Formation of disk- and bowl-shaped igneous Ca,Al-rich inclusions: Constraints from their morphology, textures, mineralogy and modelling

1C.Lorenz,1M.Ivanova,2A.Krot,3V.Shuvalov
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.07.005]
1Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Kosygin St. 19, Moscow, 119991, Russia
2University of Hawai‘i at Mānoa, 1680 East-West Road, Honolulu, HI, 96822, USA
3Institute for Dynamics of Geosphere, Russian Academy of Sciences, Moscow, Russia
Copyright Elsevier

Calcium-aluminum-rich inclusions (CAIs) are the oldest Solar System solids dated that formed by evaporation, condensation, aggregation and, sometimes, melting processes near the protoSun, and were subsequently dispersed throughout the protoplanetary disk by still poorly-understood mechanism(s). Here we report on the discovery of disk- and bowl-shaped centimeter-sized igneous CAIs in CV (Vigarano type) carbonaceous chondrites. Igneous CAIs of these shapes are not expected for crystallization of melt droplets in a low gravity field of the protoplanetary disk. We have tested several models for the formation of disk- and bowl-shaped igneous CAIs including: collision, aerodynamic deformation and shock flattening. We conclude that these CAIs resulted from aerodynamic deformation of CAI-like melt droplets and propose the following multistage formation scenario: (1) nearly complete melting and acceleration of CAIs at <30 km/s in the CAI-forming region having approximately solar dust/gas ratio; (2) aerodynamic deformation, ablation, deceleration, solidification at ˜30–40 K/min, Wark-Lovering rims formation, and deceleration of the CAIs entering a dust-rich inner disk wall; (3) radial drift of the solidified deformed CAIs towards the Sun; (4) heating and partial melting of the deformed CAIs by solar radiation that preserve their morphology; (5) cooling and crystallization of CAIs at ˜2 K/h; (5) radial transport of CAIs from their formation region to the outer disk.

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