Mineralogy of the deep lower mantle in the presence of H2O

Qingyang Hu; Jin Liu; Jiuhua Chen; Bingmin Yan; Yue Meng; Vitali B Prakapenka; Wendy L Mao; Ho-Kwang Mao

doi:10.1093/nsr/nwaa098

Mineralogy of the deep lower mantle in the presence of H₂O

Natl Sci Rev. 2020 May 13;8(4):nwaa098. doi: 10.1093/nsr/nwaa098. eCollection 2021 Apr.

Authors

Qingyang Hu¹, Jin Liu¹, Jiuhua Chen², Bingmin Yan¹, Yue Meng³, Vitali B Prakapenka⁴, Wendy L Mao⁵, Ho-Kwang Mao¹

Affiliations

¹ Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094, China.
² Center for Study of Matter under Extreme Conditions, Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33199, USA.
³ High Pressure Collaborative Access Team (HPCAT), X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439, USA.
⁴ Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60437, USA.
⁵ Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA.

Abstract

Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800-2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H₂O to form a previously unknown (Mg, Fe)O₂H _x (x ≤ 1) phase. The (Mg, Fe)O₂H _x has a pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg_0.6Fe_0.4)O or beyond 2300 km for (Mg_0.7Fe_0.3)O. The (Mg, Fe)O₂H _x is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg²⁺, Fe³⁺ and H⁺). This important phase has a number of far-reaching implications including extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM and an internal source for modulating oxygen in the atmosphere.

Keywords: ferropericlase; high pressure; lower mantle; mantle mineralogy; water-mantle interaction.