[S-IT22] [EE] 核-マントルの相互作用と共進化

[SIT22-05] Experimental investigation of high-pressure phase transitions in AlOOH and FeOOH

*西 真之^1,2、桑山 靖弘³、土屋 旬^1,2、土屋 卓久^1,2 (1.愛媛大学地球深部ダイナミクス研究センター、2.東京工業大学地球生命研究所、3.東京大学)

キーワード：含水鉱物、高圧

Hydrogen is transported into deep Earth’s mantle regions as a form of hydrous minerals via subduction of oceanic plates. Recently discovered CaCl₂-type hydroxides such as (Mg,Si)OOH phase H, delta-AlOOH, and their solid solutions were reported to have large P–T stability fields that encompass conditions representative of the lower mantle, implying the possibility that surface water may be transported as far as the core–mantle boundary. However, although Epsilon-FeOOH has CaCl₂-type structure as well, the solid solution of FeOOH component in CaCl₂-type structure has not been studied. Since FeOOH was recently reported to decompose under the lower-mantle conditions to form FeO₂ releasing H₂, FeOOH could be a key component that strongly affect the stability of CaCl₂-type hydroxide. Here, we report the results of in-situ X-ray diffraction and theoretical studies on AlOOH and FeOOH using a laser-heated diamond anvil cell technique at up to ~200 GPa. In contrast to the previous work suggesting the dehydration of FeOOH in the middle of the lower mantle, we report the formation of a pyrite-type FeOOH that is significantly denser than the surrounding mantle and stable to conditions representative of its base. Furthermore, delta-AlOOH and CaCl₂-type (Al,Fe)OOH also transform to a pyrite-type structure at higher pressures. Based on these experimental and theoretical results, the stability of hydrous phase in the lower mantle and deep interiors of icy planets will be discussed.