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).

Highlights-Density of liquid Fe-S alloys has been measured under high pressure as a function of temperature by in situ X-ray diffraction in multi-anvil press.-Thermal expansion of liquid Fe-S alloys has been determined up to 7 GPa and 2200 K.-Top-down crystallization is the most likely scenario for Fe-FeS cores of planetesimals and small planets.

We developed methods to use synthesized boron-doped diamond (BDD) as a heater in a multi-anvil high-pressure apparatus. The synthesized BDD heater could stably generate an ultra-high temperature without the issues (anomalous melt, pressure drop, and instability of heating) arising from oxidation of boron into boron oxide and graphite-diamond conversion. We synthesized BDD blocks and tubes with boron contents of 0.5-3.0 wt. % from a mixture of graphite and amorphous boron at 15 GPa and 2000 °C. The electrical conductivity of BDD increased with increasing boron content. The stability of the heater and heating reproducibility were confirmed through repeated cycles of heating and cooling. Temperatures as high as ∼3700 °C were successfully generated at higher than 10 GPa using the BDD heater. The effect of the BDD heater on the pressure-generation efficiency was evaluated using MgO pressure scale by in situ X-ray diffraction study at the SPring-8 synchrotron. The pressure-generation efficiency was lower than that using a graphite-boron composite heater up to 1500 tons. The achievement of stable temperature generation above 3000 °C enables melting experiments of silicates and determination of some physical properties (such as viscosity) of silicate melts under the Earth's lower mantle conditions.

We have developed a high-pressure furnace assembly with a commercially available chemical-vapor-deposition synthesized boron-doped diamond heater consisting of four strips for large-volume multi-anvil presses (LVPs). This assembly successfully generated temperatures up to 2990 K at 15 GPa. It also has highly reproducible power–temperature relations, enabling us to estimate temperature from power reliably. It can be used for experiments above 9 GPa and is particularly useful for synchrotron x-ray experiments because of the x-ray transparency. It is also competitive in price. This technique is, thus, practical in various LVP experiments in the diamond-stability field.

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A rapid-quench technique for multi-anvil high-pressure-temperature experiments Review of Scientific Instruments 91, 065105 (2020);