Two‐stage spin transition of iron in FeAl‐bearing phase D at lower mantle

Abstract

AbstractHydrous magnesium silicate phase D plays a key role in the transport of water from the upper to the lower mantle via subducted slabs. Here we report pressure dependence hyperfine and lattice parameters of FeAl‐bearing phase D up to megabar pressures using synchrotron nuclear forward scattering and X‐ray diffraction in a diamond anvil cell at room temperature. FeAl‐bearing phase D undergoes a two‐stage high‐spin to low‐spin transition of iron for Fe2+ at 37–41 GPa and for Fe3+ at 64–68 GPa. These transitions are accompanied by an increase in density and a significant softening in the bulk modulus and bulk velocity at their respective pressure range. The occurrence of the dense low‐spin FeAl‐bearing phase D with relatively high velocity anisotropies in deep‐subducted slabs can potentially contribute to small‐scale seismic heterogeneities in the middle‐lower mantle beneath the circum‐Pacific area.