Petrogenesis of silicate inclusions in the Weekeroo Station IIE iron meteorite: differentiation,remelting, and dynamic mixing |
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| Institution: | 1. Planetary Geosciences Institute, Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996, USA;2. Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA;3. Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA;1. Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark;2. Department of Earth & Planetary Science, University of Tokyo, Tokyo, Japan;1. Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, 1400 S Lake Shore Dr, Chicago, IL 60605, USA;2. Chicago Center for Cosmochemistry, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA;3. Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA;4. Astrogeobiology Laboratory, Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden;5. WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706-1692, USA;6. Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, MSJ514, Los Alamos, NM 87545, USA;1. Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064, United States;2. Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, United States;3. School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States;1. Department of Earth & Planetary Sciences, University of Tennessee, 1621 Cumberland Ave., 602 Strong Hall, Knoxville, TN 37996, United States;2. Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX 78712, United States;3. Oden Institute of Computational Sciences and Engineering, University of Texas at Austin, 201 E 24th St., Austin, TX 78712, United States;1. Institut für Geologie, Universität Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland;2. Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany |
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| Abstract: | The Weekeroo Station IIE iron meteorite contains a variety of felsic and mafic inclusions enclosed in an FeNi-metal host. Petrographic, EMP, and SIMS data suggest that the petrogenesis of the silicates was complex, and included differentiation, remelting, FeO-reduction, and dynamic mixing of phases.Differentiation produced a variety of olivine-free inclusion assemblages, ranging from pyroxene + plagioclase + tridymite with peritectic compositions, to coarse orthopyroxene, to plagioclase + tridymite and its glassy equivalent. Individual phases have similar trace-element abundances and patterns, despite large variations in inclusion textures, modes, and bulk compositions, probably as a result of mechanical separation of pre-existing phases in an impact event that dynamically mixed silicates with the metallic host. Trace-element data imply that augite and plagioclase grains in different inclusions crystallized from the same precursor melt, characterized by relatively unfractionated REE abundances of ～20–30 × CI-chondrites except for a negative Eu anomaly. Such a precursor melt could have been produced by ～2–5% equilibrium partial melting of an H-chondrite silicate protolith, or by higher degrees of partial melting involving subsequent fractional crystallization. Glass appears to have formed by the remelting of pre-existing plagioclase and orthopyroxene, in a process that involved either disequilibrium or substantial melting of these phases. During remelting, silicate melt reacted with the FeNi-metal host, and FeO was reduced to Fe-metal. Following remelting and metal-silicate mixing, inclusions apparently cooled at different rates in a near-surface setting on the parent body; glass- or pigeonite-bearing inclusions cooled more rapidly (≥2.5°C/hr between 1000–850°C) than pigeonite-free, largely crystalline inclusions.The results of this study point to two likely models for forming IIE iron meteorites, both involving collision between an FeNi-metal impactor and either a differentiated or undifferentiated silicate-rich target of H-chondrite affinity. Each model has difficulties and it is possible that both are required to explain the diverse IIE group. |
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