[S-IT18] Planetary cores: Structure, formation, and evolution

[SIT18-11] Possibility of percolation of Fe-S melts in asteroidsPossibility of percolation of Fe-S melts in asteroids

*Takumi Miura¹, Hidenori Terasaki², Tadashi Kondo¹, Osamu Ohtaka¹, Takashi Yoshino³ (1.Department of Earth and Space Science, Graduate School of Science, Osaka university, 2. Department of Earth Sciences, Okayama University, 3.Institute for Planetary Materials, Okayama University)

Keywords:Percolation, Asteroid, Core formation

Many asteroids and planetesimals have been reported to differentiate to metallic cores and silicate mantles (e.g., Barrat et al., 2015). Due to negligible accretional heating in these small bodies, percolation of liquid iron alloy through solid silicates may play an important role for the core formation process in asteroids. Possibility of the percolation of a core-forming liquid depends on the dihedral angle between liquid iron-alloy and crystalline silicates. If the dihedral angle is less than 60°, a melt can percolate through the crystals (Bargen and Waff, 1986). In previous experiments, the dihedral angle between Fe-S melt and olivine is reported to be less than 60 ° below 2-3 GPa depending on olivine Fe#, i.e., redox condition (Terasaki et al., 2008). To understand core formation process in asteroids, it is required to measure the dihedral angle between Fe-S melt and orthopyroxene (opx) which is one of the major mantle minerals of asteroid as with olivine.

In this study, high-pressure experiments were performed up to 2.5 GPa and 1523 K using cubic multi-anvil and piston cylinder devices. Annealing durations range from 180 to 720 min at target temperatures. Starting material were mixtures synthetic opx powder (Fe#=0.23, 0.30) and Fe-S (S=40, 50 at%). Textures of recovered samples and chemical composition were analyzed using SEM-EDS. Dihedral angles were measured from backscattered electron images using Image process software. The measured dihedral angle between FeS and opx increases from 64°, which is close to the wetting boundary of 60°, at 1GPa to 92°- 94°, well above the 60° at 2.5GPa. Based on the results of the dihedral angle, textural feature, and chemical compositions, we will discuss a possibility of percolation of Fe-S melt through opx mantle in asteroid.