes, likely associated with the Pan-African Orogeny. Distinctive stratigraphic patterns of secular variation, similar to those noted in other, widely separated late Proterozoic basins, are found in carbon-isotopic compositions of carbonates from the Nama and Otavi groups. For example, in Nama Group carbonates delta 13C values rise dramatically from -4 to +5% within a short stratigraphic interval. This excursion suggests correlation with similar excursions noted in Ediacaran-aged successions of Siberia, India, and China. Enrichment of 13C (delta 13C> +5%) in Otavi Group carbonates reflects those in Upper Riphean successions of the Akademikerbreen Group, Svalbard, its correlatives in East Greenland, and the Shaler Group, northwest Canada. The widespread distribution of successions with comparable isotopic signatures supports hypotheses that variations in delta 13C reflect global changes in the isotopic composition of late Proterozoic seawater. Within the Damara basin, carbon-isotopic compositions of carbonates provide a potentially useful tool for the correlation of units between the Kalahari and Congo cratons. Carbonates depleted in 13C were deposited during and immediately following three separate glacial episodes in Namibia. The correspondence between ice ages and negative delta 13C excursions may reflect the effects of lowered sea levels; enhanced circulation of deep, cold, O2-rich seawater; and/or the upwelling of 13C-depleted deep water. Iron-formation is additionally associated with one of the glacial horizons, the Chuos tillite. Carbon-13 enriched isotopic abundances in immediately pre-glacial carbonates suggest that oceanographic conditions favored high rates of organic burial. It is likely that marine waters were stratified, with deep waters anoxic. A prolonged period of ocean stratification would permit the build-up of ferrous iron, probably from hydrothermal sources. At the onset of glaciation, upwelling would have brought 13C-depleted and iron-rich deep water onto shallow shelves where contact with cold, oxygenated surface waters led to the precipitation of ferric iron.

NASA Program Exobiology↗
NASA NTRS2005

I-Xe Dating: The Time Line of Chondrule Formation and Metamorphism in LL Chondrites

Refractory inclusions, considered to be the oldest solids formed in the solar nebula. (4567.2 0.6 Ma) [1], are common in many carbonaceous and in some ordinary and enstatite chondrites. High-precision Pb- Pb ages for CAI s and chondrules (from different meteorites) suggested that chondrule formation appeared to have started about 2 Ma later than that of CAIs [1]. However, recent 26Al/26Mg data suggest simultaneous formation of CAI s and chondrules in Allende [2]. The I-Xe ages of CAI s in Allende are about 2 Ma younger than the I-Xe ages of Allende chondrules [3] but, like all chronometers, the I-Xe system records closure time of its particular host phase. In the case of Allende CAI s, the major iodine-bearing phase is sodalite, a secondary phase presumably formed by aqueous alteration, so I-Xe reflects the post-formational processes in these objects. In chondrules the iodine host phases vary and can reflect formation and/or alteration but, to put chondrule ages on a quantative basis, some problems should first be addressed.

Pravdivtseva, O. V.↗