Compositional Controls on the Contribution of (Fe,Al)OOH Solid Solution to Core‐Mantle Boundary Heterogeneities

Hydrous oxyhydroxides such as FeOOH and AlOOH have been proposed as potential carriers of hydrogen into Earth's deep mantle. However, these phases form extensive solid solutions in natural systems, and the influence of Al incorporation on the stability and elastic properties of Fe‐bearing oxyhydroxides under lower‐mantle conditions remains insufficiently constrained. Here we investigate the phase stability and thermoelastic properties of (Fe,Al)OOH solid solutions across ε/δ‐type and pyrite‐type (Py) structures using first‐principles density functional theory calculations. An ideal two‐phase solid‐solution model constructed from the end members ε‐FeOOH, Py‐FeOOH, δ‐AlOOH, and Py‐AlOOH is used to quantify how Al incorporation modifies the ε‐to‐Py transition and its seismic expression along representative very cold slab geotherms. Our results show that Al significantly shifts the ε‐to‐Py transition to higher pressures and broadens the transition interval. For compositions containing ∼40–70% Al, the phase transition exhibits a broad transition interval of ∼20–30 GPa along the very cold slab geotherm, spanning the core‐mantle boundary (CMB) pressure range. Fe‐rich compositions produce large reductions in seismic velocities and increased density, whereas Al‐rich compositions exhibit elastic properties close to the ambient mantle. These contrasting signatures suggest that compositional variations in slab‐derived (Fe,Al)OOH may contribute to seismic heterogeneities in the lowermost mantle, potentially influencing the geophysical expression of deep‐mantle structures near the CMB.