Water modulate the Fe-Mg partitioning in Earth’s deep lower mantle

Minor amount of water may alter the chemical properties of minerals. In this talk, we review the role of water as an oxidant and explore the chemistry between water and mineral at Earth’s deep lower mantle. We consider the addition of pyrite-type (Mg,Fe)O₂, which is stable at depth of 1800 through the reaction between water and ferropericlase. Among the major lower mantle components, we find that this pyrite-type phase holds the strongest affinity to ferrous Fe. This is consistent with quenched experiments in which we observed amorphous bands enriched with Fe and an excessive amount of O, possibly derived from the decompression-induced amorphization of high-pressure samples. Our results support the existence of Fe-rich and dense pyrite-type (Mg,Fe)O₂ near the core mantle boundary. It features high density due to its Fe concentration. At the meantime, the pyrite-type phase is known to exhibit low seismic velocity. The formation of this phase suggests some seismic heterogeneities, for example the LLSVP, may have originated from the chemistry between water and surrounding minerals.