On the dynamics of a hydrous melt layer above the transition zone

The “transition zone water filter” (Bercovici and Karato, 2003) model relies on the presence of a dense hydrous melt above the 410‐km discontinuity that is formed by dehydration melting as wet wadsleyite undergoes a phase change to low‐water‐solubility olivine. Recent studies suggest that, particularly in the Pacific, there is sufficient water in the transition zone for dehydration melting to occur. Here we construct a model for melt spreading and entrainment along the 410‐km discontinuity. In particular, we propose a mechanism for melt entrainment in downwelling regimes at ambient mantle temperatures. We find: (1) that this entrainment mechanism is efficient in that it does not require melt to reach subducting slabs in order to recycle most of the upwelling water; (2) that in steady state, the efficiency of entrainment does not necessitate the existence of a thick melt layer; (3) that the entrainment of hydrous, buoyant material into the transition zone significantly affects the bulk mantle flow field, thereby diminishing entrainment efficiency and allowing the melt to spread further; (4) that these interactions help to recirculate water from the entrained regions through the transition zone; and (5) that the adjustments in bulk mantle velocity due to water entrainment are characterized by a broadening of upward flow with a corresponding decrease in amplitude. Our model predicts regions of high water content in the transition zone at ambient mantle temperatures, which may contribute to seismic observations of high attenuation near slabs.