Compound ultrarefractory CAI-bearing inclusions from CV3 carbonaceous chondrites

Authors

  • Marina A. IVANOVA,

    Corresponding author
    1. Vernadsky Institute of Geochemistry of Russian Academy of Sciences, Moscow, Russia
    2. National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, USA
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  • Alexander N. KROT,

    1. Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, Hawai’i 96822, USA
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  • Kazuhide NAGASHIMA,

    1. Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, Hawai’i 96822, USA
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  • Glenn J. MacPHERSON

    1. National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, USA
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Corresponding author. E-mail: meteorite2000@mail.ru

Abstract–

Two compound calcium-aluminum-rich inclusions (CAIs), 3N from the oxidized CV chondrite Northwest Africa (NWA) 3118 and 33E from the reduced CV chondrite Efremovka, contain ultrarefractory (UR) inclusions. 3N is a forsterite-bearing type B (FoB) CAI that encloses UR inclusion 3N-24 composed of Zr,Sc,Y-rich oxides, Y-rich perovskite, and Zr,Sc-rich Al,Ti-diopside. 33E contains a fluffy type A (FTA) CAI and UR CAI 33E-1, surrounded by Wark-Lovering rim layers of spinel, Al-diopside, and forsterite, and a common forsterite-rich accretionary rim. 33E-1 is composed of Zr,Sc,Y-rich oxides, Y-rich perovskite, Zr,Sc,Y-rich pyroxenes (Al,Ti-diopside, Sc-rich pyroxene), and gehlenite. 3N-24’s UR oxides and Zr,Sc-rich Al,Ti-diopsides are 16O-poor (Δ17O approximately −2‰ to −5‰). Spinel in 3N-24 and spinel and Al-diopside in the FoB CAI are 16O-rich (Δ17O approximately −23 ± 2‰). 33E-1’s UR oxides and Zr,Sc-rich Al,Ti-diopsides are 16O-depleted (Δ17O approximately −2‰ to −5‰) vs. Al,Ti-diopside of the FTA CAI and spinel (Δ17O approximately −23 ± 2‰), and Wark-Lovering rim Al,Ti-diopside (Δ17O approximately −7‰ to −19‰). We infer that the inclusions experienced multistage formation in nebular regions with different oxygen-isotope compositions. 3N-24 and 33E-1’s precursors formed by evaporation/condensation above 1600 °C. 3N and 33E’s precursors formed by condensation and melting (3N only) at significantly lower temperatures. 3N-24 and 3N’s precursors aggregated into a compound object and experienced partial melting and thermal annealing. 33E-1 and 33E avoided melting prior to and after aggregation. They acquired Wark-Lovering and common forsterite-rich accretionary rims, probably by condensation, followed by thermal annealing. We suggest 3N-24 and 33E-1 originated in a 16O-rich gaseous reservoir and subsequently experienced isotope exchange in a 16O-poor gaseous reservoir. Mechanism and timing of oxygen-isotope exchange remain unclear.

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