Gradually Hydrating Upper Mantle and a Moderately Wet Mantle Transition Zone Determined From Wadsleyite‐Olivine Phase Relations

The discontinuity that exists at ∼410 km depth in the Earth emerges largely from the olivine‐wadsleyite transition which also marks a barrier between the likely wet mantle transition zone (MTZ) and the likely dry upper mantle (UM). Using a consistent thermodynamic ab initio method we predict the dynamics of the two step transition for upwelling wet wadsleyite producing first a solid olivine and then a hydrous melt which we predict to be buoyant. Features of the discontinuity can be best explained by a MTZ that is moderately wet (<0.1–0.3 wt%) with isolated wetter patches (0.3–0.6 wt%) that produce large drops in V s through partial melting. Near saturation (>0.6 wt%) of the MTZ, even in localized areas, can be ruled out as this would produce unfeasibly large decreases in V s unless such wet patches were located exclusively in non‐upwelling regions. The existence of melt is a function of temperature and water content but is predicted to occur to some degree in nearly all upwelling regions preventing selective localized melting from operating as an elemental filter for different rock sources. We predict that a moderately wet MTZ leads to late melting in Earth's history and thus a disparity between UM and MTZ water budgets can be preserved to the present day even with a loss of water to the UM through buoyant hydrous rich melts.