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    The Formation and Alteration of the Renazzo-Like Carbonaceous Chondrites

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    Author
    Schrader, Devin Lee
    Issue Date
    2012
    Keywords
    gas-solid reaction
    oxygen isotope
    Renazzo-like carbonaceous chondrite
    sulfide
    Planetary Sciences
    aqueous
    chondrule
    Advisor
    Lauretta, Dante S.
    
    Metadata
    Show full item record
    Publisher
    The University of Arizona.
    Rights
    Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
    Embargo
    Release after 30-Oct-2012
    Abstract
    This study investigates the pre-accretionary formation conditions of individual minerals within chondrules and whole-rock parent asteroid processes from the Renazzo-like carbonaceous (CR) chondrites. It presents a comprehensive work on the whole-rock O-isotope composition, sulfide-bearing opaque minerals, and type-II chondrules within the CR chondrites. Whole-rock O-isotope composition and minerals present in type-II chondrules are found to be related to the degree of parent asteroid aqueous alteration. Primary minerals within chondrules, formed prior to accretion of the CR chondrite parent asteroid, are used to constrain both the environment and the conditions present during chondrule formation.Chondrule formation, as recorded by chondrules in the CR chondrites, took place under dust- and ice-rich conditions relative to solar values. Type-II (FeO-rich) chondrules contain FeO-poor fragments compositionally similar to type-I (FeO-poor) chondrules; the formation of type-II chondrules may have occurred after the formation of type-I chondrules. The dust and ice abundances present during type-II chondrule formation were higher than those of type-I chondrules, although both populations probably exchanged with the same ¹⁶O-poor gas reservoir. Both the oxygen fugacity (fo₂) and sulfur fugacity (fs₂) appear to have increased from type-I to type-II chondrule formation, and between individual type-II chondrules. The increase in fo₂ and fs₂ may be due to the dissipation of H2 in the early Solar System. Gas-solid oxidation/sulfidation of Fe,Ni metal is recorded in both type-I and type-II chondrules. This corrosion occurred either during chondrule cooling after formation, or during chondrule reheating events, and suggests that S was present in the gas phase. After chondrule formation the CR chondrite parent asteroid accreted ¹⁶O-poor ice and experienced variable degrees of aqueous alteration, possibly due to heterogeneity in accreted ice or ammonia abundances and/or differing depth within the asteroid. Individual regions of the asteroid experienced different degrees of brecciation, perhaps a result of impacts, which fragmented chondrules and mixed together material that experienced different degrees of aqueous alteration. This process resulted in the heterogeneous nature of the CR chondrites.These observations constrain the formation conditions of a minor body, the CR chondrite parent asteroid, a remnant from the earliest stages of planet formation.
    Type
    text
    Electronic Dissertation
    Degree Name
    Ph.D.
    Degree Level
    doctoral
    Degree Program
    Graduate College
    Planetary Sciences
    Degree Grantor
    University of Arizona
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    Dissertations

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