Mineralogy,petrology, and oxygen isotopic composition of Northwest Africa 12379, metal-rich chondrite with affinity to ordinary chondrites |
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| Institution: | 1. Geoscience Institute/Mineralogy, Goethe University Frankfurt, Altenhöferallee 1, 60438, Frankfurt am Main, Germany;2. Forschungsinstitut und Naturmuseum Senckenberg, Sektion Meteoritenforschung, Senckenberganlage 25, 60325, Frankfurt am Main, Germany;3. Georg-August-Universität, Geowissenschaftliches Zentrum, Goldschmidtstraße 1, 37077, Göttingen, Germany;4. Labenne Meteorites, Paris, France;5. Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI, 96822, USA;6. StarPlan – Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, Copenhagen, DK-1350, Denmark;1. Dept 5.1, Federal Institute for Materials, Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany;2. Institut für Geowissenschaften (IGW), Friedrich-Schiller-Universität Jena (FSU), Germany;1. Kingsborough Community College of the City University of New York, 2001 Oriental Blvd., Brooklyn, NY 11235, United States;2. American Museum of Natural History, United States;3. The Natural History Museum, London, United Kingdom;4. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW Washington, DC 20015-1305, United States;5. Smithsonian Institution, National Museum of Natural History, Washington, 10th & Constitution, NW Washington, DC 20560-0119, United States;6. Department of Applied Geology, Curtin University, Perth, WA 6845, Australia;1. Institut für Geowissenschaften, Universität Kiel, Germany;2. Forschungsinstitut und Naturmuseum Senckenberg, Frankfurt/Main, Germany;1. Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Kobe 657-8501, Japan;2. Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Kyoto 606-8502, Japan;1. Department of Mineral Sciences, Smithsonian Institution, National Museum of Natural History, Washington, DC 20560, USA;2. Department of Earth and Planetary Sciences, Rutgers, State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854-8066, USA;3. Center for Meteorite Studies, Arizona State University, Tempe, AZ 85287, USA;4. School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA;5. Hawai‘i Institute of Geophysics & Planetology, University of Hawai‘i at Manoa, 1680 East-West Road, POST 602 Honolulu, HI 96822, USA;6. Retired, Iowa State University, Ames, IA 50011, USA;1. Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA;2. Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL 60637, USA;3. Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, USA;4. Department of Geoscience, University of Wisconsin, Madison, WI 53706, USA;5. Division of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan;6. Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, HI, USA;7. Korea Polar Research Institute, Incheon 406-840, Republic of Korea;8. Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA;9. Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, MSJ514, Los Alamos, NM 87545, USA |
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| Abstract: | Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (～ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa18.1–28.3, average Fa24.9±3.2, PMD = 12.8; Cr2O3 = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs3.2–18.7Wo0.2–4.5; average Fs14.7±3.7Wo1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ17O ～ 0.2–1.4‰, average ～ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa27) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ17O of the ferroan olivine (～ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ18O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin. |
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| Keywords: | Metal-rich chondrites Ordinary chondrites Thermal metamorphism Classification Oxygen isotopes |
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