Resolution of impact‐related microstructures in lunar zircon: A shock‐deformation mechanism map     

Nicholas E. TIMMS  Steven M. REDDY  David HEALY  Alexander A. NEMCHIN  Marion L. GRANGE  Robert T. PIDGEON  Robert HART 《Meteoritics & planetary science》2012,47(1):120-141

Abstract– The microstructures of lunar zircon grains from breccia samples 72215, 73215, 73235, and 76295 collected during the Apollo 17 mission have been characterized via optical microscopy, cathodoluminescence imaging, and electron backscatter diffraction mapping. These zircon grains preserve deformation microstructures that show a wide range in style and complexity. Planar deformation features (PDFs) are documented in lunar zircon for the first time, and occur along {001}, {110}, and {112}, typically with 0.1–25 μm spacing. The widest PDFs associated with {112} contain microtwin lamellae with 65°/<110> misorientation relationships. Deformation bands parallel to {100} planes and irregular low‐angle (<10°) boundaries most commonly have <001> misorientation axes. This geometry is consistent with a dislocation glide system with <100>{010} during dislocation creep. Nonplanar fractures, recrystallized domains with sharp, irregular interfaces, and localized annealing textures along fractures are also observed. No occurrences of reidite were detected. Shock‐deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon. PDFs form along a limited number of specific {hkl} planes that are perpendicular to directions of high Young’s modulus, suggesting that PDFs are likely to be planes of longitudinal lattice damage. Twinned {112} PDFs also contain directions of high shear modulus. A conceptual model is proposed for the development of different deformation microstructures during an impact event. This “shock‐deformation mechanism map” is used to explain the relative timing, conditions, and complexity relationships between impact‐related deformation microstructures in zircon.  相似文献   

Fully coupled generalised hybrid-mixed finite element approximation of two-phase two-component flow in porous media. Part II: numerical scheme and numerical results     

Estelle Marchand  Torsten Müller  Peter Knabner 《Computational Geosciences》2012,16(3):691-708

We consider the modeling and simulation of compositional two-phase flow in a porous medium, where one phase is allowed to vanish or appear. The modeling of Marchand et al. (in review) leads to a nonlinear system of two conservation equations. Each conservation equation contains several nonlinear diffusion terms, which in general cannot be written as a function of the gradients of the two principal unknowns. Also the diffusion coefficients are not necessarily explicit local functions of them. For the generalised mixed finite elements approximation, Lagrange multipliers associated to each principal unknown are introduced, the sum of the diffusive fluxes of each component is explicitly eliminated and the static condensation leads to a “global” nonlinear system of equations only in the Lagrange multipliers also including complementarity conditions to cope with vanishing or appearing phases. After time discretisation, this system can be solved at each time step using a semi-smooth Newton method. The static condensation involves “local” nonlinear systems of equations associated to each element, solved also by a semismooth Newton method. The algorithm is successfully applied to 1D and 2D examples of water–hydrogen flow involving gas phase appearance and disappearance.  相似文献   

New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group (Cameroon, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton     

Sébastien Owona  Marion Tichomirowa  Lothar Ratschbacher  Joseph Mvondo Ondoa  Dieudonné Youmen  J?rg Pf?nder  Félix M. Tchoua  Pascal Affaton  Georges Emmanuel Ekodeck 《International Journal of Earth Sciences》2012,101(7):1689-1703

Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and ²⁰⁷Pb/²⁰⁶Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624?±?2?Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The ⁸⁷Rb/⁸⁶Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. ⁸⁷Rb/⁸⁶Sr isochron ages differ from sample to sample (599?±?3, 572?±?4, 554?±?5, 540?±?5?Ma) yielding the oldest Neoproterozoic age (~600?Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540?Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The ⁸⁷Rb/⁸⁶Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540?Ma) more closely to the border of the Congo Craton.  相似文献   

Cathodoluminescence microanalysis of silica and amorphized quartz     

Marion A. Stevens-Kalceff 《Mineralogy and Petrology》2013,107(3):455-469

Cathodoluminescence (CL) techniques are used to investigate the defect structures of pure synthetic silicon dioxide (SiO₂) polymorphs. Pure, synthetic Types I, II, III and IV amorphous SiO₂ polymorphs, pure, synthetic crystal α-SiO₂ and pure, synthetic amorphized crystal α-SiO₂ have been investigated and their characteristic defects have been determined and compared. The CL emission from pure SiO₂ polymorphs is generally related to local point defects in the tetrahedrally coordinated SiO₂ host lattice. A range of CL emissions associated with non bridging oxygen defects, oxygen deficient defects and the radiative recombination of self trapped excitons are observed from both the pure synthetic crystal and amorphous SiO₂ polymorphs. In addition CL emissions associated with residual concentrations of Aluminium impurities are also observed from α-SiO₂ (quartz) and Type I and II a-SiO₂ (fused quartz). Localised amorphous micro-volumes may exist within natural α-SiO₂ due to the presence of a high concentration of pre-existing or induced defects. Amorphization of α-SiO₂ diminishes the difference between the defect structures and associated CL from α-SiO₂ and a-SiO₂. Thus CL investigation of the defect structure of a-SiO₂ polymorphs provides useful insight into the microstructure of amorphized α-SiO₂.  相似文献   

Low signal-to-noise event detection based on waveform stacking and cross-correlation: application to a stimulation experiment   总被引：1，自引：1，他引：0  

Katrin Plenkers  Joachim R. R. Ritter  Marion Schindler 《Journal of Seismology》2013,17(1):27-49

We study the microseismicity (M _L ?<?2) in the region of Landau, SW Germany. Here, due to thick sediments (~3?km) and high cultural seismic noise, the signal-to-noise ratio is in general very low for microearthquakes. To gain new insights into the occurrence of very small seismic events, we apply a three-step detection approach and are able to identify 207 microseismic events (?1?<?M _L ?<?~1) with signal-to-noise ratios smaller than 3. Recordings from a temporary broadband network are used with station distances of approximately 10?km. First, we apply a short-term to long-term average detection algorithm for data reduction. The detection algorithm is affected severely by transient noise signals. Therefore, the most promising detections, selected by coinciding triggers and high-amplitude measures, are reviewed manually. Thirteen seismic events are identified in this way. Finally, we conduct a cross-correlation analysis. As master template, we use the stacked waveforms of five manually detected seismic events with a repeating waveform. This search reveals additional 194 events with a cross-correlation coefficient exceeding 0.65 which ensures a stable identification. Our analysis shows that the repeating events occurred during the stimulation of a geothermal reservoir within a source region of only about 0.5?km³. Natural background seismicity exceeding our detection level of M _L ?~?0.7 is not found in the region of Landau by our analysis.  相似文献   

Composition and heterogeneity of anorthositic impact melt at Mistastin Lake crater, Labrador     

Cassandra L. Marion  Paul J. Sylvester 《Planetary and Space Science》2010,58(4):552-573

Anorthositic impact melt rocks, their target rocks (principally anorthosite, mangerite, granodiorite) and zircon clasts from the ∼36-Ma-old, 28-km-wide Mistastin Lake crater of northern Labrador (55°53′N; 63°18′W) have been examined in order to evaluate the scale and origin of compositional heterogeneities in impact melts produced in craters of moderate size. In particular we assess whether and, if so, how the initial composition of the impact melt was modified as it entrained mineral clasts derived from the underlying rocks over which it flowed when it moved away from the shock-induced, central melting zone. A secondary goal was to determine if zircon clasts in the impact melts are present in the proportions of their target rock sources and/or the substrate lithologies over which they flowed. Chemical compositions of bulk samples of 33 melt rocks and 14 target rocks were measured by XRF and SN-ICPMS. Matrix compositions of nine samples of impact melt rocks were determined by EPMA and LA-ICPMS. Zircon grains from four samples of target rock and zircon clasts from three samples of impact melt rock were measured for multi-element composition, U-Pb age and Hf-isotopic composition by LA-(MC)-ICPMS.The data reveal compositional heterogeneities in the impact melts on the scales of both bulk samples and matrices. Bulk samples can be divided into compositions with high and low concentrations of high-field-strength elements (HFSE; Ti, Zr, Nb) and Fe, Ba, Ce and Y. High HFSE-type melt rocks formed when impact melt entrained large quantities of clasts from mangerite, which is rich in HFSE. Matrix compositions of bulk samples do not show the HFSE distinction but are affected by the introduction of low-temperature melts from the clasts to form dispersed, micron-scale silica-rich heterogeneities.The best estimate of sources of the initial impact melt is ∼73% anorthosite, ∼7% mangerite and ∼20% granodiorite, based on least-squares modeling of major-element compositions of the matrices of thinner flows. Zircon derived from anorthosite can be distinguished from zircon from mangerite and granodiorite by higher Nb/Ta and Eu/Eu* ratios and more negative initial ε Hf values. Zircon clasts >40 mm in size in the impact melt rocks are dominantly or exclusively derived from mangerite and granodiorite rather than from anorthosite. Because anorthosite was the principal source of impact melt at Mistastin, zircon may be a poor provenance indicator for anorthosite contributions to impact melts, particularly where grains smaller than 40 microns are not analyzed.  相似文献   

Distribution and chronology of Pleistocene permafrost features in France: Database and first results     

Pascal Bertran  Eric Andrieux  Pierre Antoine  Sylvie Coutard  Laurent Deschodt  Philippe Gardère  Marion Hernandez  Claude Legentil  Arnaud Lenoble  Morgane Liard  Norbert Mercier  Olivier Moine  Luca Sitzia  Brigitte Van Vliet‐Lanoë 《Boreas: An International Journal of Quaternary Research》2014,43(3):699-711

Numerous periglacial features (polygons, nets, soil stripes, ice‐wedge pseudomorphs and sand‐wedge casts, involutions) have been recorded in France by examining bibliographical sources and aerial photographs. These data show that a large part of France was affected by permafrost during the Pleistocene and only the southern Aquitaine Basin and Languedoc seem to have been beyond its maximum extent. The first OSL ages obtained from the aeolian infill of wedge structures indicate that at least two phases of thermal contraction cracking occurred in southwestern France between ～25 and 36 ka. Chronostratigraphical data from loess in northern France indicate that these episodes correspond to the formation of ice‐wedge networks associated with tundra gleys. In the latter region, two additional permafrost episodes probably occurred during the Last Glacial, the older one corresponding to the end of Marine Isotope Stage (MIS) 4 around 60 ka and the more recent one to MIS 2 around 19–16 ka. Although stratigraphical data indicate that these episodes were relatively short (about one millennium), relict permafrost may have existed for longer periods in northern France.  相似文献   

Statistical probability distribution of snow depth at the model sub‐grid cell spatial scale     

Wolf‐Dietrich Marchand  nund Killingtveit 《水文研究》2005,19(2):355-369

Comprehensive snow depth data, collected using georadar and hand probing, were used for statistical analyses of snow depths inside 1 km grid cells. The sub‐grid cell spatial scale was 100 m. Statistical distribution functions were found to have varying parameters, and an attempt was made to connect these statistical parameters to different terrain variables. The results showed that the two parameters mean and standard deviation of snow depth were significantly related to the sub‐grid terrain characteristics. Linear regression models could explain up to 50% of the variation for both of the snowcover parameters mentioned. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Impact of plasma sheath on rocket-based E-region ion measurements     

Nadia?ImtiazEmail author  Johnathan?Burchill  Richard?Marchand 《Astrophysics and Space Science》2015,355(1):23-32

We model the particle velocity distribution functions around the entrance window of the Suprathermal Ion Imager (SII). The SII sensor was mounted on a 1 m boom carried by the scientific payload of NASA rocket 36.234 as part of Joule II mission to investigate Joule heating in the E-region ionosphere. The rocket flew above Northern Alaska on 19 January 2007. The payload was spin-stabilized with a period of 1.6 s, giving an apparent rotation of the ion flow velocity in the frame of reference of the payload. The SII sensor is an electrostatic analyzer that measures two dimensional slices of the distribution of the kinetic energies and arrival-angles of low energy ions. The study is concerned with the interpretation of data obtained from the SII sensor. For this purpose, we numerically investigate ram velocity effects on ions velocity distributions in the vicinity of the SII sensor aperture at an altitudes of approximately 150 km. The electrostatic sheath profiles surrounding the SII sensor, boom and payload are calculated numerically with the PIC code PTetra. It is observed that the direction of the ion flow velocity modifies the plasma sheath potential profile. This in turn impacts the velocity distributions of NO⁺ and \(\mathrm{O}_{2}^{+}\) ions at the aperture of the particle sensor. The velocity distribution functions at the sensor aperture are calculated by using test-particle modeling. These particle distribution functions are then used to inject particles in the sensor, and calculate the fluxes on the sensor microchannel plate (MCP), from which comparisons with the measurements can be made.  相似文献   

First alkaline magmatism during Iberia–Newfoundland rifting     

Marion Grange  Urs Schärer  Guy Cornen  Jacques Girardeau 《地学学报》2008,20(6):494-503

The age and origin of alkaline rocks emplaced into the sediments of the rift‐related continental Lusitanian basin were investigated to constrain earliest magmatic activity occurring prior to oceanic plate formation between Iberia and Newfoundland. The U–Pb titanite ages are 146.5 ± 1.6 (2σSTERR), 145.3 ± 1.4 and 142.3 ± 1.0 Ma, and initial Pb isotopic ratios of feldspars lie at 18.418–18.978 for ²⁰⁶Pb/²⁰⁴Pb, at 15.594–15.925 for ²⁰⁷Pb/²⁰⁴Pb and at 37.105–39.216 for ²⁰⁸Pb/²⁰⁴Pb. Initial ⁸⁷Sr/⁸⁶Sr ratios measured in the same feldspar fractions lie at 0.705409–0.706462. This episode of magmatic activity lasting for at least 4.2 ± 2.6 Myr most likely marks a phase of maximum lithospheric thinning during which zones of weakness were created to allow deep magmas to reach the surface. Such zones are preferentially re‐activated Palaeozoic faults of the Iberian plate. The isotope data suggest that the dominant volume of alkaline magmas was generated by partial melting of the metasomatized subcontinental Iberian mantle.  相似文献