Rare Earth Element Systematics of Fine Grained Calcium-Aluminium-rich Inclusions and Amoeboid Olivine Aggregates from Efremovka (CV3)
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Rare-earth element
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A detailed, systematic petrographic survey of all (189) Ca, Al‐rich and olivine‐rich inclusions larger than ∼0.2 mm in diameter in 17 thin sections of Allende was performed by optical and scanning electron microscopy. Most of these inclusions are aggregates containing varying proportions of three distinct constituents, termed concentric objects, chaotic material, and inclusion matrix. A new classification system for Allende inclusions is proposed. Olivine‐rich inclusions occur in two textural varieties: unrimmed olivine aggregates (Type 1A inclusions) and rimmed olivine aggregates (Type 1B inclusions). We classify Ca, Al‐rich inclusions (CAI's) on the basis of the size and abundance of their constituent concentric objects (rather than their grain size) into unrimmed complex CAI's (Type 2 inclusions), rimmed complex CAI's (Type 3 inclusions), and simple CAI's (Type 4 inclusions). Several of the inclusion classes grade into each other. Some fundamental relationships among various types of Allende inclusions are discussed. We suggest that the term ‘coarse‐grained,’ even if used only in a relative sense, is too restrictive for some members of one class of inclusion (simple CAI's) and is inappropriate for most members of another class (melilite‐rich complex CAI's).
Allende meteorite
Melilite
Melt inclusions
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Abstract— Amoeboid olivine aggregates (AOAs) in the LL3.0 Semarkona chondrite have been studied by secondary ion mass spectrometry. The AOAs mainly consist of aggregates of olivine grains with interstitial Al‐Ti‐rich diopside and anorthite. Oxygen‐isotopic compositions of all phases are consistently enriched in 16 O, with δ 17,18 O = ∼−50‰. The initial 26 Al/ 27 Al ratios are calculated to be 5.6 ± 0.9 (2σ) × 10 −5 . These values are equivalent to those of AOAs and fine‐grained calcium‐aluminum‐rich inclusions (FGIs) from pristine carbonaceous chondrites. This suggests that AOAs in ordinary chondrites formed in the same 16 O‐rich calcium‐aluminum‐rich inclusion (CAI)‐forming region of the solar nebula as AOAs and FGIs in carbonaceous chondrites, and subsequently moved to the accretion region of the ordinary chondrite parent body in the solar nebula.
Diopside
Ordinary chondrite
Anorthite
Parent body
Carbonaceous chondrite
Chondrule
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Allende meteorite
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Chondrule
Murchison meteorite
Carbonaceous chondrite
Allende meteorite
Pyroxene
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Abstract— Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine‐grained aggregates of olivine and Ca, Al‐rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn‐rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas‐solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn‐rich forsterite is proposed. The Mn‐rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin. A type A calcium, aluminum‐rich inclusion (CAI) with an AOA attached to its Wark‐Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark‐Lovering rim, revealing the crystallization of olivine at the end stages of Wark‐Lovering rim formation. The Ca, Al‐rich nodules in the AOAs may be petrogenetically related to the Ca, Al‐rich minerals in Wark‐Lovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark‐Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors.
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Parent body
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Allende meteorite
Lithophile
Refractory (planetary science)
Carbonaceous chondrite
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Allende meteorite
Refractory (planetary science)
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Allende meteorite
Nepheline
Diopside
Pyroxene
Melilite
Chondrule
Sodalite
Forsterite
Pseudomorph
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