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Discussion: Using the above criterion, we found that PREM, IASP91and AK135 (RMS misfit of 0.48s, 0.52s, and 0.49s, respectively) are allmarginally unacceptable. Slight perturbations to IASP91 yielded modelsthat fit our acceptance criterion. Of all models tested, the one with thelowest RMS misfit (0.27s) used IASP91 as a base model, with a PREMlikevelocity of 8.040 km/s at the top of the core. We also tested modelscontaining a thin (10-km thick), high-velocity layer (8.23 and 8.36 km/s)beneath the CMB. These velocity values correspond with possible valuesfor a light S-rich, immiscible fluid layer for core temperatures of 3500and 4300 K [6]. Although the addition of such a layer increases the misfitfrom the best-fitting model, these two models still meet our acceptancecriteria since the misfit (0.28s and 0.30s) is still below the scatter in theobserved data. The global stacked section will improve as more databecomes available, resulting in less scatter. This will allow for strongerconstraints on the velocity structure of the outermost core.Acknowledgements: Data used for this study were obtained fromthe waveform archives of IRIS and POLARIS. This work was funded byNSERC (DWE) and OGSST (CA).References:[1] Eaton, D.W. and Kendall, J.M. 2006, Phys. Earth Planet. Int. 155,104-119[2] Souriau, A. et al. 2003, Geophys. Res. Lett. 30, doi:10.1029/2003GL017008[3] Kennett, B.L.N. and Engdahl, E.R. 1991, Geophys. J. Int. 122,429-465[4] Dziewonski, A.M. and Anderson, D.L. 1981, Phys. Earth Planet. Int.25, 297-356[5] Kennett, B.L.N. et al. 1995, Geophys. J. Int. 122, 108-124[6] Helffrich, G. and S. Kaneshima 2004, Science 306, 2239-2242.deformation fabric in Archean xenolith samples of the African Mohoat 2023 ±15 Ma, indicating deep-level flow coeval with rebound of the2020 ±3-Ma giant Vredefort impact crater. The deformed zircons arerestricted to the main mylonitic fabric, whereas undeformed zirconsin the same sample occur in the cores of garnet porphyroclasts. LREEenrichment of the deformed and isotopically disturbed ductile-zirconzones suggests participation of fluid in the up to 100 percent evacuationof radiogenic Pb from the zircon lattice. Our study demonstrates a newmethod for strain chronometry of planetary materials while offering afirst view into post-impact processes at the crust/mantle transition ofcontinental lithosphere.Acknowledgements: De Beers is gratefully acknowledged for accessto the kimberlite property for sample collection, and Dr. Daniel Schulze,University of Toronto, is thanked for guidance in xenolith research.References:[1] Wittman, A., Kenkmann, T., Schmitt, R.T., and Stöffler, D. 2006,Meteoritics and Planetary Science 41, 433-454Lovina, A New Ataxite from Bali, Indonesia. S.A. Kissin 1 , R.L.Flemming 2 , P. Corcoran 2 , P.J.A. McCausland 2 , and M. Biesinger 3 1 Dept. ofGeology, Lakehead U., Thunder Bay ON P7B 5E1 (sakissin@lakeheadu.ca); 2 Dept. of Earth Sciences, U. Western Ontario, London ON N6A 5B7(rflemmin@uwo.ca). 3 Surface Science Western, U. Western Ontario,London ON N6A 5B7Zircon Dating of Post-Impact Deep Lithosphere Flow. D.E.Moser 1 , W.J. Davis 2 , S.R. Reddy 3 , R. Flemming 1 , R.J. Hart 4 1 Dept. of EarthSciences, U. of Western Ontario, London ON N6A 5B7 (desmond.moser@uwo.ca); 2 Geological Survey of Canada, 601 Booth St, Ottawa ON K1A0E8; 3 Dept. of Applied Geology, Curtin University of Technology, GPOBox U1987, Perth WA 6845, Australia, 4 iThemba labs, Johannesburg,South Africa.Introduction: Although primarily known as an invaluable tool forU-Pb geochronology, the refractory nature of zircon (ZrSiO 4) allows it tosurvive the intense pressures and temperatures of impact environmentswhile developing a range of unique microstructures and materialchanges related to the degree of shock metamorphism and post-shockheating [1]. Zircon shock morphotypes form over a large pressure range(20 GPa to >80 GPa) and temperatures as high as the zircon meltingtemperature (1676 ºC). Microstructures include the pressure-induceddevelopment of multiple sets of parallel, micron-scale planar features,local transformation of zircon to its high-pressure polymorph Reidite,domainal conversion to diaplectic zircon glass, partial to completerecrystallization into granular zircon aggregate, and finally totalbreakdown to Baddeleyite (ZrO 2) and silica at extreme temperatures.Comparatively little is known regarding impact-related effects on zircondeep beneath impact craters. Here we report that indirect effects includecrystal-plastic deformation and trace-element alteration of zircon atthe crust-mantle boundary, as discovered in xenoliths from kimberlitecross-cutting the ancestral Vredefort impact structure, South Africa.Methods: We have combined in situ and single-zircon U-Pb isotopicdating (ID-TIMS and SHRIMP) with colour SEM cathodoluminescenceanalysis of zircon in grain mounts and petrographic thin sections. Strainanalysis of single zircons was carried out by micro-XRD and ElectronBackscatter Diffraction (EBSD) techniques.Discussion: Our results allow us to directly date a crystal-plasticFigure 5 — The 8.2-kg Lovina ataxite main mass (cm scale). The ironshows deep vugs in its bottom and lower surfaces and odd, ziggarautlikepyramids on its upper surface.Introduction: A new ataxite, Lovina, has been discovered and itsdescription submitted to the nomenclature committee of the MeteoriticalSociety. Lovina was found by Dan Richer in January 1981 on a beach inBali, Indonesia. Richer brought it to UWO where it was investigated byin situ micro-XRD [1] and discovered to contain abundant taenite withawaruite, but no kamacite. A polished thin section was prepared forpetrographic and SEM observations, which confirmed its meteoriticorigin.Description of Specimen: Lovina is an elongate football-sized 8.2-kgiron with a strongly weathered exterior featuring cm-sized pyramidalprojections on its upper surface (as oriented on beach). The pyramidshave basal ribs spaced at ~mm intervals, which are more resistantto weathering. In polished section, the ribs are not the expression of56 Building for the International Year of Astronomy (IYA2009)JRASC April / avril 2008

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