[S-IT06] Early Earth - from accumulation to formation -

[SIT06-09] hcp構造 Fe-Si-H合金の状態方程式の決定

*田川 翔¹、太田 健二¹、廣瀬 敬²、大石 泰生³ (1.東京工業大学 地球惑星科学専攻、2.東京工業大学 地球生命研究所、3.高輝度光科学研究センター)

キーワード：コアの軽元素, コア形成, 水素, シリコン, 高圧実験, ダイヤモンドアンビルセル (DAC)

The light elements in the Earth's core have not been identified yet, but hydrogen is now collecting more attention because recent planet formation theory suggests that large amount of water (e.g. 10 to 100 times seawater) should have been brought to the Earth during the late stage of its formation. Hydrogen is a strong siderophile element and thus it is possibly present in the core. The effect of hydrogen on the property of iron alloy is little known yet. Moreover, the presence of 〜 6 wt.% silicon has been also strongly supported by geochemical and cosmochemical arguments. Here we report hydrogenation of Fe_0.88Si_0.12 (6.5 wt. % Si) alloy and the compression behavior of Fe_0.88Si_0.12H_0.8 alloy to 130 GPa at room temperature. Fe_0.88Si_0.12 foil was loaded into a diamond anvil cell (DAC), and then liquid hydrogen was introduced at temperatures below 20 K. The results demonstrate that the octahedral sites of Fe-Si-H alloys are not fully occupied by hydrogen unlike the case of FeH and as a consequence Fe_0.88Si_0.12H_0.8 is formed under hydrogen-saturated condition. The compressibility of hcp Fe_0.88Si_0.12H_0.8 is similar to that of pure iron. Assuming that liquid and solid alloys have identical density and ideal solution of hydorogen and silicon in the hcp phase, we found that the observed density profile in the outer core may be reconciled with Fe_0.88Si_0.12H_0.4. It means that the amount of hydrogen corresponding to about 90 times seawater could be in the Earth's core. This study suggests that Fe-Si-H system is a plausible chemical composition of the core.