Fig. 4

The melt density across the mantle transition zone. a Melt density compared with the density of co-existing solids. The broken line shows the solid density. Black and red curves are the melt density based on the first-principles calculations of dry and hydrous (Mg_0.75,Fe_0.25)₂SiO₄ melts (with hatched regions showing the influence of different compositions). A green curve is the density of dry MORB determined by (Sanloup et al. 2013) and a blue curve is the density of hydrous peridotite melt determined by (Sakamaki 2017) from high-pressure experiments. The gray region shows the effects of a change in FeO content by ± 5%, and the pink region shows the effects of water by ± 2 wt.%. b A schematic diagram showing how density of melt and solid changes across a phase transformation. Structural change in a solid is abrupt, but it is gradual in a melt. This leads to higher density of a melt than a solid above the transition pressure, but below the transition pressure, melt density tends to be lighter than the solid density