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    Chemical Reduction of Silicates by Meteorite Impacts and Lightning Strikes

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    Author
    Sheffer, Abigail Anne
    Issue Date
    2007
    Keywords
    Lunar Regolith Agglutinate
    Meteorite Impact
    Fulgurite
    Tektite
    Chemical Modeling
    Mossbauer Spectroscopy
    Advisor
    Melosh, Henry Jay
    Committee Chair
    Melosh, Henry Jay
    
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    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
    A suite of lightning strike glasses and unmelted starting materials has been studied by electron microscope and Mossbauer spectroscopy to determine Fe oxidation states. Nine of eleven samples are reduced compared to the starting materials; four of the glasses contain Fe0. Only one sample contained evidence of reduction by carbon, and the results support the reduction of Fe as intrinsic to the rapid, high temperature processing during lightning strikes.A thermodynamic modeling code is used to model the formation of moldavite tektites and the reduction of Fe from sediments around the Ries crater. During isentropic cooling from a strong shock, Fe3+ is reduced to Fe2+ at all modeled conditions. The best matches to an average moldavite composition and the compositions of the Bohemian and Bohemian:Radomilice sub-strewn fields occur with a mixture of surface and subsurface sands along a 4500 J/kg-K isentropic cooling path, consistent with an asteroid impact. The Lusatian and Moravian sub-strewn fields are better represented by starting materials of entirely surface sands, consistent with the uppermost layers of surface material having traveled the farthest from the impact.The thermodynamic code is also used to investigate the formation of lunar regolith agglutinates and reduction of Fe to Fe0. Forming Fe0 requires assuming Fe0 is miscible in silicate liquid at elevated temperatures and pressures. When Fe0 is included in the liquid solution, it is stable at modeled conditions. Simple separation of liquid from vapor is not sufficient to reproduce agglutinate glass. When the vapor phase is allowed to partially redeposit and some Fe0 is directly condensed from vapor, the resulting liquid better reproduces mare agglutinate glasses. This model cannot reproduce highland agglutinate glass, because the Al concentration remains too high in the liquid. The best match to mare glass is produced using the <10 µm fraction of the mare soil along the 8000 J/kg-K cooling isentrope at 100 bars, 4370 K with 95% vapor redeposition and 50% of the Fe(g) directly condensed as Fe0. The reduced fulgurite samples and the results of the impact models suggest that Fe reduction is intrinsic to the rapid, high temperature processing of silicates.
    Type
    text
    Electronic Dissertation
    Degree Name
    PhD
    Degree Level
    doctoral
    Degree Program
    Planetary Sciences
    Graduate College
    Degree Grantor
    University of Arizona
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