Microstructures and Formation History of Melilite-Rich Calcium-Aluminum-Rich Inclusions from the ALHA77307 CO3.0 Chondrite

Jangmi Han, Adrian J. Brearley

Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 21 October 2016


“We have studied four melilite-rich calcium-aluminum-rich inclusions (CAIs) from the Allan Hills A77307 CO3.0 chondrite using transmission electron microscopy with the focused ion beam sample preparation technique. This type of CAI represents one of the dominant types of refractory inclusions in CO3 chondrites. Individual melilite-rich CAIs 04 to 07 record complex formational histories involving high-temperature gas-solid condensation that occurred under both equilibrium and disequilibrium conditions. CAI 04 contains two texturally- and compositionally-distinct occurrences of perovskite: fine-grained perovskite within a melilite-rich core and aggregates of perovskite grains that surround the core. The textural and compositional differences suggest that the perovskite aggregates condensed after core formation under different conditions. CAI 05 consists of a compact melilite-rich core surrounded by a porous mantle, and likely formed by at least two different condensation events under different conditions. In CAI 06, complex intergrowth layers surrounding a melilite-rich core indicate reaction of spinel and melilite with a nebular gas to form Al-Ti-rich diopside following core formation. CAI 07 is dominated by melilite with a narrow compositional range and equilibrated textures, suggesting its formation by condensation over a limited temperature range. Collectively, we infer that the melilite-rich inclusions formed by a generalized sequence of high-temperature gas-solid condensation that involved: (1) formation of CAI cores by aggregation of primary equilibrium condensate grains (i.e., perovskite, spinel, and melilite), (2) back-reactions of the primary core minerals with a nebular gas under disequilibrium conditions, forming diopside that evolves in composition from Al-Ti-rich at the interface with the inclusion core to Al-Ti-poor on the exterior of the inclusions. The change in formation conditions may have been achieved by transport and injection of the core materials into a region of a partially-condensed gas that still contained refractory elements in the gas phase.”