The texture of a fine-grained calcium–aluminium-rich inclusion (CAI) in three dimensions and implications for early solar system condensation

Abstract

A 16mm fine-grained spinel-rich calcium–aluminium-rich inclusion (CAI) from the Allende CV3 meteorite was analysed using nano-computed tomography and scanning electron microscopy on uncoated chips and a polished thin section. The CAI is composed of spinel nodules surrounded by anorthite and Al-rich diopside rims. Minor secondary minerals including hedenbergite and nepheline are also present. The uncoated chips contain abundant wollastonite needles that are only rarely observed in the thin section. Nano-computed tomography shows that the structure of the CAI is a branching interconnected network of nodules, most of which are attached to each other in three dimensions. However some nodules are unattached to the rest of the CAI. The texture suggests that the CAI formed by condensation from a gas, and condensation and aggregation of nodules occurred contemporaneously, implying a high density of newly-formed dust. One portion of the CAI is compact and rich in melilite, with a composition and texture dissimilar to the bulk of the inclusion. We infer that this is a melilite-rich mantle of the same CAI that has experienced melting on one side.