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The solidification of a deep magma ocean occurred early in Earth’s history. Although the initial amount of H2O in Earth’s magma ocean is predicted to be low (e.g., <3000 ppm), as an incompatible element it becomes highly enriched (e.g. >10 wt%) in the final few percent of crystallization. In order to understand how a hydrous magma ocean would crystallize at the top of the lower mantle, we determined liquidus phase relations in the MgO-FeOCaO-Al2O3-SiO2-H2O system at 24 GPa. We find that the bridgmanite (brg) + stishovite (st) + melt and bridgmanite (brg) + ferropericlase (fp) + melt cotectic boundary curves trend to Mg-rich melt compositions with decreasing temperature and extend to very high H2O contents (~80 mol% H2O). The brg+st+melt curve is a subtraction curve at < ~18 mol% H2O and a reaction curve at higher H2O contents, whereas the brg+fp+melt is a subtraction curve throughout its length. The density of melts along the two cotectics leads to neutral buoyancywith respect to shallow lower mantle and transition zone minerals at H2O contents up to ~25 mol%. A transient melt-rich layer can form at the top of the lower mantle during late-stage crystallization in a mushy magma ocean when melt percolation dominates. When crystallization exceeds ~98%, hydrous melts (>25 mol% H2O) become buoyant and can percolate into and hydrate the mantle transition zone. 

Abstract A viscosity jump of one to two orders of magnitude in the lower mantle of Earth at 800–1,200-km depth is inferred from geoid inversions and slab-subducting speeds. This jump is known as the mid-mantle viscosity jump 1,2 . The mid-mantle viscosity jump is a key component of lower-mantle dynamics and evolution because it decelerates slab subduction 3 , accelerates plume ascent 4 and inhibits chemical mixing 5 . However, because phase transitions of the main lower-mantle minerals do not occur at this depth, the origin of the viscosity jump remains unknown. Here we show that bridgmanite-enriched rocks in the deep lower mantle have a grain size that is more than one order of magnitude larger and a viscosity that is at least one order of magnitude higher than those of the overlying pyrolitic rocks. This contrast is sufficient to explain the mid-mantle viscosity jump 1,2 . The rapid growth in bridgmanite-enriched rocks at the early stage of the history of Earth and the resulting high viscosity account for their preservation against mantle convection 5–7 . The high Mg:Si ratio of the upper mantle relative to chondrites 8 , the anomalous 142 Nd: 144 Nd, 182 W: 184 W and 3 He: 4 He isotopic ratios in hot-spot magmas 9,10 , the plume deflection 4 and slab stagnation in the mid-mantle 3 as well as the sparse observations of seismic anisotropy 11,12 can be explained by the long-term preservation of bridgmanite-enriched rocks in the deep lower mantle as promoted by their fast grain growth.