Phase Relations in the Fe-Si-H Ternary up to 125 GPa and 3700K: Implications for the Structure and Chemistry of Planetary Cores 

<p>Light elements play a key role in the chemical and physical processes of planetary Fe-rich metallic cores [1].&#160; H and Si are believed important candidates in planetary cores and previous estimates indicate as much as 0.6 wt% H and 13 wt% Si in the Earth&#8217;s core [2, 3]. However, existing studies are on Fe-H or Fe-Si binary systems and knowledge on Fe-Si-H ternary at high pressure and temperature is still limited [4, 5]. We conducted a series of experiments to understand the impact of hydrogen on Fe-Si alloy system. Fe-Si alloys with three compositions, Fe-9Si (9 wt% Si), Fe-16Si (16 wt% Si), and FeSi (33.3 wt% Si), reacted with H separately up to 125 GPa and 3700 K in diamond-anvil cells by combining pulsed laser heating with high-energy synchrotron X-ray diffraction. Results show little H solubility in B20 and B2 phases of FeSi (0.3 wt% and <0.1 wt% H, respectively) up to 62 GPa, which is significantly smaller than H solubility in Fe metal (1.8 wt% H) [6]. The low H solubility in these phases is likely because of their highly distorted interstitial sites which are not favorable for H incorporation. We found that the low-Si alloys (Fe-9Si and Fe-16Si) convert into FeH_x (fcc or dhcp), FeSi (B20 or B2), and Fe-Si-H ternary phases up to 125 GPa and 3700 K. Particularly, a Fe₅Si₃H_x phase is stable below 43 GPa and the cubic FeH₃ can appear after reactions above 100 GPa. These results indicate that H alters the behavior of the Fe-Si system severely. Considering the various sizes and masses of planets in the solar and exoplanetary systems, the planetary cores can have a wide range of Si contents. If Fe-droplets in early magma ocean contain much Si, Si could limit the amount of H incorporated in the core. On the other hand, for cores with low Si, crystallization at the solid-liquid core boundary may result in formation of separate H-rich and Si-rich crystals in the solid core, potentially inducing heterogeneities in the region [7].&#160;</p><p><strong>References:</strong></p><p>1. Shahar, A., et al., <em>What makes a planet habitable?</em> Science, 2019. <strong>364</strong>(6439): p. 434-435.</p><p>2. Tagawa, S., et al., <em>Experimental evidence for hydrogen incorporation into Earth&#8217;s core.</em> Nature Communications, 2021. <strong>12</strong>(1): p. 2588.</p><p>3. Hirose, K., B. Wood, and L. Vo&#269;adlo, <em>Light elements in the Earth&#8217;s core.</em> Nature Reviews Earth & Environment, 2021. <strong>2</strong>(9): p. 645-658.</p><p>4. Terasaki, H., et al., <em>Hydrogenation of FeSi under high pressure.</em> American Mineralogist, 2011. <strong>96</strong>(1): p. 93-99.</p><p>5. Tagawa, S., et al., <em>Compression of Fe&#8211;Si&#8211;H alloys to core pressures.</em> Geophysical Research Letters, 2016. <strong>43</strong>(8): p. 3686-3692.</p><p>6. P&#233;pin, C.M., et al., <em>New iron hydrides under high pressure.</em> Physical review letters, 2014. <strong>113</strong>(26): p. 265504.</p><p>7. Deuss, A., <em>Heterogeneity and anisotropy of Earth's inner core.</em> Annual Review of Earth Planetary Sciences, 2014. <strong>42</strong>: p. 103-126.</p>