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Whole‐rock 26 Al‐ 26 Mg systematics of amoeboid olivine aggregates from the oxidized CV3 carbonaceous chondrite Allende
Author(s) -
OLSEN M. B.,
KROT A. N.,
LARSEN K.,
PATON C.,
WIELANDT D.,
SCHILLER M.,
BIZZARRO M.
Publication year - 2011
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2011.01256.x
Subject(s) - allende meteorite , olivine , melilite , geology , nepheline , parent body , geochemistry , chondrite , petrography , isochron , mineralogy , meteorite , spinel , astrobiology , physics , paleontology
– We report on mineralogy, petrography, and whole‐rock 26 Al‐ 26 Mg systematics of eight amoeboid olivine aggregates (AOAs) from the oxidized CV chondrite Allende. The AOAs consist of forsteritic olivine, opaque nodules, and variable amounts of Ca,Al‐rich inclusions (CAIs) of different types, and show evidence for alteration to varying degrees. Melilite and anorthite are replaced by nepheline, sodalite, and grossular; spinel is enriched in FeO; opaque nodules are replaced by Fe,Ni‐sulfides, ferroan olivine and Ca,Fe‐rich pyroxenes; forsteritic olivine is enriched in FeO and often overgrown by ferroan olivine. The AOAs are surrounded by fine‐grained, matrix‐like rims composed mainly of ferroan olivine and by a discontinuous layer of Ca,Fe‐rich silicates. These observations indicate that AOAs experienced in situ elemental open‐system iron‐alkali‐halogen metasomatic alteration during which Fe, Na, Cl, and Si were introduced, whereas Ca was removed from AOAs and used to form the Ca,Fe‐rich silicate rims around AOAs. The whole‐rock 26 Al‐ 26 Mg systematics of the Allende AOAs plot above the isochron of the whole‐rock Allende CAIs with a slope of (5.23 ± 0.13) × 10 −5 reported by Jacobsen et al. (2008). In contrast, whole‐rock 26 Al‐ 26 Mg isotope systematics of CAIs and AOAs from the reduced CV chondrite Efremovka define a single isochron with a slope of (5.25± 0.01) × 10 −5 (Larsen et al. 2011). We infer that the excesses in 26 Mg* present in Allende AOAs are due to their late‐stage open‐system metasomatic alteration. Thus, the 26 Al‐ 26 Mg isotope systematics of Allende CAIs and AOAs are disturbed by parent body alteration processes, and may not be suitable for high‐precision chronology of the early solar system events and processes.