JpGU-AGU Joint Meeting 2020

講演情報

[E] 口頭発表

セッション記号 S (固体地球科学) » S-IT 地球内部科学・地球惑星テクトニクス

[S-IT26] 核ーマントルの相互作用と共進化

コンビーナ:太田 健二(東京工業大学理学院地球惑星科学系)、飯塚 毅(東京大学)、河合 研志(東京大学大学院理学系研究科地球惑星科学専攻)、土屋 卓久(愛媛大学地球深部ダイナミクス研究センター)

[SIT26-07] 高温高圧下における地球深部含水相(Alに富んだphase D)の弾性波速度測定

*井上 徹1,2徐 超文2Gre'aux Steeve2野田 昌道1孫 偉2桑原 秀治2肥後 裕司3 (1.広島大学大学院理学研究科地球惑星システム学専攻、2.愛媛大学地球深部ダイナミクス研究センター、3.(公財)高輝度光科学研究センター)

キーワード:高圧含水相、弾性波速度測定、地球深部、高温高圧

Water plays an important role in the Earth’s interior, which affects the physical and chemical properties of the surrounding materials, for instance, elasticity, rheology, electrical conductivity, and melting behaviors (e.g. [1,2]). The dense hydrous magnesium silicates (DHMSs) are the primary water carriers into the mantle transition zone (MTZ) and could play an important role in delivering water down to the middle part of the lower mantle (LM) (e.g. [3,4]). Among DHMSs, phase D (PhD) is regarded as a potential carrier of water from the MTZ to the LM [4,5]. Recently, it was clarified that PhD can accommodate significant amount of Al2O3 (56.6 wt%) and the Al-bearing PhD is stable up to 2273 K at 26 GPa [6], which may possibly exist in a realistic slab composition and temperature.
To interpret seismic anomalies related to the presence of hydrated regions, direct velocity measurement of hydrous phases is needed. However, the sound velocity of PhD has been poorly investigated, because ultrasonic measurement requires well sintered samples with high purity, which was difficult to achieve for PhD. In this work, we succeeded to synthesize well-sintered Al-bearing PhD, and report the longitudinal (VP) and shear (VS) velocities, as well as the density of the Al-bearing PhD up to 22 GPa and 1300 K.
A polycrystalline Al-bearing PhD sample was synthesized at 24 GPa and 1373 K for 1 hour, from a mixture of MgO, SiO2, Mg(OH)2 and Al(OH)3 by using the 2000-ton Kawai-type apparatus installed at the Geodynamics Research Center (GRC), Ehime University. The recovered sample appeared well-sintered, which is accompanied by little porosity, chemical homogeneity, and is free of accessory phase.
In situ ultrasonic experiments were conducted at the BL04B1 beamline of SPring-8 at high pressures and high temperatures up to 22 GPa and 1300 K. The sample was first squeezed at room temperature and then heated to high temperature. Then the temperature was decreased to 300 K in step of 200 K. The press load was increased after the first data collection cycle, and then, the temperature was increased to high temperature to start another data collection cycle during cooling.
The room temperature data up to 20.5 GPa were fitted to a third-order Birch-Murnaghan equation of state and yielded KT0 = 143 (5) GPa, and KT0'= 5.8 (7), which is much lower KT0 than that of Mg-PhD (KT0 = 166 (3) GPa). The compressional (VP) and shear (VS) velocities increase with increasing pressure while they decrease with increasing temperature up to 22 GPa and 1300 K. Modeled velocities of Al-bearing PhD in hydrous pyrolite along normal mantle geotherm are calculated and will be presented.

Refefrences
[1] T. Inoue: Phys. Earth Planet. Inter., 85, 237 (1994).
[2] T. Yoshino et al.: Earth Planet. Sci. Lett., 288, 291 (2009).
[3] E. Ohtani et al.: Phys. Earth Planet. Inter., 124, 105 (2001).
[4] M. Nishi et al.: Nature Geosci., 7, 224 (2014).
[5] D.J. Frost and Y. Fei.: Phys. Chem. Minerals, 26, 415 (1999).
[6] M.G. Pamato et al.: Nature Geosci., 8, 75 (2014).