[S-IT16] Planetary cores: Structure, formation, and evolution

[SIT16-06] Experimental study on trapped melt in iron meteorite: Implication for core solidification in planetesimals

*Hidenori Terasaki¹, Shiori Matsubara¹, Takashi Yoshino², Satoru Urakawa¹, Geoffrey Bromiley³, Ian Butler³ (1.Faculty of Science, Okayama University, 2.Institute for planetary Materials, Okayama University, 3.School of Geoscience, The University of Edinburgh)

Keywords:Core, Planetesimal, Iron meteorite

Magmatic iron meteorites are considered to have experienced fractional crystallization and are derived from the core of the meteorite parent body. The magmatic iron meteorites consist mainly of Fe-Ni alloys and contain some amounts of light elements, such as sulfur. Sulfur in the iron meteorites is observed as troilite nodules in Fe-Ni alloy. To explain the origins of these nodules, it has been reported that Fe-Ni-S melts were “trapped” in the grain boundaries of solid Fe-Ni and the trapped melts were remained and crystallized as a secondary solid phase, troilite (Wasson, 1999) or troilite and solid Fe-Ni (Chabot & Zhang, 2021). To constrain the behavior of trapped melts in the solid cores during core crystallization, in this study, we investigated the wetting property between solid Fe and liquid Fe–S at 0.5-3.0 GPa based on 2D and 3D textural observations.
The Fe-S melts in this study show interconnected channels at the Fe grain boundaries, whereas trapped melts in iron meteorites have a rounded shape in many cases (Wasson 1999). The difference in melt shape could be explained by the process of grain growth. The large grain growth rate of Fe may drag the melt at the grain boundaries, even with high melt mobility. Subsequently, the shape of the melt could be modified over a long period of core crystallization.
In the range of the dihedral angle of 30-48° obtained in this study, 8.8-19.3 vol% of the Fe-S melt can be stably stranded at the Fe grain boundary, which is higher than the amount of “trapped melt” observed in iron meteorite. Excess amounts of the melt would migrate away from the solid core and the stranded melt would be reduced to below the minimum-energy melt fraction over tens and hundreds million years of core crystallization in planetesimals.