Petrologic Structure of a Hydrous 410 Km Discontinuity

We examine a simple thermodynamic model relating the effect of pressure and H₂O content on the relative stabilities of olivine, wadsleyite, and hydrous melt through the upper mantle/transition zone boundary. As noted previously, the strong preference of H₂O for wadsleyite relative to olivine means that H₂O increases the depth interval over which the two polymorphs coexist and displaces the boundary to shallower depths. Olivine and wadsleyite become increasingly hydrous with decreasing depth through the transformation interval, and if the available H₂O exceeds the storage capacity of olivine, then partial melting will occur at the shallowest portions of the interval. Increases in H₂O beyond that necessary to generate incipient melt result in thinning of the transformation interval, but additional shoaling. In the extreme case where melting occurs throughout the melting interval, the transformation interval will be as thin as for the dry case (i.e., ~7 km). These calculations may serve as a starting point for dynamical calculations of possible melting at the 410 km discontinuity and as a guide for interpretation of observational geophysical characteristics of putative hydrous features at the transition zone/upper mantle boundary.