Water, on the other hand, possesses no primary host phases which are stable throughout much of the mantle. In the lower mantle, the solubility of H in ferromagnesian pv is expected to increase with increasing Fe and Al substitution, and the solubility of H in stishovite also rises with increasing Al substitution. The possibly restricted nature of H 2 O transport and storage in the mantle is suggested both by the apparent absence of broadened seismic discontinuities in the transition zone and the absence of OH in olivine and orthopyroxene inclusions from diamonds. Furthermore, it is through such substitutions that ferric iron enters lower mantle phases, allowing important reduction-oxidation reactions with volatile species such as CO 2 . In terms of seismic structure, such phenomena may lead to complexity at the base of the transition zone, the top of the lower mantle, and the bottom of the lower mantle near the core-mantle boundary.
Significant uncertainties remain, however.
A general absence of experimental reversals means that the stability fields of many phases have not been clearly delineated. We are very grateful to the numerous researchers whose work is reviewed herein, many of whom kindly provided preprints of their work. Schematic representation of energy versus depth for silicate perovskite and various oxide assemblages. Disproportionation occurs when an oxide assemblage attains a lower energy than the silicate perovskite. Go to Craig's home page .