日本地球惑星科学連合2022年大会

講演情報

[J] 口頭発表

セッション記号 S (固体地球科学) » S-MP 岩石学・鉱物学

[S-MP26] 鉱物の物理化学

2022年5月26日(木) 09:00 〜 10:30 201B (幕張メッセ国際会議場)

コンビーナ:大平 格(学習院大学 理学部 化学科)、コンビーナ:柿澤 翔(広島大学大学院先進理工系科学研究科)、座長:大平 格(学習院大学 理学部 化学科)

09:00 〜 09:15

[SMP26-01] Crustal chemistry and compressibility of Fe2+- and oxygen vacancy-rich aluminous bridgmanite: Implications for lower mantle structure and dynamics

★Invited Papers

*石井 貴之1Giacomo Criniti2、宮島 延吉2、Alexander Kurnosov2、Nicki Siersch4、Glazyrin Konstantin3、Hu Qingyang1桂 智男2、Boffa Ballaran Tiziana2 (1.北京高圧科学研究中心、2.バイロイト大学、3.ドイツ電子放射光施設、4.Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie)

キーワード:ブリッジマナイト、玄武岩、下部マントル、マルチアンビルプレス

Seismological studies have inferred the presence of small scale (~10 km) dense bodies in the Earth’s lower mantle through seismic scattering. A numerical modeling also demonstrated the presence of basaltic crust in the lower mantle and the increase of its amount with increasing depth. These studies suggest that significant amounts of subducted basaltic crusts have been accumulated in the lower mantle, which can affect the physical properties and chemical evolution of the lower mantle and mantle dynamics. In this study, we present crystal chemistry of bridgmanitein mid-ocean ridge basalt (MORB) system up to 52 GPa at 2000 K using our advanced multi-anvil technology. We also synthesized single crystals of bridgmanites with compositions similar to basaltic one at 27 GPa and conducted a compression experiment using a diamond anvil cell.
We found a significant compositional change in bridgmanite with pressure in MORB system. Electron energy-loss spectroscopy shows that basaltic Al-rich bridgmanite has significantly high ferrous iron (75-85%), which is a striking contrast to relatively Al-poor peridotitic bridgmanite (~30%). This result is opposite to previous studies which suggested that the ferric iron content in bridgmanite increases with increasing Al content. The oxygen vacancy component MgAlO2.5 is ~10 mol.% below 40 GPa, which is much higher than that in peridotitic bridgmanite (~2-3%). Above 40 GPa, this component disappears whereas the A-site vacancy component of Fe2/3SiO3 appears and the charge-couple Al2O3 component increases. The charge-coupled component of FeAlO3 are limited only 2-10.mol% despite the high Fe and Al content and decreases with pressure, implying less importance of this component in basaltic bridgmanite.
Synthesized bridgmanite single crystals have 75% ferrous iron and ~20-30 mol.% of the oxygen vacancy component, whcih is similar to the compositions of basaltic bridgmanite. The single crystal structure analysis shows almost no iron in the octahedral B-site. A compression experiment shows no spin transition in the bridgmanite up to 60 GPa. The synthesized bridgmanite has a pressure derivative of isothermal bulk modulus (K'T0 ) of 3.1(1), which is relatively low compared with other compositional bridgmanites (~4), indicating that oxygen vacancy-bearing bridgmanite is compressible. The disappearance of the oxygen vacancy component can increase bulk sound velocity and viscosity of basaltic crust, respectively, possibly explaining slab stagnation and plume thinning between 660 km and 1000 km depth. The presence of ferrous iron-rich bridgmanite in the deep lower mantle may also contribute to features of large low-shear-velocity provinces.