Evolution of a hydrous silicate melt layer above the mantle transition zone

Recent research suggests that a dense hydrous silicate melt may form and accumulate at the 410-km discontinuity due to dehydration melting. A recent model by Leahy and Bercovici (2007) predicts that accumulated melt could form a steady-state structure that efficiently recycles water into the transition zone via a mechanism that relies upon viscous spreading of the melt along the discontinuity into regions of downgoing mantle flow. In these regions, melt is advected into the wadsleyite stability field, and the ensuing crystallization enriches the melt in water relative to the overlying olivine. The overlying solid then melts to restore equilibrium. This mechanism is hypothesized to permit the melt to spread further into mantle downwelling regions, allowing the establishment of a steady-state water and silicate cycle. Here we present a dynamic investigation of this mechanism to address whether the melt reaction or gravitational collapse governs the evolution of the melt layer towards steady state, whether transient or steady state solutions are more relevant on time scales controlling upper mantle chemistry, and whether the steady solutions are stable.