Keywords:Pyrite-structured FeOOH, Deep water, Mantle, Core
We investigated the phase relations of the Fe-H2O system at high pressures based on in situ X-ray diffraction experiments and first-principles calculations and demonstrate that FeHx and FeO are present at pressures less than ~78 GPa. A recently reported pyrite-structured FeO2 was identified in the Fe-H2O system at pressures greater than ~78 GPa after laser-heating. The phase observed in this study has a unit-cell volume 8%-11% larger than that of FeO2, produced in the Fe-O binary system reported previously, suggesting that hydrogen might be retained in a FeO2Hx crystal structure. Our observations indicate that H2O is likely introduced into the deep Earth through reaction between iron and water during the accretion and separation of the metallic core. Additionally, reaction between Fe and H2O would occur at the core-mantle boundary, given water released from hydrous subducting slabs that intersect with the metallic core. Accumulation of volatile-bearing iron compounds may provide new insights into the enigmatic seismic structures observed at the base of the lower mantle.
Acknowledgements: This study was supported by the JSPS KAKENHI (Grant Numbers JP15H05748 to E.O., JP15H05828 to A.S., and JP15H05834 to J.T.). J.T. was also supported in part by MEXT as “Exploratory Challenge on Post-K computer” (Frontiers of Basic Science: Challenging the Limits). E.O. was also partly supported by the Ministry of Education and Science of the Russian Federation (Project 14B25.31.0032 to E.O.). S.K. was partly supported by KAKENHI Grant Numbers 26247089, 15H05831, 16K13902. This work was also supported by the JSPS Japanese-German Graduate Externship.