Isotopic fractionation of zinc in tektites |
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| Authors: | Frederic Moynier Pierre Beck Fred Jourdan Qing-Zhu Yin Uwe Reimold Christian Koeberl |
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| Institution: | 1. Department of Earth and Planetary Sciences, Washington University in St Louis, One Brookings Drive, St Louis, MO 63130, USA;2. Department of Geology, University of California Davis. One Shields Avenue, Davis, CA 95616, USA;3. Laboratoire de Planetologie, Universite Joseph Fourier, CNRS/INSU, Bat. Physique D, BP 53, 38041 Grenoble Cedex 9, France;4. Western Australian Argon Isotope Facility, Department of Applied Geology & JdL-CMS, Curtin University of Technology, GPO Box U1987, Perth, WA 6845; Australia;5. Museum of Natural History (Mineralogy), Humboldt University Berlin, Invalidenstrasse 43, 10115 Berlin, Germany;6. Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria;1. Institut für Planetologie, Westfälische Wilhelms – Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany;2. Origins Laboratory, Department of Geophysical Sciences, The University of Chicago, IL 60637, USA;1. Faculty of Science, Vrije Universiteit, Amsterdam, The Netherlands;2. The Geophysical Laboratory, Carnegie Institution of Science, Washington D.C., United States;3. Institute of Mineralogy, University of Münster, Germany;4. Faculty of Geosciences, Utrecht University, The Netherlands;1. Geology & Geophysics, School of Ocean, Earth Science & Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA;2. Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science & Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA;3. Australian Synchrotron, Clayton, Victoria 3168, Australia;4. Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, USA;5. School of Earth, Atmosphere & Environment, Monash University, Melbourne, Victoria 3800, Australia;1. Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstrasse 74-100, 12249 Berlin, Germany;2. Universität Bayreuth, Bayerisches Geoinstitut, 95440 Bayreuth, Germany;1. Department of Environment, Earth and Ecosystems, Centre for Earth, Planetary, Space & Astronomical Research, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK;2. Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland;3. Bristol Isotope Group, School of Earth Sciences, University of Bristol, Wills Memorial Building, BS8 1RJ, UK;1. School of Earth and Atmospheric Sciences, Queensland University of Technology, Australia;2. Isotope Geochemistry, Department of Geosciences, Eberhard-Karls University of Tuebingen, Germany;3. Department of Geology, University of Johannesburg, South Africa |
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| Abstract: | Tektites are terrestrial natural glasses produced during a hypervelocity impact of an extraterrestrial projectile onto the Earth's surface. The similarity between the chemical and isotopic compositions of tektites and terrestrial upper continental crust implies that the tektites formed by fusion of such target rock. Tektites are among the driest rocks on Earth. Although volatilization at high temperature may have caused this extreme dryness, the exact mechanism of the water loss and the behavior of other volatile species during tektite formation are still debated. Volatilization can fractionate isotopes, therefore, comparing the isotope composition of volatile elements in tektites with that of their source rocks may help to understand the physical conditions during tektite formation.For this study, we have measured the Zn isotopic composition of 20 tektites from four different strewn fields. Almost all samples are enriched in heavy isotopes of Zn compared to the upper continental crust. On average, the different groups of tektites are isotopically distinct (listed from the isotopically lightest to the heaviest): Muong-Nong type indochinites (δ66/64Zn = 0.61 ± 0.30‰); North American bediasites (δ66/64Zn = 1.61 ± 0.49‰); Ivory Coast tektites (δ66/64Zn = 1.66 ± 0.18‰); the Australasian tektites (others than the Muong Nong-type indochinites) (δ66/64Zn = 1.84 ± 0.42‰); and Central European moldavites (δ66/64Zn = 2.04 ± 0.19‰). These results are contrasted with a narrow range of δ66/64Zn = 0–0.7‰ for a diverse spectrum of upper continental crust materials.The elemental abundance of Zn is negatively correlated with δ66/64Zn, which may reflect that isotopic fractionation occurred by evaporation during the heating event upon tektite formation. Simple Rayleigh distillation predicts isotopic fractionations much larger than what is actually observed, therefore, such a model cannot account for the observed Zn isotope fractionation in tektites. We have developed a more realistic model of evaporation of Zn from a molten sphere: during its hypervelocity trajectory, the molten surface of the tektite will be entrained by viscous coupling with air that will then induce a velocity field inside the molten sphere. This velocity field induces significant radial chemical mixing within the tektite that accelerates the evaporation process. Our model, albeit parameter dependent, shows that both the isotopic composition and the chemical abundances measured in tektites can be produced by evaporation in a diffusion-limited regime. |
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