Phase-field simulation for crystallization of a highly supercooled forsterite-chondrule melt droplet

Abstract

Chondrules are submillimeter-sized and spherical-shaped crystalline grains consisting mainly of silicate material observed in chondritic meteorites. We numerically simulated pattern formation of a forsterite (Mg2SiO4)-chondrule in the melt droplet using a phase-field method. Because of the large surface-to-volume ratio, the surface cooling term was introduced in the framework of this method. We reproduced an unique crystal growth pattern inside the droplet composed of two distinguishable parts; the rim that covers whole droplet surface, and dendrite inside the droplet. It was found that the rim was formed when there is a large temperature difference of ∼100 K between the center and surface of the droplet due to the large cooling flux at the surface. In order to obtain the temperature difference, we derived temperature distribution of the droplet analytically, and concluded that the rim was formed only when the droplet cools rapidly at a rate of Rcool∼103 K s−1. However, when the surface cooling was so large as the temperature at the droplet center still remains above the melting point, no dendrite was obtained, though the rim was formed. The double structure captures the distinctive features of barred-olivine textures observed in natural chondrules.

Compound Chondrule Formation in Optically Thin Shock Waves

“…The latter hypothesis, called "dust enrichment", originates from the assumption that porphyritic chondrules, which are the main type among all chondrules, may be formed with a low cooling rate (∼ 10 −3 -1 K s −1 ; Desch et al 2012, and references therein). This assumption originates from the results of classical furnace-based crystallization experiments (e.g., ; however, several estimations based on some chondrule features, such as overgrowth thicknesses on relict grains (e.g., Wasson & Rubin 2003) and rim formation for barred olivine chondrules (Miura et al 2010b), give much higher cooling rates (∼ 200-2000 K s −1 ; Miura & Yamamoto 2014). Moreover, porphyritic textures may be reproduced by multiple melting processes (e.g., Rubin 2010) and they can also be formed via supercooled precursors (e.g., Srivastava et al 2010;Seto et al 2017).…”

The quasi-universality of chondrule size as a constraint for chondrule formation models

“…as A = L c (T L − T ) /T L(Kirkpatrick 1975) with T L the liquidus temperature and L c the latent heat of crystallization (1.7 × 10 −19 J per silicate tetrahedron for pure forsterite(Miura et al 2010)) and injecting equation (A.2) yields, after a Taylor expansion of the exponential:…”

Anisotropy function of kinetic coefficient for phase-field simulations: Reproduction of kinetic Wulff shape with arbitrary face angles

“…The kinetic coefficient is one of the most difficult parameters to be determined experimentally. Miura et al [34,35] adopted m A ¼ 0:4 cm s À1 K À1 to simulate solidification from a droplet of pure forsteritic composition. However, in the case of Mg-Fe olivine, the crystal growth velocity is limited by slow iron diffusion in liquid phase, so the kinetics at the solid-liquid interface would not play an important role.…”