R3: High-pressure science and deep Earth’s material

[R3-P-08] High-temperature phase relations of hydrous aluminosilicates in the AlOOH-AlSiO₃OH system

*Goru TAKAICHI¹, Masayuki Nishi², Youmo Zhou³, Shinichi Machida⁴, Ginga Kitahara⁵, Akira Yoshiasa⁶, Tetsuo Irifune⁷, Toru Inoue¹ (1. Hiroshima Univ. Adv. Sci. Eng., 2. Osaka Univ. Sci., 3. China Univ. Geosci., 4. CROSS, 5. KEK, 6. Kumamoto Univ., 7. Ehime Univ. GRC)

Keywords:

Hysrous aluminosilicates,Deep water cycle,Multi-anvil apparatus

The slabs that subduct from the trench into the Earth's interior are composed by sediments, basalts, and peridotite. Each rock layer is thought to contain hydrous minerals of various compositions. Hydrous magnesium silicates (DHMSs) are stable over a wide pressure range in the low-temperature part of peridotite layer . In basalt layer, Al-rich DHMS and Fe-Ti oxyhydroxides are the major water carriers. In sediment layer, hydrous aluminosilicates can retain water at higher temperatures than DHMS. In particular, phase Egg, one of the hydrous aluminosilicates, was found naturally as a diamond inclusion [1]. Therefore, it is important to investigate the high-temperature phase relations of hydrous aluminosilicates for understanding the water cycle in the Earth’s deep mantle.In this study, the phase relations of hydrous aluminosilicates in the AlOOH-AlSiO₃OH system were investigated at 22 GPa and 1400–2275 K using a multi-anvil apparatus. Based on the X-ray diffraction measurements and composition analysis of the recovered samples, we found that the AlSiO₄H phase egg forms a solid solution with δ-AlOOH above 1500 K. Additionally, at temperatures above 1800 K, two unknown hydrous aluminosilicates with compositions Al_2.03Si_0.97O₆H_2.03 (Phase I) and Al_2.11Si_0.88O₆H_2.11 (Phase II) appeared, depending on the bulk composition of the starting materials. Although the crystal structure has not been determined, both phases are not consistent with Aluminous phase D [2] or the recently reported Phase Psi [3], based on X-ray diffraction patterns. These phases can host large amount of water at least up to 2275 K, exceeding the typical mantle geotherm. Thus, the extreme thermal stability of hydrous aluminosilicates suggests that subducted sediments could be a possible water reservoir from the mantle transition zone to uppermost lower mantle. In the presentation, we will also introduce recent work on the phase relations of hydrous aluminosilicates.

[1] R. Wirth et al., EPSL 259 (2007) 384-399.
[2] MG. Pamato et al., Nat. Geosci. 8 (2015) 75-79.
[3] B. Wang et al., Nat. Commun. 16 (2025) 1038

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