Intermittent negative velocity gradients adjacent to the 410 and 660 km discontinuities

Multiple seismic indications of an intermittent low-velocity layer extending up to 70 km above the olivine-to-wadsleyite reaction at the 410 km seismic discontinuity are sufficient to acknowledge it as a true feature. In a recent analysis of western U.S. seismic data, negative velocity gradients (NVG) extending 25-65 km above the 410 km discontinuity are identified in 11 out of 18 stacks of receiver functions from back azimuth corridors that contain 36-98 station-years of data. For the 11 identified NVG, phasing analysis finds the correct Ps phase move-out and thus confidence in phase identification is high. Seismic modeling suggest a 1% near neutrally buoyant melt fraction can extend up to 70 km above the 410 km discontinuity via surface tension to explain the NVG signals. Hence, the leading hypothesis to explain the origin of this NVG is the water-filter model of Bercovici and Karato (2003) that predicts the production of a dense hydrous melt that can gravitationally segregate atop the olivine to wadsleyite mineral reactions near 410 km depth. A newer seismic observation using EarthScope Transportable Array data is the finding of an intermittent NVG below the 410 and 660 km discontinuities in an image volume that extends from the American west coast to the Mississippi River. We suggest that these new intermittent NVG also can be explained by the water-filter model. The NVG beneath the 410 is produced via pressure induced solidification due to downward advection of a melt layer above the 410. The quenching of hydrous melt layer would be predicted to make a low-velocity wadsleyite layer. The NVG beneath the 660 requires the downflow of wet mantle to produce hydrous melt due to the water solubility reduction of the lower mantle mineral assemblage with respect to ringwoodite. This below 660 NVG would be transient as the melt beneath the 660 would be expected to percolate upwards on a short time scale. In summary, these NVG interpretations are consistent with whole-mantle convection that concentrates water in the transition zone due to the low solubility of water for olivine above the 410 and perovskite below the 660.