11:00 〜 13:00
[SIT19-P02] Reaction mechanism between hydrous bridgmanite and metallic iron: implications for the origin of ultralow-velocity zones
キーワード：地震波超低速度領域、マルチアンビルプレス実験、ブリッジマナイト、フェロペリクレース
Water (hydrogen) can be dissolved into the structures of most mantle minerals in hydrous minerals and also nominally anhydrous minerals such as olivine, wadsleyite, and bridgmanite. Recent experimental studies suggest these minerals deliver a certain amount of water to the Earth’s interior via the plate subduction. High-resolution seismic tomography images demonstrated that some subducting plates penetrate into the deep lower mantle; therefore, this deep-water subduction processes may allow the reaction between water and iron at the bottom of the mantle and may form some Fe-rich reaction layers at the core-mantle boundary (CMB) (e.g., Liu et al., 2017; Nishi et al., 2020). Furthermore, the sound velocity of the Fe-rich layers can explain the seismic structure of ultralow velocity zones (ULVZs), which represent the dense domains that are characterized by variable thicknesses in the range 5–40 km at the base of the lower mantle (e.g., McNamara et al., 2010; Garnero and Helmberger, 1998). However, slow atomic diffusivities of mantle minerals may prevent to form such visible huge reaction layers. Here, we show the experimental results on the reaction mechanisms between hydrous bridgmanite and metallic iron under pressure.
A multi-anvil apparatus was used to achieve homogeneous temperature distributions within large sample volumes. We used bridgmanite which includes 0.45 wt.% of water, and metallic iron (Fe). We observed the formation of FeO through the reaction between hydrous bridgmanite and metallic iron. This experimental result had not been seen without water, indicating that water may drive the core-mantle interaction. The reaction would cause the partial melt of minerals at the core-mantle boundary because FeO component should lower the solidus of the MgO-FeO system (Fu et al, 2018).
Minerals that are stable in the lower mantle have a little water and they may deliver water continuously to the bottom of the mantle through the mantle downflow. Therefore, we calculated the effect of water on the seismic structure of CMB based on the reaction between water and iron, which we observed in the experiment. The seismic structure of ULVZ can be explained if we assume that 29%~53% mass of water in the earth’s oceans is used to the reaction and also there is the partial melt rich in FeO. The diffusion-controlled reaction under pressure is slow for mantle silicates (Holzapfel et al, 2005). However, increase of FeO at CMB would enhance the reaction kinetics by forming partial melt and interconnected network of ferropericlase in the mantle (Holzapfel et al, 2003; Van Orman et al, 2003). This result suggests that the water in the deep mantle has a significant role in explaining the mechanism to form the ULVZs.
A multi-anvil apparatus was used to achieve homogeneous temperature distributions within large sample volumes. We used bridgmanite which includes 0.45 wt.% of water, and metallic iron (Fe). We observed the formation of FeO through the reaction between hydrous bridgmanite and metallic iron. This experimental result had not been seen without water, indicating that water may drive the core-mantle interaction. The reaction would cause the partial melt of minerals at the core-mantle boundary because FeO component should lower the solidus of the MgO-FeO system (Fu et al, 2018).
Minerals that are stable in the lower mantle have a little water and they may deliver water continuously to the bottom of the mantle through the mantle downflow. Therefore, we calculated the effect of water on the seismic structure of CMB based on the reaction between water and iron, which we observed in the experiment. The seismic structure of ULVZ can be explained if we assume that 29%~53% mass of water in the earth’s oceans is used to the reaction and also there is the partial melt rich in FeO. The diffusion-controlled reaction under pressure is slow for mantle silicates (Holzapfel et al, 2005). However, increase of FeO at CMB would enhance the reaction kinetics by forming partial melt and interconnected network of ferropericlase in the mantle (Holzapfel et al, 2003; Van Orman et al, 2003). This result suggests that the water in the deep mantle has a significant role in explaining the mechanism to form the ULVZs.