The link between water and ferric iron in Earth's lower mantle

As the most massive geochemical reservoir, the lower mantle affects Earth's budget of volatile elements, including hydrogen or H2O. The properties of minerals in Earth's lower mantle are further affected by changes in the electronic configurations of iron cations, i.e., spin transitions. Potential feedback between deep H2O retention and spin transitions in mantle minerals, however, remain unexplored. We constrained the excess energy arising from the spin transition of ferric iron in high-pressure oxyhydroxide phases by a combination of high-pressure experiments. Our results show that the spin transition of ferric iron may stabilize oxyhydroxide phases in the lower mantle even at low H2O concentrations. In addition, the spin transition expands the thermal stability of high-pressure oxyhydroxides and creates a geochemical link between H2O and ferric iron in Earth's lower mantle that might be sampled by rising plumes and reflected in related magmas produced at the surface.