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    Northwest Africa 1500: Plagioclase-bearing monomict ureilite or ungrouped achondrite?

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    Author
    Goodrich, Cyrena Anne
    Wlotzka, Frank
    Ross, D. Kent
    Bartoschewitz, Rainer
    Issue Date
    2006-01-01
    Keywords
    isotope anomaly
    Nucleosynthesis
    Astrophysics
    Short-lived isotopes
    
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    Citation
    Goodrich, C. A., Wlotzka, F., Ross, D. K., & Bartoschewitz, R. (2006). Northwest Africa 1500: Plagioclase‐bearing monomict ureilite or ungrouped achondrite?. Meteoritics & Planetary Science, 41(6), 925-952.
    Publisher
    The Meteoritical Society
    Journal
    Meteoritics & Planetary Science
    URI
    http://hdl.handle.net/10150/656147
    DOI
    10.1111/j.1945-5100.2006.tb00496.x
    Additional Links
    https://meteoritical.org/
    Abstract
    Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (100-500 micrometers) olivine (95-96%), augite (2-3%), and chromite (0.6-1.6%) in an equilibrated texture. Plagioclase (0.7-1.8%) occurs as poikilitic grains (up to 3 mm) in vein-like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl-apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65-73, with a strong peak at Fo 68-69. Most grains are reverse-zoned, and also have ~10-30 micrometer reduction rims.In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo is greater than or equal to 75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite-bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augite-bearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800-1000 degrees C, from two-pyroxene and olivine-chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history.
    Type
    Article
    text
    Language
    en
    ISSN
    1945-5100
    ae974a485f413a2113503eed53cd6c53
    10.1111/j.1945-5100.2006.tb00496.x
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    Meteoritics & Planetary Science, Volume 41, Number 6 (2006)

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