Linking the Spin Transition of Ferric Iron in δ-(Al,Fe)OOH to Water Storage in the Lower Mantle

As the most massive geochemical reservoir, the lower mantle affects the Earth's budget of volatile elements, including hydrogen or H₂O. The properties of minerals in the lower mantle are further affected by changes in the electronic configurations of iron cations, that is, by spin transitions. The feedback between spin transitions and potential storage of H₂O in solid hydrous phases in the lower mantle, however, remains unexplored. By combining high-pressure nuclear resonant inelastic X-ray scattering and high-pressure high-temperature X-ray diffraction experiments, we constrained the thermal equation of state of δ-(Al,Fe)OOH, a member of the phase H solid solution. Based on the derived thermal equation of state of δ-(Al,Fe)OOH and the underlying thermodynamic model, we calculate the excess Gibbs free energy that arises from the spin transition of ferric iron in this compound and evaluate the effect on phase equilibria. The results of our analysis show that the spin transition of ferric iron in phase H may significantly reduce the thermodynamic activity and hence the concentration of H₂O in a coexisting hydrous melt. As a consequence, nominally anhydrous minerals of the lower mantle may become dehydrated in the presence of phase H. Our analysis further suggests that, under certain conditions, the spin transition may expand the thermal stability of Fe³⁺-bearing phase H and create a geochemical link between the storage of H₂O in phase H and ferric iron in the lower mantle.