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Microstructural evidence for complex formation histories of amoeboid olivine aggregates from the ALHA 77307 CO 3.0 chondrite
Author(s) -
Han Jangmi,
Brearley Adrian J.
Publication year - 2015
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/maps.12439
Subject(s) - anorthite , olivine , spinel , diopside , melilite , chondrite , mineralogy , geology , annealing (glass) , geochemistry , materials science , metallurgy , meteorite , astrobiology , paleontology , physics
The microstructures and compositions of olivine and refractory components in six amoeboid olivine aggregates ( AOA s) in the Allan Hills A77307 CO 3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique with transmission electron microscopy. In the AOA s, refractory components (perovskite, melilite, spinel, anorthite, and Al‐Ti‐bearing diopside) provide evidence of a high degree of textural and compositional heterogeneity, suggesting that these phases have formed by disequilibrium gas–solid condensation at high temperatures under highly dynamic conditions. We infer different possible reactions of early‐condensed solid minerals (perovskite and spinel) with a nebular gas, forming diopside with wide ranges of Al and Ti contents and/or anorthite. The progressive, incomplete consumption of spinel in these reactions may have resulted in the Cr enrichment in the remaining, unreacted spinel in the AOA s. In contrast to the refractory components, olivines in the AOA s have equilibrated textures with 120° triple junctions, indicating that the AOA s were subjected to high‐temperature annealing after agglomeration of olivine and refractory components. Because the AOA s consist of fine‐grained olivine grains with numerous pores, the annealing is constrained by experimental data to have occurred for a short duration of the order of a few hours to tens of hours depending on the annealing temperature. In comparison, the effects of annealing on the refractory components are minimal, probably due to pinning of grain boundaries in the multiphase assemblages that inhibited grain growth.