During its early stages, the Earth's mantle was partially molten into a magma ocean thanks to the heat released through collisions (Tonks and Melosh, 1993; Canup, 2004, 2008). With time, this magma ocean progressively cooled down and crystallised to shape the initial structure and chemical composition of the primitive solid Earth. Although such a crystallisation is a major event in the Earth's geological record, it is still not fully constrained.

Using a cutting-edge experimental and analytical protocol of melting followed by controlled crystallisation in the laser-heated diamond anvil cell (Nabiei et al., 2021) combined with analytical transmission electron microscopy, we measured the composition of the first solids (liquidus phase) that crystallised from a pyrolitic melt at pressures between 55 and 133 GPa by quantitative STEM-EDXS. We also constrained the residual melts' compositions and the equilibrium phases allowing us to determine the experimental melting phase diagrams of pyrolite across the lower mantle. We additionally inversed these experimental data to constrain the Margules' parameters which were then used in a thermodynamic model to predict the crystallisation sequence of a pyrolitic melt at any depth in the lower mantle (Boukaré et al., 2015). Such an experimentally constrained thermodynamic model can be further implemented in dynamical evolution models to predict the behaviour of the crystallising magma ocean.