LIME silicates in amoeboid olivine aggregates in carbonaceous chondrites

Indicator of nebular and asteroidal processes

Mutsumi Komatsu, Timothy Jay Fagan, Takashi Mikouchi, Michail I. Petaev, Michael E. Zolensky

    Research output: Contribution to journalArticle

    7 Citations (Scopus)

    Abstract

    MnO/FeO ratios in olivine from amoeboid olivine aggregates (AOAs) reflect conditions of nebular condensation and can be used in concert with matrix textures to compare metamorphic conditions in carbonaceous chondrites. LIME (low-iron, Mn-enriched) olivine was identified in AOAs from Y-81020 (CO3.05), Kaba (CV~3.1), and in Y-86009 (CV3), Y-86751 (CV3), NWA 1152 (CR/CV3), but was not identified in AOAs from Efremovka (CV3.1-3.4) or Allende (CV>3.6). According to thermodynamic models of nebular condensation, LIME olivine is stable at lower temperatures than Mn-poor olivine and at low oxygen fugacities (dust enrichment <10× solar). Although this set of samples does not represent a single metamorphic sequence, the higher subtypes tend to have AOA olivine with lower Mn/Fe, suggesting that Mn/Fe decreases during parent body metamorphism. Y-81020 has the lowest subtype and most forsteritic AOA olivine (Fo<inf>>95</inf>) in our study, whereas Efremovka AOAs are slightly Fe-rich (Fo<inf>>92</inf>). AOA olivines from Kaba are mostly forsteritic, but rare Fe-rich olivine precipitated from an aqueous fluid. A combination of precipitation of Fe-rich olivine and diffusion of Fe into primary olivine grains resulted in iron-rich compositions (Fo<inf>97-59</inf>) in Allende AOAs. Variations from fine-grained, nonporous matrix toward higher porosity and coarser lath-like matrix olivine can be divided into six stages represented by (1) Y-81020, Efremovka, NWA 1152; (2) Y-86751 lithology B; (3) Y-86009; (4) Kaba; (5) Y-86751 lithology A; (6) Allende. These stages are inferred to represent general degree of metamorphism, although the specific roles of thermally driven grain growth and diffusion versus aqueous dissolution and precipitation remain uncertain.

    Original languageEnglish
    Pages (from-to)1271-1294
    Number of pages24
    JournalMeteoritics and Planetary Science
    Volume50
    Issue number7
    DOIs
    Publication statusPublished - 2015 Jul 1

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    carbonaceous chondrites
    carbonaceous chondrite
    olivine
    silicates
    silicate
    iron
    lithology
    indicator
    matrix
    condensation
    matrices
    fugacity

    ASJC Scopus subject areas

    • Geophysics
    • Space and Planetary Science

    Cite this

    LIME silicates in amoeboid olivine aggregates in carbonaceous chondrites : Indicator of nebular and asteroidal processes. / Komatsu, Mutsumi; Fagan, Timothy Jay; Mikouchi, Takashi; Petaev, Michail I.; Zolensky, Michael E.

    In: Meteoritics and Planetary Science, Vol. 50, No. 7, 01.07.2015, p. 1271-1294.

    Research output: Contribution to journalArticle

    Komatsu, Mutsumi ; Fagan, Timothy Jay ; Mikouchi, Takashi ; Petaev, Michail I. ; Zolensky, Michael E. / LIME silicates in amoeboid olivine aggregates in carbonaceous chondrites : Indicator of nebular and asteroidal processes. In: Meteoritics and Planetary Science. 2015 ; Vol. 50, No. 7. pp. 1271-1294.
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    abstract = "MnO/FeO ratios in olivine from amoeboid olivine aggregates (AOAs) reflect conditions of nebular condensation and can be used in concert with matrix textures to compare metamorphic conditions in carbonaceous chondrites. LIME (low-iron, Mn-enriched) olivine was identified in AOAs from Y-81020 (CO3.05), Kaba (CV~3.1), and in Y-86009 (CV3), Y-86751 (CV3), NWA 1152 (CR/CV3), but was not identified in AOAs from Efremovka (CV3.1-3.4) or Allende (CV>3.6). According to thermodynamic models of nebular condensation, LIME olivine is stable at lower temperatures than Mn-poor olivine and at low oxygen fugacities (dust enrichment <10× solar). Although this set of samples does not represent a single metamorphic sequence, the higher subtypes tend to have AOA olivine with lower Mn/Fe, suggesting that Mn/Fe decreases during parent body metamorphism. Y-81020 has the lowest subtype and most forsteritic AOA olivine (Fo>95) in our study, whereas Efremovka AOAs are slightly Fe-rich (Fo>92). AOA olivines from Kaba are mostly forsteritic, but rare Fe-rich olivine precipitated from an aqueous fluid. A combination of precipitation of Fe-rich olivine and diffusion of Fe into primary olivine grains resulted in iron-rich compositions (Fo97-59) in Allende AOAs. Variations from fine-grained, nonporous matrix toward higher porosity and coarser lath-like matrix olivine can be divided into six stages represented by (1) Y-81020, Efremovka, NWA 1152; (2) Y-86751 lithology B; (3) Y-86009; (4) Kaba; (5) Y-86751 lithology A; (6) Allende. These stages are inferred to represent general degree of metamorphism, although the specific roles of thermally driven grain growth and diffusion versus aqueous dissolution and precipitation remain uncertain.",
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