On the origin of rim textures surrounding anhydrous silicate grains in CM carbonaceous chondrites


  • Lauren BROWNING,

    Corresponding author
    1. Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA
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    • Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, Texas 78238–5166, USA

  • Harry Y. McSWEEN Jr.,

    1. Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410, USA
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  • Michael E. ZOLENSKY

    1. Earth Science and Solar System Exploration Division, NASA Johnson Space Center, Houston, Texas 77058, USA
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Abstract— Fine-grained, optically opaque rims coat individual olivine and pyroxene grains in CM matrices and chondrules. Bulk chemical analyses and observations of these rims indicate the presence of phyllosilicates and disseminated opaques. Because phyllosilicates could not have survived the chondrule formation process, chondrule silicate rims must have formed entirely by late-state aqueous reactions. As such, these textures provide a useful benchmark for isolating alteration features from more complex CM matrix materials. Both chondrule silicate and matrix silicate rims exhibit morphological features commonly associated with advancing stages of replacement reactions in terrestrial serpentinites. Contacts between many matrix silicate rims and the adjacent matrix materials suggest that these rims formed entirely by aqueous reactions in a parent-body setting. This contrasts with previous assertions that rim textures can only form by the accretion of nebular dust but does not imply an origin for the rims surrounding other types of CM core components, such as chondrules.