Alloys in Fe-Si-H Ternary at High Pressures-Temperatures and Their Implications for the Planetary Cores

Finding habitable planets has drawn great interest from astrophysicists, Earth scientists as well as astrobiologists. Some key processes in the deep interior can affect the habitability, such as dynamo in a Fe-rich metallic core [1], where light elements play a key role. A recent study suggested that 0.30.6 wt% H can be incorporated into the Earths core during early differentiation [2]. Si is another important light element candidate for the planetary cores [3]. However, knowledge on Fe-Si-H ternary is still limited [4, 5]. We conducted a series of experiments to understand the impact of hydrogen on Fe-Si alloy system. We studied Fe with 9 wt% Si (Fe-9Si), 16 wt% Si (Fe-16Si), and 33.3 wt% Si (FeSi) separately in a hydrogen medium up to 65 GPa and 3500 K in diamond-anvil cells by combining pulsed laser heating with high-energy synchrotron X-ray diffraction. We found very little H solubility in B20- and B2-FeSi (0.3 wt% and <0.1 wt% H, respectively) compared with Fe metal (1.8 wt% H). The low H solubility in these phases is likely because of their highly distorted interstitial sites which are not favorable for H incorporation. Low-Si alloys (Fe-9Si and Fe-16Si) converted into FeH (fcc or dhcp), FeSi (B20 or B2), and hexagonal Fe5Si3H3 at 28.6-42.2 GPa after heating. Therefore, H alters the behavior of the Fe-Si system severely and Si may limit the H solubility in Fe metal. If Fe-droplets contain much Si in early magma ocean, Si could limit the amount of H incorporated in the core. In addition, crystallization at the solid-liquid boundary in the low-Si core may result in formation of separate H-rich and Si-rich crystals in the solid core, potentially explaining seismic heterogeneities in the region [6]. [1] Shahar, A., et al., Science, 2019. 364(6439): p. 434-435. [2] Tagawa, S., et al., Nature Communications, 2021. 12(1): p. 2588. [3] Hirose, K., S. Labrosse, and J. Hernlund, Annual Review of Earth and Planetary Sciences, 2013. 41: p. 657-691. [4] Terasaki, H., et al., American Mineralogist, 2011. 96(1): p. 93-99. [5] Tagawa, S., et al., Geophysical Research Letters, 2016. 43(8): p. 3686-3692. [6] Deuss, A., Annual Review of Earth Planetary Sciences, 2014. 42: p. 103-126.