Solubility of hydrogen and ferric iron in rutile and TiO 2 (II): Implications for phase assemblages during ultrahigh‐pressure metamorphism and for the stability of silica polymorphs in the lower mantle

The solubility of hydrogen in Fe 2 O 3 ‐doped rutile and TiO 2 (II) at 1100°C has been experimentally determined. H incorporation in rutile is coupled to substitution of Fe 3+ onto the octahedral Ti 4+ site. In contrast, TiO 2 (II) contains no structurally‐incorporated hydrogen. The dominant Fe 3+ incorporation mechanism in both phases is unrelated to H content, and involves substitution of Fe 3+ onto octahedral Ti 4+ sites, charge‐balanced by oxygen vacancies. Substitution of Fe 2 O 3 into TiO 2 (II) stabilizes the structure to much lower pressures than in the pure TiO 2 system. Results indicate that Al‐bearing stishovite could act as an important carrier of water in subducting oceanic crust, but that formation of the post‐stishovite phase with the α‐PbO 2 structure would represent a significant dehydration event at the base of the lower mantle.