MICROSTRUCTURAL CONSTRAINTS ON THE FORMATIONAL AND THERMAL HISTORIES OF REFRACTORY INCLUSIONS IN CO3 CHONDRITES
Refractory inclusions, calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), of variable textures and mineralogies in CO3 chondrites have been investigated in detail using the focused ion beam (FIB) sample preparation technique combined with transmission electron microscopy (TEM) in order (1) to characterize the microstructures, compositions and textural relationships of their various constituent phases at the micrometer to nanometer scales and (2) to provide additional constraints on their formational and subsequent thermal histories both in the solar nebula and on their asteroidal parent bodies. In particular, refractory inclusions in CO3 chondrites are very fine-grained so that they are sensitive recorders of both primary formational and secondary alteration processes, which can only be adequately characterized by TEM. This work represents the first microstructural study of AOAs and CAIs from two CO3 chondrites, Allan Hills A77307 and Kainsaz. The microstructural and compositional characteristics of AOAs and CAIs in the pristine CO3.0 chondrite, Allan Hills A77307, provide evidence that various high-temperature condensation and gas-solid reactions occurred in the solar nebula under both equilibrium and disequilibrium conditions. Diverse gas-solid reactions reflect localized differences in the primary mineral assemblages in individual AOAs and CAIs that controlled reaction pathways. Despite large variations in their texture and mineralogy, mineralogically- and compositionally-zoned refractory components in AOAs, spinel-pyroxene CAIs and melilite-rich CAIs may have formed by a similar sequence of high-temperature condensation and gas-solid reactions, as follows; (1) the formation of the cores of the inclusions from primary equilibrium condensates (i.e., perovskite, spinel and melilite), (2) disequilibrium, probably short-lived reactions of the primary core minerals with a nebular gas under highly dynamic conditions, forming diopside with remarkably wide compositional ranges over submicrometer distances and (3) direct equilibrium condensation of pure diopside from a nebular gas, forming the outermost rim on the exterior of the inclusions. In spinel-hibonite CAIs, the crystallographic orientation relationships between spinel and hibonite provide evidence of the preferential epitaxial nucleation and growth of spinel on hibonite surfaces after hibonite condensation, rather than melilite. This process contributed to the rarity of melilite in this type of CAI. In the Kainsaz CO3.2 chondrite, the widespread, but heterogeneous occurrence of Fe enrichments at the outer margins of AOAs and along grain boundaries of olivines in AOAs provide evidence that mild parent body thermal metamorphism occurred at relatively low temperatures. However, due to incomplete textural and chemical equilibration of the whole rock, Kainsaz AOAs still retain primary nebular compositional and textural characteristics.
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