Thermochemical heterogeneities detected today in the Earth's mantle could arise from ongoing partial melting in different mantle regions. A major open question, however, is the level of chemical stratification inherited from an early magma-ocean (MO) solidification. Here we show that the MO crystallized homogeneously in the deep mantle, but with chemical fractionation at depths around 1000 km and in the upper mantle. Our arguments are based on accurate measurements of the viscosity of melts with forsterite, enstatite and diopside compositions up to~30 GPa and more than 3000 K at synchrotron X-ray facilities. Fractional solidification would induce the formation of a bridgmanite-enriched layer at~1000 km depth. This layer may have resisted to mantle mixing by convection and cause the reported viscosity peak and anomalous dynamic impedance. On the other hand, fractional solidification in the upper mantle would have favored the formation of the first crust.

Identifying the magma-ocean (MO) solidification type is vital to understand the evolution of the Earth's mantle leading to the present-day isotopic heterogeneity. The Earth is believed to have experienced largescale melting owing to massive energy released during the accretion and differentiation, which formed MOs with various depths (e.g., Elkins-Tanton, 2012 and reference therein). The most important MO was caused by a giant Moon-forming impact (e.g., Tonks & Melosh, 1993), which may have reached the lower mantle's bottom. The solidification of this MO produced the initial mantle structure that evolved to the present-day mantle. If the solidification was fractional, the initial structure should already have had substantial chemical heterogeneity. If equilibrium solidification occurred, the present heterogeneity (e.g., Hofmann, 1997), which isotope geochemistry suggests, should have been produced later. Thus, an assumption of the solidification type leads to an entirely different scenario of mantle evolution. Based on phase relations and elemental partitioning at pressures less than 26 GPa, geochemical models showed that even ∼10 wt% of bridgmanite segregation can substantially deviates the ratios of minor and trace elements from the observed ranges, suggesting a maximum fractionation of ∼10 wt% (Ito et al., 2004;Liebske, Corgne, et al., 2005;Walter et al., 2004). However, no conclusion was made for the solidification type. Although the solidification type has been investigated by geodynamic modeling, poorly constrained physical parameters prevent the modeling from giving a definitive conclusion (Solomatov, 2007).