2:45 PM - 3:00 PM
[SIT16-10] Iron-water exchange at the earth’s core-mantle boundary
Keywords:ultralow velocity zones, multi-anvil apparatus experiment, bridgmanite, magnesiowustite, water, high pressure
According to the seismological observations, the earth’s core-mantle boundary (CMB) is a chemically heterogeneous region as the silicate mantle and the liquid iron alloy of the Earth's core contact under extremely high-pressure and high-temperature conditions. Ultralow velocity zones (ULVZs) are locally distributed at the CMB and represent the dense domains that have variable thicknesses in the range of 5–40 km at the base of the lower mantle (e.g., McNamara et al., 2010; Garnero and Helmberger, 1998). The sound velocity of the Fe-rich layers can explain its seismic structure. However, slow diffusivities of mantle silicate minerals under pressure likely prevent the formation of huge reaction layers.
We focused on the water which may drive the chemical reactions at the CMB. Recent experimental studies suggest that plate subduction delivers a certain amount of water which is incorporated in the crystal structure, may be delivered to the deep earth by plate subduction. Here, we show the experimental results of the reaction mechanisms between bridgmanite and metallic iron under hydrous and pressure condidtions.
A multi-anvil apparatus (Orange-3000) was used to achieve homogeneous temperature distribution within large sample volumes. These experiments were carried out at 25 GPa and 1473 K to 2573 K. We used bridgmanite polycrystalline which includes 0.45 wt.% of water, and metallic iron (Fe) as starting materials.
We observed the formation of (Fe,Mg)O-rich layer through the reaction between these samples. On the other hand, no reaction layer was observed in anhydrous condition. This result indicates that water drives the reaction between iron and bridgmanite, which may occur at CMB. The thickness of the reaction layer depends on the amount of water. Accordingly, this reaction can be explained in two stages. First, a chemical reaction between H2O in bridgmanite polycrystalline and iron (H2O+3Fe→2FeH+FeO). Second, FeO partitioning into bridgmanite (FeO+MgSiO3→(Fe,Mg)O+(Fe,Mg)SiO3). These chemical reactions occurred not only when the iron was solid, but also when it was liquid.
The diffusion rate of Fe in (Fe,Mg)O is extremely fast to be compared to that in silicate minerals. Therefore, over the age of the Earth, the reaction layers may be formed on the order of km at the CMB. We calculated the effect of water on the seismic structure of the CMB based on the reaction between water and iron, which we observed in the experiment. The seismic structure of ULVZs can be explained if we assume that 6×1020 kg of water (~40% mass of water in the earth’s oceans) is used for the reaction and a partial FeO-rich melt occurs. This result suggests that the water in the deep mantle has a significant role in the formation of the ULVZs.
We focused on the water which may drive the chemical reactions at the CMB. Recent experimental studies suggest that plate subduction delivers a certain amount of water which is incorporated in the crystal structure, may be delivered to the deep earth by plate subduction. Here, we show the experimental results of the reaction mechanisms between bridgmanite and metallic iron under hydrous and pressure condidtions.
A multi-anvil apparatus (Orange-3000) was used to achieve homogeneous temperature distribution within large sample volumes. These experiments were carried out at 25 GPa and 1473 K to 2573 K. We used bridgmanite polycrystalline which includes 0.45 wt.% of water, and metallic iron (Fe) as starting materials.
We observed the formation of (Fe,Mg)O-rich layer through the reaction between these samples. On the other hand, no reaction layer was observed in anhydrous condition. This result indicates that water drives the reaction between iron and bridgmanite, which may occur at CMB. The thickness of the reaction layer depends on the amount of water. Accordingly, this reaction can be explained in two stages. First, a chemical reaction between H2O in bridgmanite polycrystalline and iron (H2O+3Fe→2FeH+FeO). Second, FeO partitioning into bridgmanite (FeO+MgSiO3→(Fe,Mg)O+(Fe,Mg)SiO3). These chemical reactions occurred not only when the iron was solid, but also when it was liquid.
The diffusion rate of Fe in (Fe,Mg)O is extremely fast to be compared to that in silicate minerals. Therefore, over the age of the Earth, the reaction layers may be formed on the order of km at the CMB. We calculated the effect of water on the seismic structure of the CMB based on the reaction between water and iron, which we observed in the experiment. The seismic structure of ULVZs can be explained if we assume that 6×1020 kg of water (~40% mass of water in the earth’s oceans) is used for the reaction and a partial FeO-rich melt occurs. This result suggests that the water in the deep mantle has a significant role in the formation of the ULVZs.