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    Cubanite and associated sulfides in CI chondrites and Comet 81P/Wild 2: Implications for aqueous processing

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    Author
    Berger, Eve L.
    Issue Date
    2011
    Keywords
    cubanite
    Meteorites
    Stardust Mission
    sulfides
    Planetary Sciences
    CI chondrites
    Comets
    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.
    Abstract
    The discovery of Ni-, Cu-, and Zn-bearing Fe-sulfides from comet 81P/Wild 2 represents the strongest evidence, in the Stardust collection, of grains that formed in an aqueous environment. Crystalline sulfide assemblages from Wild 2 and the hydrothermally altered CI chondrite Orgueil were investigated. Structural and compositional characterizations of the sulfide grains from both collections reveal similarities. The Stardust samples include a cubanite (CuFe₂S₃) grain, a pyrrhotite [(Fe,Ni)₁₋ₓS]/pentlandite [(Fe,Ni)₉S₈] assemblage, and a pyrrhotite/sphalerite [(Fe,Zn)S] assemblage. Similarly, the CI-chondrite sulfides include individual cubanite and pyrrhotite grains, cubanite/pyrrhotite assemblages, pyrrhotite/pentlandite assemblages, as well as possible sphalerite inclusions within pyrrhotite grains. The cubanite is the low temperature orthorhombic form, which constrains temperature to a maximum of 210°C. The Stardust and Orgueil pyrrhotites are the 4C monoclinic polytype, which is not stable above ~250°C. The combinations of cubanite and pyrrhotite, as well as pyrrhotite and pentlandite, signify even lower temperatures. The crystal structures, compositions, and petrographic relationships of these sulfides constrain formation and alteration conditions. Taken together, these constraints attest to low-temperature hydrothermal processing. The hydrothermal conditions under which cubanite forms were investigated through thermodynamic modeling and experimental synthesis. A thermodynamic model for cubanite was developed to constrain its formation environment. Cubanite was synthesized under hydrothermal conditions consistent with those predicted for the CI-chondrite parent body. The similarity between Stardust and CI-chondrite sulfides suggest similar fluid conditions may have existed on the comet at some point in its history. The presence of cubanite in the Stardust collection has implications for large scale issues such as: heat sources in the comet-forming region; aqueous activity on cometary bodies; and the extent and mechanisms of radial mixing of material in the early nebula. The Wild 2 sulfides are most likely the products of low-temperature aqueous alteration and provide evidence of radial mixing of material from the inner solar system to the comet-forming region and possible secondary aqueous processing on the cometary body.
    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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