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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

The Pasamonte unequilibrated eucrite: Pyroxene REE systematic and major-, minor-, and trace-element zoning

We are evaluating the trace-element concentrations in the pyroxenes of Pasamonte. Pasamonte is a characteristic member of the main group eucrites, and has recently been redescribed as a polymict eucrite. Our Pasamonte sample contained eucritic clasts with textures ranging from subophitic to moderately coarse-grained. This study concentrates on pyroxenes from an unequilibrated, coarse-grained eucrite clast. Major-, minor-, and trace-element analyses were measured for zoned pyroxenes in the eucritic clast of Pasamonte. The major- and minor-element zoning traverses were measured using the JEOL 733 electron probe with an Oxford-Link imaging/analysis system. Complemenatry trace elements were then measured for the core and rim of each of the grains by SIMS. The trace elements analyzed consisted of eight REE, Sr, Y, and Zr. These analyses were performed on a Cameca 4f ion probe. The results of the CI chondrite normalized (average CI trace-element analyses for several grains and the major- and minor-element zoning patterns from a single pyroxene grain are given. The Eu abundance in the cores of the pyroxenes represents the detection limit and therefore the (-Eu) anomaly is a minimum. Major- and minor-element patterns are typical for igneous zoning. Pyroxene cores are Mg enriched, whereas the rims are enriched in Fe and Ca. Also, Ti and Mn are found to increase, while Cr and Al generally decrease in core-to-rim traverses. The cores of the pyroxenes are more depleted in the Rare Earth Elements (REE) than the rims. Using the minor- and trace-element concentrations of bulk Pasamonte and the minor- and trace-element concentrations from the cores of the pyroxenes in Pasamonte measured in this study, we calculated partition coefficients between pyroxene and melt. This calculation assumes that bulk Pasamonte is representative of a melt composition.

Pun, A.↗
NASA NTRS2002

The Late Pliocene Eltanin Impact: Documentation From Sediment Core Analyses

The expeditions ANT-XII/4 (1995) and ANT-XVIII/5a (2001) of the RV POLARSTERN collected extensive bathymetric and seismic data sets as well as sediment cores from an area in the Bellingshausen Sea (eastern Pacific Southern Ocean) that allow the first comprehensive geoscientific documentation of an asteroid impact into a deep ocean (approx. 5 km) basin, named the Eltanin impact. Impact deposits have now been recovered from a total of more than 20 sediment cores collected in an area covering about 80,000 km2. Combined biomagnetostratigraphic dating places the impact event into the earliest Matuyama Chron, a period of enhanced climate variability. Sediment texture analyses and studies of sediment composition including grain size and microfossil distribution reveal the pattern of impact- related sediment disturbance and the sedimentary processes immediately following the impact event. The pattern is complicated by the San Martin Seamounts (approx. 57.5 S, 91 W), a large topographic elevation that rises up to 3000 m above the surrounding abyssal plain in the area affected by the Eltanin impact. The impact ripped up sediments as old as Eocene and probably Paleocene that have been redeposited in a chaotic assemblage. This is followed by a sequence sedimented from a turbulent flow at the sea floor, overprinted by fall-out of airborne meteoritic ejecta that settled trough the water column. Grain size distribution reveals the timing and interaction of the different sedimentary processes. The gathered estimate of ejecta mass deposited over the studied area, composed of shock-melted asteroidal material and unmelted meteorites including fragments up to 2.5 cm in diameter, point to an Eltanin asteroid larger than the 1 km in diameter size originally suggested as a minimum based on the ANT-XII/4 results. This places the energy released by the impact at the threshold of those considered to cause environmental disturbance at a global scale and it makes the impact a likely transport mechanism explaining the presence of extinct Cenozoic microfossils in the transantarctic Sirius Unit. Although a crater structure representing Eltanin ground zero has not been discovered, the distribution pattern of sediment disturbance and ejecta deposits now allows to better determine the central target area north of the San Martin Seamounts.

Gersonde, R.↗
NASA NTRS1974

A general model for the textural evolution of lunar soil

The investigation reported is concerned with the qualitative expansion of an earlier model developed by Lindsay (1971, 1972, 1973). Questions regarding the maturity of lunar soils are examined. A multivariate model related to textural maturity is considered, taking into account mean and standard deviations, the asymmetry of the grain size curve, the kurtosis anomaly, and pyroclastic materials. It is concluded that the mean grain size and the standard deviation of the lunar soil can be used as general indicators of the textural maturity of the soil anywhere on the lunar surface.

Lindsay, J. F.↗
NASA NTRS1996

Search for Past Life on Mars: Possible Relict Biogenic Activity in Martian Meteorite ALH84001

Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest the PAHs are indigenous to the meteorite. High resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and Fe-monosulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.

McKay, David S.↗
NASA NTRS1996

Search for past life on Mars: possible relic biogenic activity in martian meteorite ALH84001

Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. High-resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and Fe-sulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features, including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.

NASA Discipline Number 52-10↗
NASA NTRS2009

Late Reduction Textures in Almahata Sitta Ureilite

The Almahata Sitta ureilite, derived from asteroid 2008 TC3, consists of many individual fragments recovered from the Nubian dessert strewn field [1]. Like most ureilites, it contains abundant carbon and exhibits examples of disequilibrium textures that record a late reduction event accompanied by rapid cooling (tens of degC/h) from high temperatures (1150-1300 C). Variations in Fe/Mg of silicate minerals are accompanied by variations in Fe/Mn, indicating loss of Fe into metal [2]. In coarser-grained fragments of Almahata Sitta, olivine exhibits irregular high mg# rims in contact with networks of interstitial metal 5- 20 microns in typical thickness. This is a common ureilite texture thought to be driven by the reaction of graphite to a CO gas phase and the concurrent reduction of FeO in olivine to Fe metal, with excess silica going primarily into pyroxene (2MgFeSiO4 + C approaches MgSiO4 + MgSiO3 + 2Fe + CO) [3, see also 4,5,6]. Other fragments of Almahata Sitta exhibit anomalous textures such as fine grain size, high porosity, and abundant graphite. Within these fragments pyroxene locally exhibits high-mg# rims in contact with metal and a discreet silica phase, suggesting that the reduction mechanism MgFeSi2O6 + C approaches MgSiO3 + Fe + SiO2 + CO. Metals in Almahata Sitta are particularly unaltered in comparison to ureilite finds. Variations in minor and trace element composition of this metal might partly result from localized dilution as iron is supplied by reduction of silicates.

Herrin, J. S.↗
DOE OSTI2025

Production of Creep-Resistant FeCrAl-ODS

Four ODS FeCrAl alloys with base alloy composition Fe-16Cr-4Al-2Mo-0.2Y (wt.%) were mechanically alloyed with minor additions of CeO 2 and Y 2 O 3 additions and were successfully extruded into rectangular-shaped bars. Two oxide compositions were explored, which were designated NC with oxide addition of 0.5CeO 2 plus 0.2Yin solution of the base alloy powder and PC with additions of 0.5CeO 2 and 0.7Y 2 O 3 to elevate the O level. The microstructure of both alloys revealed by SEM and EBSD to consist of very small grains with sizes of ~450-520 nm and weak texture components. Sub-size tensile specimens fabricated from both alloys were tensile tested from room temperature to 800ºC. The results for both alloys from room temperature to 400ºC showed very high strengths, but low ductilities, especially uniform strain. From 500ºC to 800ºC, the strengths decreased rapidly, and the ductilities increased significantly. The reason for the rapid decreases in tensile strengths above 500ºC for both alloys is not clear, but will require advanced characterization techniques including atom probe tomography, to determine the size and number density of the nano-size oxide particles, and TEM/EFTEM, for investigating the dispersion of nano-size oxide particles on grain boundaries to assess the Zener grain boundary pinning effect, which will hinder grain coarsening at high temperatures that may enhance the Hall Petch strengthening mechanism.

36 MATERIALS SCIENCE↗
NASA NTRS1982

Intergrown mica and montmorillonite in the Allende carbonaceous chondrite

High resolution transmission electron microscopy (HRTEM) observations were made of a mixture of mica and montmorillonite from fine-grained calcium, aluminum inclusions (CAI) in the Allende C3(V) meteorite. A petrographic thin section having a diameter of 4 mm contained CAI fragments ranging from less than 1 to 50 microns. The observed textural and chemical characteristics placed the inclusion in the fine-grained alkali-rich spinel aggregate category of Warks' (1979) classifications of CAIs and as type 3 in Kornacki's classifications of fine grains in Allende. Chemical analyses were performed on the phyllosilicate grains observed in the TEM scan by means of an X ray observed, and the proximity to the matrix boundary suggests a metamorphism which included aqueous alteration at a relatively low temperature.

Tomeoka, K.↗
DOE OSTI2020

Grain detachment and transport clogging during mineral dissolution in carbonate rocks with permeable grain boundaries

In this paper, we explore the effect of permeable grain boundaries on carbonate rock dissolution in order to improve our understanding of grain detachment and migration during reactive flow. To do so we investigated the effects of grain size and enhanced reactivity along grain boundaries on global dissolution kinetics. Variations of permeability and porosity were calculated, and exponential relationships were observed in fractured rocks. Our model employed a reactive transport framework based on the Darcy-Brinkman equation to simulate calcite dissolution in carbonate rocks composed of microporous grains and, included fluid transport along grain boundaries. The model includes fluid flow, solute transport by advection–diffusion, heterogeneous reaction between fluid and minerals and grain detachment with subsequent grain transport in macropores. The migration of solid particles due to dissolution was based on cluster analysis and local movement, and the results show that grain detachment can lead to significant decreases in permeability due to the clogging of transport pathways. Microporous media with smaller grain sizes (fine grains in this study) showed a higher average reaction rate than those with coarser grains. In addition, as the overall rates of geochemical processes are commonly affected by the presence of texture heterogeneities such as fractures, a single fracture (macropore) introduced into a microporous matrix composed of permeable grains and grain boundaries was modelled as a large channel connecting the inlet and outlet of the simulation domain. It was found that macropore clogging by grain detachment temporary decreased permeability, lowering the long-term global reaction rates. Lastly, we observed that increases in flow rate can reduce detachment and fracture clogging by reducing local dissolution along grain boundaries.

58 GEOSCIENCES↗
DOE OSTI2021

Evolution of texture and internal stresses within polycrystalline rock salt using in situ 3D synchrotron computed tomography and 3D X-ray diffraction

Over time rock salt caverns have been extensively used as reliable repositories for hazardous waste such as nuclear waste, oil or compressed gases. Undisturbed rock salt deposits in nature are usually impermeable and have very low porosity. However, rock salt formations under excavation stresses can develop crack networks, which increase their porosities; and in the case of a connected crack network within the media, rock salt may become permeable. Although the relationship between the permeability of rock salt and the applied stresses has been reported in the literature, a microscopic study that investigates the properties influencing this relationship, such as the evolution of texture and internal stresses, has yet to be conducted. This study employs in situ 3D synchrotron micro-computed tomography and 3D X-ray diffraction (3DXRD) on two small-scale polycrystalline rock salt specimens to investigate the evolution of the texture and internal stresses within the specimens. The 3DXRD technique measures the 3D crystal structure and lattice strains within rock salt grains. The specimens were prepared under 1D compression conditions and have shown an initial {111} preferred texture, a dominant {110}$\langle1\bar{1}0\rangle$ slip system and no fully connected crack network. The {111} preferred texture under the unconfined compression experiment became stronger, while the {111}$\langle1\bar{1}0\rangle$ slip system became more prominent. The specimens did not have a fully connected crack network until applied axial stresses reached about 30 MPa, at a point where the impermeability of the material becomes compromised due to the development of multiple major cracks.

36 MATERIALS SCIENCE↗
NASA NTRS1978

Dynamic crystallization and kinetic melting of the lunar soil

Dynamic crystallization and kinetic melting tests have been conducted for a lunar soil that resembles the bulk composition of the matrices of melt-rock breccias. The melting tests indicate that plagioclase and olivine can exist for finite times above their equilibrium liquidus temperatures; within 20-30 C below its liquidus temperature the sample is choked with crystals. A range of textures from radiate to porphyritic pyroxene-plagioclase granophyric to subophitic to intergranular to intersertal to very fine-grained poikilitic is found in the dynamic crystallization experiments. The number of nuclei, as deduced from the textures, increases as the melting temperature deceases from above the liquidus to below.

Lofgren, G. E.↗
DOE OSTI2020

In situ indentation and high cycle tapping deformation responses in a nanolaminate crystalline/amorphous metal composite

The incorporation of nanostructured and amorphous metals into modern applications is reliant on the understanding of deformation and failure modes in constrained conditions. To study this, a 105 nm crystalline Cu/160 nm amorphous Cu 45 Zr 55 (at.%) multilayer structure was fabricated with the two crystalline layers sputter deposited between the top-middle-bottom amorphous layers and prepared to electron transparency. The multilayer was then in situ indented either under a single load to a depth of ~ 100 nm (max load of ~ 100 μN) or held at 20 μN and then repeatedly indented with an additional 5 μN up to 20,000 cycles in a transmission electron microscope to compare the deformation responses in the nanolaminate. For the single indentation test, the multilayer showed serrated load-displacement behavior upon initial indentation inductive of shear banding. Additionally, at an indentation depth of ~ 32 nm, the multilayer exhibited perfect plastic behavior and no strain hardening. Both indented and fatigue-indented films revealed diffraction contrast changes with deformation. Subsequent Automated Crystal Orientation Mapping (ACOM) measurements confirmed and quantified global texture changes in the crystalline layers with specifically identified grains revealing rotation. Using a finite element model, the in-plane displacement vectors under the indent mapped conditions where ACOM determined grain rotation was observed, indicating the stress flow induced grain rotation. Furthermore, the single indented Cu layers also exhibited evidence of deformation induced grain growth, which was not evident in the fatigue-indented Cu based multilayer. Finally, the single indented multilayer retained a significant plastic crater in the upper most amorphous layer that directly contacted the indenter; a negligible crater impression in the same region was observed in the fatigued tested multilayer. These differences are explained by the different loading methods, applied load, and deformation mechanisms experienced in the multilayers.

36 MATERIALS SCIENCE↗
NASA NTRS1972

The Haveroe ureilite - Petrographic notes.

Review of the mineralogy and texture of the Haveroe and of some of the six other known ureilites, and assessment of the implications of their observed characteristics. In common with other ureilites, Haveroe shows mineralogical and textural evidence of preterrestrial shock. The olivines and metal grains have been recrystallized, the fabric includes angular voids, and graphite-rich areas are present that have been partially converted to diamonds with some indication of lonsdaleite. On the basis of such features, a subdivision of ureilites into particular types is proposed.

Marvin, U. B.↗
NASA NTRS1989

On reported occurrences of shock-deformed clasts in the volcanic ejecta from Toba caldera, Sumatra

Reports of shock-deformed phenocrysts from the Toba ignimbrite deposits, Sumatra, have prompted considerable debate over whether shock-deformation products are clear evidence of a meteorite impact origin for the K/T boundary deposits as well as terresrial 'cryptoexplosion' structures. Evidence presented in favor of volcanically induced shock at Toba includes kinked biotites and rare occurrences of single set of lamellae in quartz grains but rests most heavily upon occurrences of mosaic extinction patterns in plagioclase phenocrysts. The present analysis of several of the same Toba samples reveals that these mosaic patterns ae related to distinct compositional zoning and cannot be attributed to deformation of the crystal lattice that shock would produce. Additionally, in more than 200 quartz grains examined, no occurrences of microdeformation features or mosaic textures similar to those associated with known impact structures and the K/T boundary are detected. It is concluded that evidence of shock deformation in the Toba deposits has not been demonstrated.

Sharpton, V. L.↗
NASA NTRS1992

Soil texture and granulometry at the surface of Mars

The microtexture of the near-surface Martian soil was sensed with three diagnostic parameters: (1) the albedo A at normal incidence and phase angle 5 degrees, which relates to the composition of the top surface exposed layer; (2) the polarization parameter b characterizes the texture of the top surface layer in terms of grain size; and (3) the thermal inertia parameter I which refers to the soil compaction through the first few decimeters below the top surface sensed by polarimetry, in terms of size for the pieces making a granular regolith. Parameter b was derived from instrument VPM on board the Soviet spacecraft MARS-5, inertial I is from IRTM on the American Viking, and albedo A from both. The polarimetric scans racked strips covering two contrasted regions, the dark hued Mare Erythraeum, and the adjacent bright orange Thaumasia. Erythraem is characterized everywhere by a same type of terrain, despite the large geomorphological diversity of the surface. There is an ubiquitous coating or mantling with small dark grains, of both albedo 12.7 percent and particle size 10 to 20 microns, above a subsurface dislocation in pieces around 300 to 600 microns. A simple model is with sand-size particles completely coated with 15 micron dark grains.

Dollfus, A.↗
NASA NTRS1993

Mineralogy of chondritic interplanetary dust particle impact residues from LDEF

A detailed structural and compositional analysis of several impactor residues was performed utilizing transmission electron microscopy, energy dispersive spectroscopy, and electron diffraction. Residues from the interior of several craters in gold surfaces were removed with a tungsten needle, mounted in EMBED-812 epoxy, and ultramicrotomed. The presence in these residues of equilibrated ferromagnesian minerals, recrystallization textures, glass, and melted metal and sulfide bodies decorating grain boundaries is indicative of varying degrees of shock metamorphism in all impact residues we have characterized.

Barrett, R. A.↗
NASA NTRS1994

Comparison of the LEW88516 and ALHA77005 martian meteorites: Similar but distinct

By mineral and bulk compositions, the Lewis Cliff (LEW) 88516 meteorite is quite similar to the ALHA77005 martian meteorite. These two meteorites are not paired because their mineral compositions are distinct, they were found 500 km apart in ice fields with different sources for meteorites, and their terrestrial residence ages are different. Minerals in LEW88516 include: olivine, pyroxenes (low- and high-Ca), and maskelynite (ater plagioclase); and the minor minerals chromite, whitlockite, ilmenite, and pyrrhotite. Mineral grains in LEW88516 range up to a few mm. Texturally, the meteorite is complex, with regions of olivine and chromite poikilitically enclosed in pyroxene, regions of interstitial basaltic texture, and glass-rich (shock) veinlets. Olivine compositions range from Fo(sub 64) to Fo(sub 70), (avg. Fo(sub 67)), more ferroan and with more variation than in ALHA77005 (Fo(sub 69) to Fo(sub 73)). Pyroxene compositions fall between En(sub 77)Wo(sub 4) and En(sub 65)Wo(sub 15) and in clusters near En(sub 63)Wo(sub 9) and En(sub 53)Wo(sub 33), on average more magnesian and with more variation than in ALHA77005. Shock features in LEW88516 range from weak deformation through complete melting. Bulk chemical analyses by modal recombination of electron microprobe analyses, instrumental neutron activation, and radiochemical neutron activation confirm that LEW88516 is more closely related to ALHA77005 than to other known martian meteorites. Key element abundance ratios are typical of martian meteorites, as is it nonchondritic rare earth pattern. Differences between the chemical compositions of LEW88516 and ALHA77005 are consistent with slight differences in the proportions of their constituent minerals and not from fundamental petrogenetic differences. Noble gas abundances in LEW88516, like those in ALHA77005, show modest excesses of Ar-40 and Xe-129 from trapped (shock-implanted) gas. As with other ALHA77005 and the shergottite martian meteorites (except EETA79001), noble gas isotope abundances in LEW88516 are consistent with exposure to cosmic rays for 2.5-3 Ma. The absence of substantial effects of shielding from cosmic rays suggest LEW88516 spent this time as an object no larger than a few cm in diameter.

Treiman, A. H.↗
NASA NTRS2004

Lunar and Planetary Science XXXV: Carbonaceous Chondrites

The titles in this section include: 1) A Quantitative NMR Analysis of Phosphorus in Carbonaceous and Ordinary Chondrites; 2) An Infrared Study of the Matrices of CI1 and CM2 Chondrites; 3) A Study of the Morphology, Composition and Mineral Associations of Fe-Ni Sulphides in CM Carbonaceous Chondrites; 4) Aqueous Alteration Mineralogy in CM Carbonaceous Chondrites; 5) In Situ Location and Characterization of Carbon-bearing Phases in Carbonaceous; 6) Chondrites: Insights from Yamato 791198, a Weakly-altered CM2 Chondrite; 7) Unique Texture in EET 83389: Clues to Formation of Fine Grained Rims in CM Chondrites; 8) Amoeboid Olivine Aggregates in the NWA 760 CV3 Chondrite; 9) Petrologic Type of CV3 Chondrites as Revealed by Raman Spectroscopy of Organic Matter; 10) A First Look at Acfer 324: Evidence for Another CR 3 Chondrite? 11) Hydrogen Isotopic Composition of the Bencubbin Meteorite; 12) The Fountain Hills Meteorite: A New CBa Chondrite from Arizona; 13) Shock Effects in the Metal-rich Chondrites QUE 94411, Hammadah al Hamra 237 and Bencubbin; 14) Mineralogy and Petrology of Al-rich Objects in the CH Carbonaceous Chondrite North West Africa 739.

Source record↗