Xenoliths and microxenoliths in H chondrites: Sampling the zodiacal cloud in the asteroid Main Belt

Authors

  • Giacomo BRIANI,

    Corresponding author
    1. Dipartimento di Fisica e Astronomia, Università di Firenze, Largo E. Fermi 2, 50125, Firenze, Italy
    2. Laboratoire de Minéralogie et Cosmochimie du Muséum, UMR7202, Muséum National d’Histoire Naturelle – CNRS, 61 rue Buffon, 75005, Paris, France
    3. Present address: Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse (CSNSM), CNRS-IN2P3, Université Paris Sud XI, Bat. 104, 91045 Orsay Campus, France
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  • Matthieu GOUNELLE,

    1. Laboratoire de Minéralogie et Cosmochimie du Muséum, UMR7202, Muséum National d’Histoire Naturelle – CNRS, 61 rue Buffon, 75005, Paris, France
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  • Michèle BOUROT-DENISE,

    1. Laboratoire de Minéralogie et Cosmochimie du Muséum, UMR7202, Muséum National d’Histoire Naturelle – CNRS, 61 rue Buffon, 75005, Paris, France
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  • Michael E. ZOLENSKY

    1. Astromaterial Research and Exploration Science – NASA Johnson Space Center, Houston, Texas 77058, USA
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Corresponding author. E-mail: giacomo.briani@csnsm.in2p3.fr

Abstract

Abstract– Xenoliths are inclusions of a given meteorite group embedded in host meteorites of a different group. Xenoliths with dimensions between a few μm and about 1 mm (microxenoliths) are “meteorite-trapped” analogues of micrometeorites collected on the Earth. However, they have the unique features of sampling the zodiacal cloud (1) at more ancient times than those sampled by micrometeorites and (2) at larger distances from the Sun (corresponding to the asteroid Main Belt) than that sampled by micrometeorites (1 AU). Herein we describe a systematic search for new xenoliths and microxenoliths in H chondrites, aimed at determining their abundance in these ordinary chondrites, analyzing their mineralogy, and searching for possible correlations with host meteorite properties. Sixty-six sections from 40 meteorites have been analyzed. Twenty-four new xenoliths have been discovered. About 87% of them are microxenoliths (i.e., <1 mm), only three are >1 mm in their largest dimension. All the newly discovered xenoliths and microxenoliths are composed of carbonaceous chondritic material. Hence, the zodiacal cloud was dominated by carbonaceous material even in past epochs. All the new xenoliths and microxenoliths have been found in regolith breccias. Hydrous-phase-rich xenoliths and microxenoliths in H4 and H5 chondrites attest that their embedding happened after the end of the thermal metamorphism. All these data suggest that xenoliths and microxenoliths were embedded when their host meteorites were part of the parent body regolith. This, combined with the H chondrite impact age distribution, attests that the embedding may have happened as early as 3.5 Gyr ago.

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