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  • 1
    Electronic Resource
    Electronic Resource
    [s.l.] : Macmillian Magazines Ltd.
    Nature 425 (2003), S. 65-69 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The Late Devonian genus Ichthyostega was for many decades the earliest known tetrapod, and the sole representative of a transitional form between a fish and a land vertebrate. However, despite being known since 1932 (ref. 1) from a large collection of specimens, its morphology ...
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0003-2697
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1520-6904
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Abstract In order to study the thermal structure of active thrust belts, we have developed a numerical model of conductive heat transfer between thrust sheets during deformation. Our finite difference approach alternates small, instantaneous increments of displacement and isotherm translation with conductive relaxation of perturbed isotherms. In each step, conduction occurs for a length of time equal to the displacement increment divided by the thrust velocity. Computer simulations demonstrate that conductive heat transfer is significant during deformation and that temperatures in hanging-wall rocks decrease while temperatures in foot-wall rocks increase over distances of up to 10 km from the thrust surface. When the effects of internal heat production are also calculated, heating of foot-wall rocks exceeds cooling of hanging-wall rocks. Rocks located between two thrusts may experience a complicated temperature–time path of early heating followed by cooling. These models help to explain the rapid metamorphism of rocks in the Taconian thrust belt in the northern Appalachians of New England soon after deposition of the youngest sediments.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Macroscopic textures resulting from different atomic-scale mechanisms for metamorphic crystallization display different degrees of order, clustering, intergrowth and relative isolation of porphyroblasts. Data on the sizes and locations of thousands of crystals in a three-dimensional volume are required to identify reliably the mechanisms governing nucleation and growth of porphyroblasts from these textural features. These data can now be acquired by means of high-resolution computed X-ray tomography. Numerical models that simulate porphyroblast formation governed by either interface-controlled or diffusion-controlled reaction mechanisms indicate that quantitative textural analysis can discriminate between these possibilities. These numerical models also allow a comparison between textures predicted for different crystallization mechanisms and textures measured in natural samples, from which inferences can be drawn concerning the relative importance of these mechanisms in nature. An independent test of the validity of such inferences is possible for porphyroblasts such as garnet that may preserve prograde growth zoning and allow the examination of normalized radius–rate relations.
    Type of Medium: Electronic Resource
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  • 6
    Publication Date: 2003-01-01
    Description: High-resolution and ultra-high-resolution X-ray computed tomography are rapid, non-destructive and extremely powerful techniques for three-dimensional examination and measurement of a great variety of geological materials and specimens with sizes from several millimetres to several decimetres. A review of recent applications in petrology, meteoritics and palaeontology, which utilized an instrument optimized for geological studies (High-Resolution X-ray Computed Tomography Facility of the University of Texas at Austin), documents an abundance of novel scientific results and illuminates the potential for still broader application of these techniques in the earth sciences.
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  • 7
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    Unknown
    International Society for Reef Studies
    In:  In: Proceedings 9th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000. , ed. by Moosa, M. K., Soemodihardjo, S. and Soegiarto, A. International Society for Reef Studies, [s.l.], pp. 365-371.
    Publication Date: 2019-06-26
    Description: Density bands in corals have long been considered a valuable tool for reconstructing past environmental and climatic conditions. X-radiographs reveal density banding within a skeletal slab, but provide little information about the skeletal variability causing banding. The skeletal architecture of Diploria strigosa was analyzed by X-radiography, X-ray computed tomography, and image analysis to identify the specific skeletal elements responsible for density banding. Threedimensional skeletal reconstructions, density-band reconstructions, and skeletal animations were created to assess the apparent changes in skeletal structure associated with density banding. Measurements were made of the dissepiments, thecae, septa, and columellae to determine how element size related to density banding. Dissepiment spacing and thecal wall thickness exhibited no consistent variation relative to density. Density bands were associated with thickening of septal and columellar structures. X-ray computed tomography provided an effective tool for revealing density banding as well as measuring variations in skeletal elements.
    Type: Book chapter , NonPeerReviewed
    Format: text
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  • 8
    Publication Date: 2019-07-12
    Description: The L chondrite Patuxent Range (PAT) 41 91501 is an 8.5-kg unshocked, homogeneous, igneous-textured impact melt that cooled slowly compared to other meteoritic impact melts in a crater floor melt sheet or sub-crater dike. We conducted mineralogical and tomographic studies of previously unstudied mm- to cm-sized metal-sulfide-vesicle assemblages and chronologic studies of the silicate host. Metal-sulfide clasts constitute about 1 vol.%, comprise zoned taenite, troilite and pentlandite, and exhibit a consistent orientation between metal and sulfide and of metal-sulfide contacts. Vesicles make up approximately 2 vol.% and exhibit a similar orientation of long axes. Ar-39-Ar-40 measurements date the time of impact at 4.461 +/- 0.008 Gyr B.P. Cosmogenic noble gases and Be-10 and Al-2l activities suggest a pre-atmospheric radius of 40-60 cm and a cosmic ray exposure age of 25-29 Myr, similar to ages of a cluster of L chondrites. PAT 91501 dates the oldest known impact on the L chondrite parent body. The dominant vesicle-forming gas was S2 (approximately 15-20 ppm), which formed in equilibrium with impact-melted sulfides. The meteorite formed in an impact melt dike beneath a crater, as did other impact melted L chondrites, such as Chico. Cooling and solidification occurred over approximately 2 hours. During this time, approximately 90% of metal and sulfide segregated from the local melt. Remaining metal and sulfide grains oriented themselves in the local gravitational field, a feature nearly unique among meteorites. Many of these metal sulfide grains adhered to vesicles to form aggregates that may have been close to neutrally buoyant. These aggregates would have been carried upward with the residual melt, inhibiting further buoyancy-driven segregation. Although similar processes operated individually in other chondritic impact melts, their interaction produced the unique assemblage observed in PAT 91501.
    Keywords: Lunar and Planetary Science and Exploration
    Format: application/pdf
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  • 9
    Publication Date: 2019-07-13
    Description: Established contemporary conservation methods within the fields of Natural and Cultural Heritage encourage an interdisciplinary approach to preservation of heritage material (both tangible and intangible) that holds "Outstanding Universal Value" for our global community. NASA's lunar samples were acquired from the moon for the primary purpose of intensive scientific investigation. These samples, however, also invoke cultural significance, as evidenced by the millions of people per year that visit lunar displays in museums and heritage centers around the world. Being both scientifically and culturally significant, the lunar samples require a unique conservation approach. Government mandate dictates that NASA's Astromaterials Acquisition and Curation Office develop and maintain protocols for "documentation, preservation, preparation and distribution of samples for research, education and public outreach" for both current and future collections of astromaterials. Documentation, considered the first stage within the conservation methodology, has evolved many new techniques since curation protocols for the lunar samples were first implemented, and the development of new documentation strategies for current and future astromaterials is beneficial to keeping curation protocols up to date. We have developed and tested a comprehensive non-destructive documentation technique using high-resolution image-based 3D reconstruction and X-ray CT (XCT) data in order to create interactive 3D models of lunar samples that would ultimately be served to both researchers and the public. These data enhance preliminary scientific investigations including targeted sample requests, and also provide a new visual platform for the public to experience and interact with the lunar samples. We intend to serve these data as they are acquired on NASA's Astromaterials Acquisistion and Curation website at http://curator.jsc.nasa.gov/. Providing 3D interior and exterior documentation of astromaterial samples addresses the increasing demands for accessability to data and contemporary techniques for documentation, which can be realized for both current collections as well as future sample return missions.
    Keywords: Computer Programming and Software; Lunar and Planetary Science and Exploration
    Type: JSC-CN-32836 , Lunar and Planetary Sciences Conference; 16-20 Mar. 2015; The Woodlands, TX; United States
    Format: application/pdf
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  • 10
    Publication Date: 2019-07-13
    Description: Providing web-based data of complex and sensitive astromaterials (including meteorites and lunar samples) in novel formats enhances existing preliminary examination data on these samples and supports targeted sample requests and analyses. We have developed and tested a rigorous protocol for collecting highly detailed imagery of meteorites and complex lunar samples in non-contaminating environments. These data are reduced to create interactive 3D models of the samples. We intend to provide these data as they are acquired on NASA's Astromaterials Acquisition and Curation website at http://curator.jsc.nasa.gov/.
    Keywords: Computer Programming and Software; Lunar and Planetary Science and Exploration
    Type: JSC-CN-31842 , Annual Meeting of the Meteoritical Society; 7-12 Sep. 2014; Casablanca; Morocco
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