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All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Preparation and optical properties of GA(x)IN(1-x)P alloys

The solution crystallization method was used to obtain Ga(z)In(1-x)P alloys for all values of x desired between zero and 1. The method of preparation makes it possible to crystallize the solid at a constant temperature. The points obtained by cathodoluminescence are nearly in straight lines. The optical absorption thresholds are confirmed in the results and make it possible to define the nature of the transitions except when x is in the neighborhood of 0.65. These determinations agree with those of Hilsum and Porteus (1968), but are not in agreement with those obtained by Lorenz et al, (1968).

Rodot, H.↗
NASA NTRS1987

Intrusions in the active layer of channeled-substrate-planar laser diodes

Channeled-substrate-planar laser diodes from various wafers were observed to develop a node or dark spot in their near-field patterns after failure during lifetesting. 'Node' devices from each wafer were angle-lapped and stained in an attempt to uncover a physical mechanism for this common failure symptom. Optical microscopic examination of the beveled cross sections revealed that an intrusion, in the form of a spike at the tip of the zinc diffusion front, had penetrated into the active layer of these diodes. The node observed in the near-field pattern appears to be in a position correspondig to the location of the spike in the active region. In addition, cathodoluminescence measurements on other 'node' diodes revealed a dark line region approximately 2-micron-wide running parallel to and in the middle of the 5-micron contact stripe. It is believed that this dark line region is representative of an area of nonradiative recombination which occurs in the portion of the active layer containing the intrusion.

Slavin, S. E.↗
NASA NTRS1989

Compositions of isolated forsterites in Ornans (C3O)

The Ornans (C3O) carbonaceous chondrite shows many examples of isolated olivine grains which are Mg-rich and show cathodoluminescence (CL). Using backscattered electron imaging to scan and select Mg-rich isolated olivine grains, and a CL detector, the chemical zoning within these grains was investigated together with its relation to CL and to chemical zoning of grains within a Fe-rich chondrule. Composition profiles from the core to the rim of five isolated forsterites showed details of Al, Ca, Sc, Ti, V, Cr, Mn, and Fe zoning with respet ot position and CL color.

Steele, Ian M.↗
NASA NTRS1991

Dynamic crystallization characteristics of enstatite chondrite chondrules

Although the chemical properties of enstatite and ordinary chondrites are distinctly different, they both contain chondrules with a similar array of textures. This similarity suggests like origins. Textural studies using chondrule compositions from ordinary chondrites suggest that these chondrules have an igneous origin: either by crystallization from melts or from partial melts of crystalline material. In contrast, the cathodoluminescence (CL) properties of the enstatite from enstatite chondrites were interpreted to mean that mechanical aggregation played an important part in their formation. An alternative interpretation of these CL properties, however, suggests that variations in the minor element content of the enstatite, a probable result of igneous fractionation processes, could also produce different CL colors. An attempt was made to evaluate the two models by performing dynamic crystallization experiments on an average enstatite chondrule composition and by looking at the resultant CL. The textures grown on experimentally crystallized E-chondrite melts confirm that formational processes are similar to those for the ordinary chondrites with the obvious exception of the oxidation state.

Lofgren, Gary E.↗
NASA NTRS1991

Diamond film deposition using microwave plasmas under low pressures

Microwave plasma depositions of diamond films have been investigated under low pressures of 10 mTorr to 10 Torr, at low substrate temperatures of 400 to 750 C, using high methane concentrations of 5 to 15 percent and oxygen concentrations of 5 to 10 percent in hydrogen plasmas. The deposition system consists of a microwave plasma chamber, a downstream deposition chamber, and a RF induction-heated sample stage. The deposition system can be operated in either high-pressure microwave or electron cyclotron resonance (ECR) modes by varying the sample stage position. Cathodoluminescence (CL) studies on diamond films deposited at 10 Torr pressure show that CL emissions at 430, 480, 510, 530, 560, 570 and 740 nm can be employed to characterize the quality of diamond films. High-quality, well-faceted diamond films have been deposited at 10 Torr and 600 C using 5 percent CH4 and 5 percent O2 in H2 plasmas; CL measurements on these films show very low N impurities and no detectable Si impurities. Diamond nucleation on SiC has been demonstrated by depositing well-faceted diamond crystallites on SiC-coated Si substrates.

Shing, Y. H.↗
NASA NTRS1993

Aluminum acceptor four particle bound exciton complex in 4H, 6H, and 3C SiC

Evidence is presented for a four particle acceptor complex in 3C, 6H, and 4H SiC, obtained in low-temperature photoluminescence and cathodoluminescence experiments. The new lines were observed in p-type films lightly doped with aluminum, of 6H, 4H, and 3C SiC grown on the silicon (0001) face of 6H SiC under special conditions. The lines increase in intensity as more aluminum is added during growth. The multiplicity of observed lines is consistent with symmetry-based models similar to those which have been proposed to describe 4A centers in p-type zincblende semiconductors.

Clemen, L. L.↗
NASA NTRS1992

Low-pressure microwave plasma nucleation and deposition of diamond films

Low-pressure microwave plasma nucleation and deposition of diamond films were investigated in the pressure range 10-mtorr to 10 torr, at substrate temperatures 400-750 C and with CH4 and O2 concentrations in H2 plasma of 2-15 percent and 2-10 percent, respectively. The experiments were performed in a microwave plasma system consisting of a microwave plasma chamber, a downstream deposition chamber, and an RF induction heated sample stage. Scanning electron microscopy of diamond films deposited at 600 C with 5 percent CH4 and 5 percent O2 in H2 plasmas showed high-quality well faceted crystallites of 1/2 micron size. Cathodoluminescence measurements of these films showed very few nitrogen impurities and no detectable silicon impurities.

Shing, Y. H.↗
NASA NTRS1991

Optical and structural characterization of InAs/GaAs quantum wells

Three InAs/GaAs single quantum wells of two-, three-, and four-monolayer thickness were characterized using optical and structural techniques. The results of high-resolution transmission electron (HRTEM) microscopy and optical studies which combine absorption, photoluminescence (PL), photoreflectance, and cathodoluminescence are presented. Using the polarization modulated absorptance technique, we observed two absorption features in our samples at 77 K. On the basis of their polarization properties and comparison with an envelope function calculation, these structures are assigned to transitions between the confined heavy-hole and confined and unconfined electron levels. Photoreflectance spectra of the three-monolayer sample in 77-300 K range show only the fundamental quantum well transition. The temperature dependence of this transition is approximately linear with a slope of 2.2 x 10 exp -4 eV/K, which is significantly lower than in both constituent materials. Comparison to the absorption data reveals that the PL spectra are affected by the carrier diffusion and therefore do not provide direct measure of the exciton density of states. The HRTEM images indicate that, while the interfaces of the two-monolayer sample are smooth and the well thickness is uniform, the four-monolayer sample has uneven interfaces and contains domains of two, three, and four monolayers.

Ksendzov, A.↗
NASA NTRS1993

Experimentally reproduced relict enstatite in porphyritic chondrules of enstatite chondrite composition

Experiments are presented that test a model for the origin of porphyritic pyroxene (PP) chondrules in enstatite chondrites that contain phenocrysts of enstatite with blue cathodoluminescence (CL) set in a matrix of radial, dendritic enstatite with red CL. Established one-atmosphere, gas-mixing techniques were used. Relict enstatite phenocrysts with blue CL in a matrix of coarsely radial to dendritic enstatite with red CL were successfully produced. The relict crystals are preserved in runs with a melt time of 36 minutes or less at 1537 C. The relicts remain angular with smooth crystal/melt interfaces, and thus melting has occurred uniformly. Partial melting does occur along fractures produced when the blue CL enstatite was initially grown and cooled through the proto/ortho enstatite transition with the attendent volume change. There is either reaction with the melt and diffusion of Mn and Cr into the blue CL En, or there is an overgrowth of red CL En along the fractures. The bulk of the relicts remain blue. The melt enclosing the relicts crystalized to a coarsely radial to dendritic to micro porphyritic texture comprised of enstatite that has a bright red CL with decreasing melt time. The blue CL En has Mn and Cr contents at or below detection limits of the electron probe as described in earlier studies and in natural blue CL En. In the red CL En in this study, the Mn, Al2O3, and Cr are at previously observed levels and the levels change rapidly.

Lofgren, Gary E.↗
NASA NTRS1994

The occurrence of blue luminescing enstatite in E3 and E4 chondrites

Two compositional types of enstatite that emit cathodoluminescence (CL) are known to exist in E3 and E4 chondrites. The first type consists of the most common enstatites that are relatively FeO-poor and emit a red CL. Their CL is apparently activated by the presence of MnO and Cr2O3 in concentrations of 0.2 and 0.6 weight percent. The second type of enstatite is nearly FeO-free, contains no MnO or Cr2O3 and emits a blue CL. The origin of these two types of enstatite and their accompanying chemical and CL differences has long been a subject of discussion. Leitch and Smith first observed to two types and felt the compositional differences were too great to have formed under the same conditions. They postulated the two types of enstatite formed on separate parent bodies and were mixed when these bodies collided. McKinley et al. observed a continuous range of compositions between blue luminescing and red luminescing enstatites and concluded the two types of enstatite formed evidence that blue luminescing pyroxenes were relics that did not completely melt during the heating event which melted other precursor grains, and are distinct from the red CL pyroxene in the chondrules in E chondrites. In order to further clarify the nature and origin of the pyroxene that emits blue CL, the sections listed in another work were examined for the occurrence of blue luminescing enstatite.

Dehart, John M.↗
NASA NTRS1994

The evolution of enstatite and chondrules in unequilibrated enstatite chondrites: Evidence from iron-rich pyroxene

FeO-rich (Fs(sub 6)-34) pyroxene lacking cathodoluminescence (CL), hereafter black pyroxene, is a major constituent of some of the chondrules and fragments in unequilibrated (type 3) enstatite chondrites (UECs). It contains structurally oriented zones of Cr-, Mn-, V-rich, FeO-poor enstatite with red CL, associated with mm-sized blebs of low-Ni, Fe-metal and, in some cases, silica. These occurrences represent clear evidence of pyroxene reduction. The black pyroxene is nearly always rimmed by minor element (Cr, Mn, V)-poor enstatite having a blue CL. More commonly, red and blue enstatites, unassociated with black pyroxene, occur as larger grains in chondrules and fragments, and these constitute the major silicate phases in UECs. The rare earth element (REE) abundance patterns of the black pyroxene are LREE-depleted. The blue enstatite rims, however, have a near-flat to LREE-enriched pattern, approx. 0.5-4x chondritic. The petrologic and trace element data indicate that the black pyroxene is from an earlier generation of chondrules that formed in a nebular region that was more oxidizing than that of the enstatite chondrites. Following solidification, these chondrules experienced a more reducing nebular environment and underwent reduction. Some, perhaps most, of the red enstatite that is common throughout the UECs may be the product of solid-state reduction of black pyroxene. The blue enstatite rims grew onto the surfaces of the black pyroxene and red enstatite as a result of condensation from a nebular gas. The evolutionary history of some of the enstatite and chondrules in enstatite chondrites can be expressed in a four-stage model that includes: Stage 1. Formation of chondrules in an oxidizing nebular environment. Stage 2. Solid-state reduction of the more oxidized chondrules and fragments to red enstatite in a more reducing nebular environment. Stage 3. Formation of blue enstatite rims on the black pyroxene as well as on the red enstatite. Stage 4. Reprocessing, by various degrees of melting, of many of the earlier-formed materials.

Weisberg, Michael K.↗
NASA NTRS1993

Roosevelt County 075: A petrologic chemical and isotopic study of the most unequilibrated known H chondrite

Roosevelt County (RC) 075 was recovered in 1990 as a single 258-gram stone. Classification of this meteorite is complicated by its highly unequilibrated nature and its severe terrestrial weathering, but we favor H classification. This is supported by O isotopes and estimates of the original Fe, Ni metal content. The O isotopic composition is similar to that of a number of reduced ordinary chondrites (e.g., Cerro los Calvos, Willaroy), although RC 075 exhibits no evidence of reduced mineral compositions. Chondrule diameters are consistent with classification as an L chondrite, but large uncertainties in chondrule diameters of RC 075 and poorly constrained means of H, L and LL chondrites prevent use of this parameter for reliable classification. Other parameters are compromised by severe weathering (e.g., siderophile element abundances) or unsuitable for discrimination between unequilibrated H, L and LL chondrites (e.g., Co in kamacite delta C-13). Petrologic subtype 3.2 +/- 0.1 is suggested by the degree of olivine heterogeneity, the compositions of chondrule olivines, the thermoluminescence sensitivity, the abundances and types of chondrules mapped on cathodoluminescence mosaics, and the amount of presolar SiC. The meteorite is very weakly shocked (S2), with some chondrules essentially unshocked and, thus, is classified as an H3.2(S2) chondrite. Weathering is evident by a LREE enrichment due to clay contamination, reduced levels of many siderophile elements, the almost total loss of Fe, Ni metal and troilite, and the reduced concentrations of noble gases. Some components of the meteorite (e.g., type IA chondrules, SiC) appear to preserve their nebular states, with little modification from thermal metamorphism. We conclude that RC 075 is the most equilibrated H chondrite yet recovered and may provide additional insights into the origin of primitive materials in the solar nebula.

Mccoy, T. J.↗
NASA NTRS1995

Chemical and physical studies of type 3 chondrites 12: The metamorphic history of CV chondrites and their components

The induced thermoluminescence (TL) properties of 16 CV and CV-related chondrites, four CK chondrites and Renazzo (CR2) have been measured in order to investigate their metamorphic history. The petrographic, mineralogical and bulk compositional differences among the CV chondrites indicate that the TL sensitivity of the approximately 130 C TL peak is reflecting the abundance of ordered feldspar, especially in chondrule mesostasis, which in turn reflects parent-body metamorphism. The TL properties of 18 samples of homogenized Allende powder heated at a variety of times and temperatures, and cathodoluminescence mosaics of Axtell and Coolidge, showed results consistent with this conclusion. Five refractory inclusions from Allende, and separates from those inclusions, were also examined and yielded trends reflecting variations in mineralogy indicative of high peak temperatures (either metamorphic or igneous) and fairly rapid cooling. The CK chondrites are unique among metamorphosed chondrites in showing no detectable induced TL, which is consistent with literature data that suggests very unusual feldspar in these meteorites. Using TL sensitivity and several mineral systems and allowing for the differences in the oxidized and reduced subgroups, the CV and CV-related meteorites can be divided into petrologic types analogous to those of the ordinary and CO type 3 chondrites. Axtell, Kaba, Leoville, Bali, Arch and ALHA81003 are type 3.0-3.1, while ALH84018, Efremovka, Grosnaja, Allende and Vigarano are type 3.2-3.3 and Coolidge and Loongana 001 are type 3.8. Mokoia is probably a breccia with regions ranging in petrologic type from 3.0 to 3.2. Renazzo often plots at the end of the reduced and oxidized CV chondrite trends, even when those trends diverge, suggesting that in many respects it resembles the unmetamorphosed precursors of the CV chondrites. The low-petrographic types and low-TL peak temperatures of all samples, including the CV3.8 chondrites, indicates metamorphism in the stability field of low feldspar (i.e., less than 800 C) and a metamorphic history similar to that of the CO chondrites but unlike that of the ordinary chondrites.

Guimon, R. Kyle↗
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 NTRS2004

Chondrites and the Protoplanetary Disk, Part 2

Contents include the following: On the Dynamical Evolution of a Nebula and Its Effect on Dust Coagulation and the Formation of Centimeter-sized Particles. The Mineralogy and Grain Properties of the Disk Surfaces in Three Herbig Ae/Be Stars. Astrophysical Observations of Disk Evolution Around Solar Mass Stars. The Systematic Petrology of Chondrites: A Consistent Approach to Assist Classification and Interpretation. Understanding Our Origins: Formation of Sun-like Stars in H II Region Environments. Chondrule Crystallization Experiments. Formation of SiO2-rich Chondrules by Fractional Condensation. Refractory Forsterites from Murchison (CM2) and Yamato 81020 (CO3.0) Chondrites: Cathodoluminescence, Chemical Compositions and Oxygen Isotopes. Apparent I-Xe Cooling Rates of Chondrules Compared with Silicates from the Colomera Iron Meteorite. Chondrule Formation in Planetesimal Bow Shocks: Physical Processes in the Near Vicinity of the Planetesimal. Genetic Relationships Between Chondrules, Rims and Matrix. Chondrite Fractionation was Cosmochemical; Chondrule Fractionation was Geochemical. Chondrule Formation and Accretion of Chondrite Parent Bodies: Environmental Constraints. Amoeboid Olivine Aggregates from the Semarkona LL3.0 Chondrite. The Evolution of Solids in Proto-Planetary Disks. New Nickel Vapor Pressure Measurements: Possible Implications for Nebular Condensates. Chemical, Mineralogical and Isotopic Properties of Chondrules: Clues to Their Origin. Maximal Size of Chondrules in Shock-Wave Heating Model: Stripping of Liquid Surface in Hypersonic Rarefied Gas Flow. The Nature and Origin of Interplanetary Dust: High Temperature Components. Refractory Relic Components in Chondrules from Ordinary Chondrites. Constraints on the Origin of Chondrules and CAIs from Short-lived and Long-lived Radionuclides. The Genetic Relationship Between Refractory Inclusions and Chondrules. Contemporaneous Chondrule Formation Between Ordinary and Carbonaceous Chondrites. Chondrules and Isolated Grains in the Fountain Hills Bencubbinite. Implications of Chondrule Formation in a Gas of Solar Composition. Implications of Meteoritic Cl-36 Abundance for the Origin of Short-lived Radionuclides in the Early Solar System. Size Sorting and the Chondrule Size Spectrum. Comparative Study of Refractory Inclusions from Different Groups of Chondrites. In Situ Investigation of Mg Isotope Distributions in an Allende CAI by Combined LA-ICPMS and SIMS Analyses Photochemical Speciation of Oxygen Isotopes in the Solar Nebula.

Source record↗
NASA NTRS2005

Lunar and Planetary Science XXXVI, Part 14

Contents include the following: Destruction of Presolar Silicates by Aqueous Alteration Observed in Murchison CM2 Chondrite. Generation of Chondrule Forming Shock Waves in Solar Nebula by X-Ray Flares. TEM and NanoSIMS Study of Hydrated/Anhydrous Phase Mixed IDPs: Cometary or Asteroidal Origin? Inflight Calibration of Asteroid Multiband Imaging Camera Onboard Hayabusa: Preliminary Results. Corundum and Corundum-Hibonite Grains Discovered by Cathodoluminescence in the Matrix of Acfer 094 Meteorite. Spatial Extent of a Deep Moonquake Nest A Preliminary Report of Reexamination. Modal Abundances of Carbon in Ureilites: Implications for the Petrogenesis of Ureilites. Trapped Noble Gas Components and Exposure History of the Enstatite Chondrite ALH84206. Deep-seated Crustal Material in Dhofar Lunar Meteorites: Evidence from Pyroxene Chemistry. Numerical Investigations of Kuiper Belt Binaries. Dust Devils on Mars: Effects of Surface Roughness on Particle Threshold. Hecates Tholus, Mars: Nighttime Aeolian Activity Suggested by Thermal Images and Mesoscale Atmospheric Model Simulations. Are the Apollo 14 High-Al Basalts Really Impact Melts? Garnet in the Lunar Mantle: Further Evidence from Volcanic Glass Beads. The Earth/Mars Dichotomy in Mg/Si and Al/Si Ratios: Is It Real? Dissecting the Polar Asymmetry in the Non-Condensable Gas Enhancement on Mars: A Numerical Modeling Study. Cassini VIMS Preliminary Exploration of Titan s Surface Hemispheric Albedo Dichotomy. An Improved Instrument for Investigating Planetary Regolith Microstructure. Isotopic Composition of Oxygen in Lunar Zircons Preliminary Design of Visualization Tool for Hayabusa Operation. Size and Shape Distributions of Chondrules and Metal Grains Revealed by X-Ray Computed Tomography Data. Properties of Permanently Shadowed Regolith. Landslides in Interior Layered Deposits, Valles Marineris, Mars: Effects of Water and Ground Shaking on Slope Stability. Mars: Recent and Episodic Volcanic, Hydrothermal, and Glacial Activity Revealed by Mars Express High Resolution Stereo Camera (HRSC). The Cratering Record of the Saturnian Satellites Phoebe, Tethys, Dione and Iapetus in Comparison: First Results from Analysis of the Cassini ISS Imaging Data. Joint Crossover Solutions of Altimetry and Image Data on 433 Eros. The Martian Soil as a Geochemical Sink for.

Source record↗
NASA NTRS2009

Electron Backscatter Diffraction (EBSD) Analysis and U-Pb Geochronology of the Oldest Lunar Zircon: Constraining Early Lunar Differentiation and Dating Impact-Related Deformation

The evolution of the early moon was dominated by two processes (i) crystallization of the Lunar Magma Ocean (LMO) and differentiation of potassium-rare earth element-phosphorous-rich residual magma reservoir referred to as KREEP, and (ii) an intense meteorite bombardment referred to as lunar cataclysm . The exact timing of these processes is disputed, and resolution relies on collection and interpretation of precise age data. This study examines the microstructure and geochronology of zircon from lunar impact breccias collected during the Apollo 17 mission. A large zircon clast within lunar breccia 72215,195 shows sector zoning in optical microscopy, cathodoluminescence (CL) imaging and Raman mapping, and indicates that it was a relict fragment of a much larger magmatic grain. Sensitive high resolution ion microprobe (SHRIMP) U-Pb analysis of the zircon shows that U and Th concentration correlate with sector zoning, with darkest CL domains corresponding with high-U and Th (approx.150 and approx.100 ppm respectively), and the brightest-CL sectors containing approx.30-50 ppm U and approx.10-20 ppm Th. This indicates that variations in optical CL and Raman properties correspond to differential accumulation of alpha-radiation damage in each sector. Electron backscatter diffraction (EBSD) mapping shows that the quality of electron backscatter patterns (band contrast) varies with sector zoning, with the poorest quality patterns obtained from high-U and Th, dark-CL zones. EBSD mapping also reveals a deformation microstructure that is cryptic in optical, CL and Raman imaging. Two orthogonal sets of straight discrete and gradational low-angle boundaries accommodate approx.12 misorientation across the grain. The deformation bands are parallel to the crystallographic {a}-planes of the zircon, have misorientation axes parallel to the c-axis, and are geometrically consistent with formation by dislocation creep associated with <100>{010} slip. The deformation bands are unlike curved morphology of crystal-plastic microstructures in tectonically deformed terrestrial zircon, and geometrically similar to dislocation microstructures reported in experimentally shocked zircon. We interpret these crystal-plastic deformation microstructures to have resulted from a significant impact, either directly from impact shock, or during ductile flow directly following the impact. The deformation bands appear to continue undeflected through the non-indexed, radiation-damaged areas of the grain, which suggests that the orientation variation predates any significant mechanical weakening from radiation damage in the grain, and therefore occurred early in its history.

Timms, Nick↗