Melting curve of iron-silicon alloy to the core-mantle boundary pressure and the thermal structure of the Earth's core

Earth's core consists of iron-Ni alloys and 10 wt.% of light elements, such as Si, S, O, C, H. It is composed of the solid inner core and the liquid outer core. Melting relation of the iron alloy is essential to estimate the thermal structure and temperature of the core, since the temperature of the inner core-outer core boundary (ICB) corresponds to the melting temperature of iron alloy at ICB pressure. Furthermore, the temperature of the outer core-mantle boundary (CMB) has to be higher than the melting temperature of iron alloy at CMB pressure. Therefore, the melting temperature of iron alloy at high pressures is significant for estimating the thermal structure of the Earth. In this study, we determined the melting temperature of Fe-17 wt.% Si alloy up to 119 GPa based on change of laser heating efficiency and the texture of the recovered samples using the sided laser heated diamond anvil cell. The measured melting curve of Fe-17 wt.% Si is slightly lower than that of pure Fe and the melting temperature was 3300 K at 120 GPa (CMB pressure). The melting temperature of the alloy is expressed as (P- P0)/a=(Tm/T0)c-1 by Simon's equation , i.e., P0= 0 GPa, Tm= 1500 K, a= 3.54+1.1, and c= 4.53+0.4. In order to clarify the solid phase of Fe-17 wt.% Si, in-situ X-ray diffraction study was also carried out at the BL13A beamline in PF of KEK. The high-pressure, and high-temperature behavior of Fe-17 wt.% Si alloy was investigated up to 96 GPa and 2000 K. Fe-17 wt.% Si has a bcc structure and there was no phase transition in the range of the present experimental pressures and temperatures. By extrapolation of the melting temperature using Simon's equation, the melting temperature of Fe-17 wt.% Si alloy is estimated to be 3300 K at the core-mantle boundary, 135 GPa, and 4000 K at the inner-core boundary around 330 GPa, which may provide the lower bound of the temperatures at CMB and ICB if the light element in the core is silicon.