Whole‐rock 26Al‐26Mg systematics of amoeboid olivine aggregates from the oxidized CV3 carbonaceous chondrite Allende

We report on mineralogy, petrography, and whole-rock26Al-26Mg systematics ofeight amoeboid olivine aggregates (AOAs) from the oxidized CV chondrite Allende. TheAOAs consist of forsteritic olivine, opaque nodules, and variable amounts of Ca,Al-richinclusions (CAIs) of different types, and show evidence for alteration to varying degrees.Melilite and anorthite are replaced by nepheline, sodalite, and grossular; spinel isenriched in FeO; opaque nodules are replaced by Fe,Ni-sulfides, ferroan olivine andCa,Fe-rich pyroxenes; forsteritic olivine is enriched in FeO and often overgrown byferroan olivine. The AOAs are surrounded by fine-grained, matrix-like rims composedmainly of ferroan olivine and by a discontinuous layer of Ca,Fe-rich silicates. Theseobservations 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 rimsaround AOAs. The whole-rock26Al-26Mg systematics of the Allende AOAs plot abovethe isochron of the whole-rock Allende CAIs with a slope of (5.23 ± 0.13)·10)5reported by Jacobsen et al. (2008). In contrast, whole-rock26Al-26Mg isotope systematicsof CAIs and AOAs from the reduced CV chondrite Efremovka define a single isochronwith a slope of (5.25± 0.01)·10)5(Larsen et al. 2011). We infer that the excesses in26Mg* present in Allende AOAs are due to their late-stage open-system metasomaticalteration. Thus, the26Al-26Mg isotope systematics of Allende CAIs and AOAs aredisturbed by parent body alteration processes, and may not be suitable for high-precisionchronology of the early solar system events and processes.