14:15 〜 14:30
[SIT19-03] Evidence of Superionic Hydrous SiO2 in the Earth’s lower mantle by In-situ High P-T Electrical Conductivity Measurement
キーワード：超イオン、含水スティショバイト、電気伝導度、沈み込むスラブ
Superionic conductors have been a matter of significant interest and studied in various fields. For example, superionic H2O ice, which is thought to exist underneath a conducting liquid layer inside icy planets such as Uranus and Neptune, has experimentally been observed. The superionic phase is a solid containing ions that are not bonded in a specific position but move in crystals, which is expected to enhance electrical conductivity significantly. It contributes to the formation and maintenance of the planetary magnetic fields of these icy planets.
Subducted slabs are thought to bring water down to the deep mantle. Observing the distribution of water is a key to understanding the bulk water abundance and water-cycling in the Earth’s interior. High-pressure SiO2 phase, one of the major constituent minerals in subducted continental and oceanic crusts, can contain a large amount of water and carry it to the lowermost mantle. The previous molecular dynamics simulations by Umemoto et al. (2016 PEPI) demonstrated a superionic character of the dense SiO2 phase.
The EC of hydrous SiO2 has been reported up to 12 GPa, in which superionic conduction was not observed. Here we performed high P-T EC measurements at lower mantle conditions using a laser-heated diamond-anvil cell (DAC) in order to observe the superionic state of hydrous CaCl2-type SiO2. We employed SiO2 stishovite containing 0.15 wt.% H2O and 4.8 wt.% Al2O3 as a starting material, which was synthesized in a multi-anvil press. The sample resistance was measured using a recently developed method that enables us to obtain the impedance of a laser-heated transparent material in the DAC. We observed a sudden increase in electrical conductivity with increasing temperature around 1500 K, which may indicate the superionic state of hydrous SiO2. We will show the details of our methods and results and discuss the change in the electrical conduction mechanism in the presentation.
Subducted slabs are thought to bring water down to the deep mantle. Observing the distribution of water is a key to understanding the bulk water abundance and water-cycling in the Earth’s interior. High-pressure SiO2 phase, one of the major constituent minerals in subducted continental and oceanic crusts, can contain a large amount of water and carry it to the lowermost mantle. The previous molecular dynamics simulations by Umemoto et al. (2016 PEPI) demonstrated a superionic character of the dense SiO2 phase.
The EC of hydrous SiO2 has been reported up to 12 GPa, in which superionic conduction was not observed. Here we performed high P-T EC measurements at lower mantle conditions using a laser-heated diamond-anvil cell (DAC) in order to observe the superionic state of hydrous CaCl2-type SiO2. We employed SiO2 stishovite containing 0.15 wt.% H2O and 4.8 wt.% Al2O3 as a starting material, which was synthesized in a multi-anvil press. The sample resistance was measured using a recently developed method that enables us to obtain the impedance of a laser-heated transparent material in the DAC. We observed a sudden increase in electrical conductivity with increasing temperature around 1500 K, which may indicate the superionic state of hydrous SiO2. We will show the details of our methods and results and discuss the change in the electrical conduction mechanism in the presentation.