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

[SIT18-06] Evaluation of Fe–S melt segregation in solid core during core crystallization in planetesimals

*Shiori Matsubara¹, Hidenori Terasaki¹, Takashi Yoshino², Satoru Urakawa¹, Kotaro Kobayashi¹, Geoffrey D. Bromiley³, Ian Butler³ (1.Graduate School of Natural Science and Technology, Okayama University, 2.Institute for Planetary Materials, Okayama University, 3.School of Geoscience, The University of Edinburgh)

Keywords:planetesimals, asteroids, core crystallization, percolation, planetary interior, iron-alloys

In differentiated planetesimals, liquid core starts to crystallize during secular cooling after core–mantle differentiation and segregation of liquid–solid iron-alloys (liquid–solid core differentiation) would occur in the core. In the top-down core crystallization process proposed for the planetesimal-sized body (Williams, 2009), some amount of Fe–S melt is likely to be “trapped” in the solid core (Scheinberg et al. 2016). Wetting property between liquid and solid iron-alloys controls whether the “trapped” melts are remained in the solid core or they can separate from solid core during core crystallization. In our previous study, the dihedral angle between Fe–S melt and solid Fe were well below the percolation threshold of 60° at 0.5–3.0 GPa, 1323 K. Based on the result, the “trapped” Fe–S melt can form interconnected networks in the solid core. Melts that form networks at solid iron grain boundaries may separate from the solid core if there is a driving force of melt migration and also if the melt migration velocity is faster than the cooling rate of the solid core.
To clarify time variation and driving force for the melt separation, in this study, we performed percolation experiments of Fe–FeS at 0.5–3.0 GPa and 1323 K using piston-cylinder apparatus and 2D and 3D textural observations were performed for recovered samples using SEM and X-ray computed tomography.
From textural observations, the Fe–S melts distributed at the Fe grain boundaries in most of the samples. Melt segregation was observed in the sample at higher pressure (3 GPa). In the cell with large temperature gradient, melt segregation was observed and the melt tends to migrate to the hot side at 1.0 GPa, 1323 K. This could be applicable for the direction of melt migration during core crystallization in the planetesimals.