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    A unique type B inclusion from Allende with evidence for multiple stages of melting

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    Author
    Simon, S. B.
    Grossman, L.
    Davis, A. M.
    Issue Date
    2005-01-01
    Keywords
    Melilite
    fassaites
    Allende
    
    Metadata
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    Citation
    Simon, S. B., Grossman, L., & Davis, A. M. (2005). A unique type B inclusion from Allende with evidence for multiple stages of melting. Meteoritics & Planetary Science, 40(3), 461-475.
    Publisher
    The Meteoritical Society
    Journal
    Meteoritics & Planetary Science
    URI
    http://hdl.handle.net/10150/655979
    DOI
    10.1111/j.1945-5100.2005.tb00394.x
    Additional Links
    https://meteoritical.org/
    Abstract
    A large (7 mm in diameter) Allende type B inclusion has a typical bulk composition and a unique structure: a fassaite-rich mantle enclosing a melilite-rich core. The core and mantle have sharply contrasting textures. In the mantle, coarse (~1 mm across), subhedral fassaite crystals enclose radially oriented melilite laths about 500 micrometers long that occur at the inclusion rim. The core consists of blocky melilite grains 20-50 micrometers across and poikilitically enclosed in anhedral fassaite grains that are optically continuous over ~1 mm. Another unique feature of this inclusion is that melilite laths also extend from the core into the mantle. Fassaite in both the core and mantle is very rich in fine-grained (1-10 micrometer) spinel. The rim laths are normally zoned (Åk 30-70) inward from the rim of the inclusion with reverse zoning over the last ~200 micrometers to crystallize. A very wide range of melilite compositions is found in the core of the inclusion, where gehlenitic grains (Åk 5-12) occur. These grains are enclosed in strongly zoned (Åk 15-70) overgrowths. The gehlenitic cores and innermost parts of the overgrowths are Na2O-free, but the outer parts of the overgrowths are not. In the laths at the rim, Na2O decreases inward from the rim, then increases. Fassaite in the core has the same range of Ti contents as that in the mantle: 29 wt% TiO2 + Ti2O3. Two melting events are required to account for the features of this inclusion. In the first event, the precursor assemblage is heated to ~1400 degrees C and melts except for gehlenitic (Åk 5-12) melilite and some spinel. These grains become concentrated in the core. During cooling, Na2O-free melilite nucleates at the rim of the inclusion and on the relict grains in the core. After open system secondary alteration, the inclusion is heated again, but only to ~1260 degrees C. Melilite more gehlenitic than Åk40 does not melt. During cooling, Na2O-bearing melilite crystallizes as small, blocky grains and laths in the core and as overgrowths on relict grains in the core and at the rim. Eventually melilite co-crystallizes with fassaite, leading to the reverse zoning observed in the laths. The coexistence in this inclusion of Na-free and Na-bearing melilite, plus a positive correlation between Na2O and ååkermanite contents in melilite in an inclusion with a bulk Mg isotopic composition that is mass-fractionated in favor of the heavy isotopes, are both consistent with at least two melting events. Several other recently described coarse-grained inclusions also have features consistent with a sequence of early, high-temperature melting, secondary alteration, and remelting at a lower temperature, suggesting that remelting of refractory inclusions was a common occurrence in the solar nebula.
    Type
    Article
    text
    Language
    en
    ISSN
    1945-5100
    ae974a485f413a2113503eed53cd6c53
    10.1111/j.1945-5100.2005.tb00394.x
    Scopus Count
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    Meteoritics & Planetary Science, Volume 40, Number 3 (2005)

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