Shape and porosity of refractory inclusions in CV3 chondrites: A micro‐computed tomography (µCT) study

Abstract

Refractory calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in chondritic meteorites are the earliest solids of our solar system, bearing the information of nebular condensation as well as accretion and asteroidal shock and metasomatism processes. While the compositions of refractory inclusions have been intensely studied for~50 years, their physical properties such as shape and porosity are poorly constrained. Here, we present a microcomputed tomography (µCT) study on 16 refractory inclusions of condensate origin in five CV3 chondrites. We find that they are prolate or triaxial in shape with very rough morphologies. The CAIs have nodular textures and are thought to form by agglomerating individual nodules via collision-induced bouncing and/or fragmentation, where the nodules were grown by gas-solid reactions during condensation. On the parent body, refractory inclusions from the CV R meteorite Leoville experienced intense shocks that led to the flattening of their shapes and lowering of their porosities. High-temperature metasomatism in CV OxA meteorites and low-temperature metasomatism in CV OxB meteorites do not seem to have large effects on the porosities of their refractory inclusions, which have similar ranges and pore-size distributions. Instead, we infer that their pores are mostly inherited from the gas-solid condensation and subsequent agglomeration processes. The porosities of CAIs are higher than those of AOAs, which is mainly due to the high-temperature sintering process of AOAs.

Formation of fused aggregates under long‐term microgravity conditions aboard the ISS with implications for early solar system particle aggregation

“…Furthermore, the natural “swan‐like” and bent inclusions shown in their study resemble aggregates formed in our experiment. The b / c ratio of the elongated, natural CAIs observed in NWA 13656 is also in accordance with the b / c ratio determined by Zhang et al (2021) and the experimental outcomes of this study. For unknown reasons, aggregates and CAIs prefer a ratio of their longest to their shortest dimension in the range of 2.…”

“…Furthermore, Zhang et al (2021) performed CT analysis of refractory inclusions of five CV3 meteorites to study their three‐dimensional morphology and found that among the six intact inclusions, three showed an elongated morphology with b / c ratios of 1.5–2.1 and two had a fractal and “swan‐like” morphology. The mean b / c ratio of 2.1 ± 2 of the elongated aggregates formed in our experiment is similar to that observed for the natural refractory inclusions in the study of Zhang et al (2021). Furthermore, the natural “swan‐like” and bent inclusions shown in their study resemble aggregates formed in our experiment.…”

“…Some igneous Type B CAIs have a disk-or bowlshaped morphology (Ivanova et al, 2008), which was interpreted to result from aerodynamical deformation and shock flattening (Lorenz et al, 2019), or more complex scenarios which include ejection from the inner solar system and remelting during reentry at hypersonic speed (Liffman et al, 2016). Further relations between the morphology of CAIs and aerodynamical effects have been proposed in the most recent study of Zhang et al (2021) in which the studied CAIs and AOAs have an elongated or a bent fractal morphology, interpreted as resulting from movement induced by, for example, radial drift.…”