9:50 AM - 10:05 AM
[SIT18-05] Melting Experiments on the Fe-C-O Ternary System Under Core Pressures
Keywords:Outer core, Inner core, Light Elements, Melting Experiments, Fe-C-O Ternary System
The light elements included in the iron core of the Earth have been explored for more than 70 years, but it still remains highly controversial. Five light elements, S, Si, C, O, and H are considered strong candidates. In particular, a combination of C and O have been paid much attention. Li et al. (2018) calculated the possible range of inner composition in Fe-S-Si-C and argued that C is an important element to explain the density and compressional and shear velocities of the inner core. Contrarily, O is known to have negligible solubility in solid Fe and thus so in the inner core. This indicates that O may be a major element in the outer core to account for the density difference between the outer core and inner core.
Here we performed melting experiments on the Fe-C-O ternary system by using laser-heating diamond-anvil cell techniques and obtained the liquidus phase relations at ~50 and ~136 GPa. By extrapolationg the present results and earlier data on the relevant Fe-O and Fe-C binary systems (Oka et al., 2019; Mashino et al., 2019) to 330 GPa, we estimated those under the inner core boundary pressure. The outer core liquid composition should be within the liquidus field of Fe to crystallize the dense inner core. We also obtained the partition coefficient of carbon DC between solid and liquid Fe.
We calculated the liquid compositions that are in equilibrium with the possible inner core compositions proposed by Li et al. (2018) by using DC, DSi, and DS. And we found that a narrow range of liquid Fe-S-Si-O-C compositions are compatible with the outer core only when inner core temperatures are 6500 K, according to previous ab initio calculations (Badro et al., 2014; Umemoto & Hirose 2020). Such inner core temperatures are unlikely because otherwise the corresponding temperatures at the core-mantle boundary cause extensive melting in the lowermost mantle. These may suggest that the inner core is not Fe-S-Si-C alloy but includes H as a major impurity element.
Here we performed melting experiments on the Fe-C-O ternary system by using laser-heating diamond-anvil cell techniques and obtained the liquidus phase relations at ~50 and ~136 GPa. By extrapolationg the present results and earlier data on the relevant Fe-O and Fe-C binary systems (Oka et al., 2019; Mashino et al., 2019) to 330 GPa, we estimated those under the inner core boundary pressure. The outer core liquid composition should be within the liquidus field of Fe to crystallize the dense inner core. We also obtained the partition coefficient of carbon DC between solid and liquid Fe.
We calculated the liquid compositions that are in equilibrium with the possible inner core compositions proposed by Li et al. (2018) by using DC, DSi, and DS. And we found that a narrow range of liquid Fe-S-Si-O-C compositions are compatible with the outer core only when inner core temperatures are 6500 K, according to previous ab initio calculations (Badro et al., 2014; Umemoto & Hirose 2020). Such inner core temperatures are unlikely because otherwise the corresponding temperatures at the core-mantle boundary cause extensive melting in the lowermost mantle. These may suggest that the inner core is not Fe-S-Si-C alloy but includes H as a major impurity element.