Chemical Reaction Between Molten Iron and MgSiO3 at the Core-Mantle Boundary Condition

Chemical reaction between the mantle and core is an important process for dissolution of the light elements in the core and the anomalous seismic structure at the core-mantle boundary (CMB). A reaction between (Mg, Fe)SiO3 perovskite and metallic iron has been experimentally investigated using laser-heated diamond-anvil cell (LHDAC)[1]. It is essential to clarify that the chemical reaction and textural relationship between iron and post-perovskite[2] at the CMB condition. Powdered natural MgSiO3 enstatite from Sri Lanka and iron powder (99.5 %) were used for the starting material. We have conducted high-pressure experiments under the conditions of the CMB, up to 139(10) GPa and 3,000(200) K using LHDAC technique. In this condition, metallic iron was coexisted with post-perovskite, which was confirmed in situ X-ray diffraction studies at Photon Factory(KEK) in Tsukuba.We made detailed TEM observations of the textures and compositions of the metallic iron and post-perovskite coexisting in the run products. Compositional analysis indicates that a significant amount of oxygen up to 6.3 wt.% and silicon up to 4.0 wt.% are dissolved in metallic iron. The solubility of silicon and oxygen in metallic iron observed in this study is consistent with [3], it can readily account for the estimated density deficit of the core [4]. The present results strongly imply that both oxygen and silicon are the most plausible candidates of the light elements in the core. The dihedral angles between molten iron and post-perovskite from TEM images of the recovered sample from 139(10) GPa and 3,000(200) K was 67 degrees, it is inconsistent with previous study [3]. The required melt fraction to form an interconnected network of liquid iron (critical melt fraction) is around 2 vol.% [5], suggesting a small amount of metallic iron can be trapped at the core-mantle boundary. [1] e.g., Knittle and Jeanloz, Science, 1991 [2]Murakami et al., Science, 2004 [3]Takafuji et al., EPSL, 2004 [4]Anderson and Isaak, PEPI, 2002 [5]Bargen and Waff, JGR, 1986