[S-IT21] 固体地球科学と材料科学の融合が切り拓く新展開

[SIT21-P06] 含水鉱物superhydrous phase Bの安定性と結晶構造へのAlの影響

*柿澤 翔^1,2、井上 徹^1,2、中野 泰斗³、黒田 みなみ⁴、栗林 貴弘⁵、坂本 直哉⁶、圦本 尚義⁷ (1.広島大学大学院先進理工系科学研究科、2.プレート収束域の物質科学研究拠点、3.愛媛大学地球深部ダイナミック研究センター、4.北海道大学大学院理学院自然史科学専攻地球惑星システム科学講座、5.東北大学大学院理学研究科地学専攻、6.北海道大学創成研究機構、7.北海道大学理学研究院自然史科学部門)

キーワード：地球深部、水、含水鉱物、superhydrous phase B

We determined the stability and chemical composition of Al-bearing superhydrous phase B at 20–24 GPa and 1400–2000 °C to discuss the mechanism of water transport in the mantle transition zone and uppermost lower mantle at temperatures close to the mantle geotherm. Superhydrous phase B[Mg₁₀Si₄O₁₄(OH)₄] is one of the important dense hydrous magnesium silicates (DHMSs) in hydrous peridotite system for understanding the deep water cycle in the Earth’s interior. It contained significant amounts of Al₂O₃, from 14 to 32 wt%, and Al-bearing superhydrous phase B remained stable, even at 2000 °C and pressures of approximately 20–24 GPa. Moreover, two types of superhydrous phase B with different chemical compositions coexisted at 20–24 GPa and 1600 °C. The Al₂O₃ and H₂O contents increased, and the MgO and SiO₂ contents decreased as the pressure and temperature increased up to 1600 °C. Above 1600 °C, the MgO and Al₂O₃ contents increased, and the SiO₂ and H₂O contents decreased as the temperature increased. We found two substitution mechanisms: (1) 2Mg²⁺ + Si⁴⁺ = 2Al³⁺ + 2H⁺ + V_Mg (Mg site vacancy) (2Mg²⁺ = Al³⁺ + H⁺ + V_Mg):(Si⁴⁺ = Al³⁺ + H⁺) = 1:1, (2) Si⁴⁺ + 16H⁺ = 4Mg²⁺ + 4Al³⁺. The maximum H₂O content of Al-bearing superhydrous phase B is 11.1(3) wt%, which is ~1.9 times larger than that of the Mg-end-member. The crystal structures of the two coexisting superhydrous phase B values are expected to be slightly different from each other. The present results indicate that Al-bearing superhydrous phase B can be stable in a subducted slab with a high Al content compared to pyrolite (e.g, chlorite) at temperatures typical of the mantle transition zone and the lower mantle. Thus, water can be transported to the lower mantle by Al-bearing superhydrous phase B in the subducting slab, even at the typical mantle geotherm.
In addtion, we investigated the crystal structure of Al-bearing superhydrous phase B (Mg_8.1Si_2.0Al_1.9H_5.9O₁₈) by single crystal X-ray diffraction (XRD), Raman and Fourier Transform infrared spectroscopy (FT-IR) to determine the Al positions and to predict the H positions in Al-bearing superhydrous phase B. As a result of the crystal structure refinement, Al was clarified to be located in octahedral Mg1, Mg2, and Si1 sites in superhydrous phase B with Pnnm space group. The result is consistent with 2Mg²⁺ + Si⁴⁺ = 2Al³⁺ + 2H⁺ + V_Mg substitution. The obtained FT-IR spectra, difference Fourier maps, and bond-valence considerations leaded to new H positions. They are located at (1) O6-H…O4, which is a part of AlO₆ octahedron, (2) O5-H…O3, which is the same as Mg-endmember superhydrous phase B, and (3) O4-H…O1, which is share edge of Mg2O₆ and Mg4O₆ octahedra.