Effect of Hydrogen Gas Pressure on Calcium-Aluminum-rich Inclusion Formation in the Protosolar Disk: a Laboratory Simulation of Open-system Melt Crystallization

Calcium-aluminum-rich inclusions (CAIs) are the oldest materials that formed in the protosolar disk. Igneous CAIs experienced melting and subsequent crystallization in the disk during which the evaporation of relatively volatile elements such as Mg and Si occurred. Evaporation from the melt would have played a significant role in the variation of chemical, mineralogical, and petrologic characteristics of the igneous CAIs. In this study, we investigated crystallization of CAI analog melt under disk-like low-pressure hydrogen (P H2) conditions of 0.1, 1, and 10 Pa to constrain the pressure condition of the early solar system in which type B CAIs were formed. At P H2 = 10 Pa, the samples were mantled by melilite crystals, as observed for type B1 CAIs. However, the samples heated at P H2 = 0.1 Pa exhibited random distribution of melilite, as in type B2 CAIs. At the intermediate P H2 of 1 Pa, type-B1-like structure formed when the cooling rate was 5 C hr-1, whereas the formation of type-B2-like structure required a cooling rate faster than 20 C hr-1. The compositional characteristics of melilite in type B1 and B2 CAIs could also be reproduced by experiments. The results of the present study suggest that P H2 required for type-B1-like textural and chemical characteristics is greater than 1 Pa. The hydrogen pressure estimated in this study would impose an important constraint on the physical condition of the protosolar disk where type B CAIs were formed.