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• Liquidus phase relations in a hydrous silicate system were determined at 24 GPa. • Melts are neutrally buoyant at 24 GPa before ∼98% crystallization of a magma ocean. • A transient melt-rich layer may form atop the lower mantle in a mushy magma ocean. The solidification of a deep magma ocean occurred early in Earth's history. Although the initial amount of H 2 O 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-FeO-CaO-Al 2 O 3 -SiO 2 -H 2 O 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 H 2 O contents (∼80 mol% H 2 O). The brg+st+melt curve is a subtraction curve at < ∼18 mol% H 2 O and a reaction curve at higher H 2 O contents, whereas the brg+fp+melt is a subtraction curve throughout its length. The density of melts along the two cotectics leads to neutral buoyancy with respect to shallow lower mantle and transition zone minerals at H 2 O 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% H 2 O) become buoyant and can percolate into and hydrate the mantle transition zone. [ABSTRACT FROM AUTHOR]