Formation and destruction of magnetite in CO3 chondrites and other chondrite groups |
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| Institution: | 1. Department of Earth, Planetary & Space Sciences, University of California, Los Angeles, CA, 90095-1567, USA;2. Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Bethel, ME, 04217, USA;3. Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210034, China;4. Chinese Academy of Sciences Center for Excellence in Comparative Planetology, China;1. Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, United States;2. Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, United States;3. Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637, United States;4. Chicago Center for Cosmochemistry, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637, United States;5. Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, United States;6. Geoscience Institute/Mineralogy, Goethe University Frankfurt, Altenhoeferallee 1, 60438 Frankfurt am Main, Germany;1. Department of Earth and Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, NM 87131, USA;2. Lunar and Planetary Institute, USRA, 3600 Bay Area Boulevard, Houston, TX 77058, USA;3. ARES, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA;4. Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA;5. Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA;1. University of Cologne, Department of Geology and Mineralogy, Zülpicher Str. 49b, 50674 Köln, Germany;2. Natural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD London, UK;3. American Museum of Natural History, Department of Earth and Planetary Sciences, NY 10024, New York, USA;4. Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA;5. Graduate School and Graduate Center of the City University of New York, USA;6. University of Wuppertal, Faculty of Mathematics and Natural Sciences, Gaußstraße 20, 42119 Wuppertal, Germany;1. School of Ocean, Earth Science and Technology, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, USA;2. Geoscience Institute/Mineralogy, Goethe University Frankfurt, Germany;3. Centre for Star and Planet Formation, University of Copenhagen, Denmark;4. Institute of Meteoritics, University of New Mexico, USA;5. Division of Geological and Planetary Sciences, California Institute of Technology, USA;6. Department of Geology, School of Earth and Environment, Rowan University, USA;7. Department of the Geophysical Sciences, The University of Chicago, USA;8. Enrico Fermi Institute, The University of Chicago, USA;9. Chicago Center for Cosmochemistry, USA;1. Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, 1680 East-West Road, Honolulu, HI 96822, USA;2. Geoscience Institute, Goethe University, 60438 Frankfurt am Main, Germany;3. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA 91125, USA;4. Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA;5. Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA;6. Chicago Center for Cosmochemistry, Chicago, IL 60637, USA;7. Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA;8. The Graduate University for Advanced Studies (SOKENDAI), Hayama, Kanagawa 240-0193 Japan;9. Earth Sciences Department, Waseda University, Shinjuku-ku, Tokyo 169-8050, Japan;10. Vernadsky Institute of Geochemistry of Russian Academy of Sciences, Kosygin St. 19, Moscow 119991, Russia;11. Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany;1. WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA;2. Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, MSJ514, Los Alamos, NM 87545, USA;3. Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan;4. Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 200 Monobe Otsu, Nankoku, Kochi 783-8502, Japan;5. Faculty of Science, Ibaraki University, Mito 310-8512, Japan;6. National Institute of Polar Research, Tokyo 190-8518, Japan;7. Kingsborough Community College and Graduate Center, The City University of New York, 2001 Oriental Boulevard, Brooklyn, NY 11235-2398, USA;8. American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192, USA |
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| Abstract: | Primitive CO3.00–3.1 chondrites contain ～2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H2 and reacts with SiO2 to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ?3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr2O3 in a few R4-6 chondrites. Magnetite grains with high Cr2O3 behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ～4 vol.% magnetite with substantial Cr2O3 (up to ～15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7. |
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| Keywords: | CO chondrites Magnetite Chromite Alteration Aqueous alteration Metallic Fe-Ni Metallic Cu |
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