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Our melting experiments were performed on samples with bulk composition ([Mg.sub.0.89] [[Fe.sub.011]).sub.2] Si[O.sub.4] at pressures from 20 to 159 GPa in a laser-heated diamond-anvil cell (DAC; Supplementary Table 1). The [...], A melt has greater volume than a silicate solid of the same composition. But this difference diminishes at high pressure, and the possibility that a melt sufficiently enriched in the heavy element iron might then become more dense than solids at the pressures in the interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation (1,2). The occurrence of such dense silicate melts in the Earth's lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core-mantle boundary region (3). Recent theoretical calculations (4) combined with estimates of iron partitioning between (Mg,Fe)Si[O.sub.3] perovskite and melt at shallower mantle conditions (5-7) suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments (8). Here we extend measurements of iron partitioning over the entire mantle pressure range, and find a precipitous change at pressures greater than ~76 GPa, resulting in strong iron enrichment in melts. Additional X-ray emission spectroscopy measurements on (M[g.sub.0.95] [Fe.sub.0.05])Si[O.sub.3] glass indicate a spin collapse around 70 GPa, suggesting that the observed change in iron partitioning could be explained by a spin crossover of iron (from high-spin to low-spin) in silicate melt. These results imply that (Mg,Fe)Si[O.sub.3] liquid becomes more dense than coexisting solid at ~1,800 km depth in the lower mantle. Soon after the Earth's formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to ~1,000 km in thickness underneath the solid mantle. We also infer that (Mg,Fe)Si[O.sub.3] perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved towards an iron-rich and silicon-poor composition, consistent with seismic inferences of structures in the core-mantle boundary region.

A phase transition of MgGe[O.sub.3] perovskite was examined at high-pressure and--temperature using synchrotron X-ray diffraction measurements. The results demonstrate that it transforms to a CaIr[O.sub.3]type post-perovskite phase above 63 GPa at 1800 K. The density increase is 1.5% at the transition pressure. These observations confirm that MgGe[O.sub.3] is a low-pressure analogue to MgSi[O.sub.3], for which a similar phase transition was recently found above 125 GPa and 2500 K. The unit-cell parameters of MgGe[O.sub.3] post-perovskite phase obtained at 300 K during decompression from 79 to 6 GPa show that the b-axis is significantly more compressible than are the a- and c-axes, which could be due to the Ge[O.sub.6]-octahedral sheet stacking structure along b. The bulk modulus was determined to be [K.sub.0] = 192([+ or -]5) GPa with a fixed pressure derivative of the bulk modulus, K' of 4.

We examined the phase relation of Fe2S to 306 GPa and 3,000 K and found that Fe2S forms an orthorhombic structure with space group Pnma above 190 GPa. Reexamination of previous X‐ray diffraction data demonstrated that hexagonal close‐packed Fe coexists with Pnma Fe2S at >2,700 K and ~290 GPa, while it concurs with CsCl (B2)‐type stoichiometric FeS at lower temperatures. Our results indicate that Fe2S is the most Fe‐rich iron sulfide above ~250 GPa where Fe3S is not stable. It is most likely that eutectic melting occurs between Fe and Fe2S at 330 GPa corresponding to inner core boundary conditions, instead of between Fe and Fe3S below 250 GPa. The compression curve of Fe2S obtained to 294 GPa shows that it is not dense enough to account for the inner core density, suggesting that the Fe‐Fe2S eutectic liquid gives the maximum sulfur concentration in the outer core. Plain Language Summary: The chemistry of the Earth's core has long remained enigmatic; the observed density and velocity profiles indicate that the core includes some light alloying elements. The present study focuses on sulfur, one of the major candidates, and experimentally investigates the phase relations in the Fe‐FeS system under high pressures relevant to the core. The main finding of this work is that 1) Fe2S adopts the orthorhombic Pnma structure above 190 GPa and 2) it is the most iron‐rich sulfide at >250 GPa where Fe3S decomposes. Our data also show that Fe2S is much lighter than the observed inner core density, suggesting that sulfur concentration in the liquid core is not high enough to crystallize Fe2S. Key Points: Fe2S undergoes a phase transition to the orthorhombic Pnma phase above 190 GPaFe and Fe2S form a eutectic system above ~250 GPa, where Fe3S is not stablePnma Fe2S is not dense enough to account for the inner core density [ABSTRACT FROM AUTHOR]

The pressure-volume-temperature (P-V-T) measurements of the B2 (CsCl-type) phase of KCl were performed at 9 -- 61 GPa/1500 -- 2600 K and up to 229 GPa at room temperature, based on synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell (DAC). The nonhydrostatic stress conditions inside the sample chamber were critically evaluated based on the platinum pressure marker. With thermal annealing by laser after each pressure increment, the deviatoric stress was reduced to less than 1% of the sample pressure even at the multi-megabar pressure range. The obtained P-V-T data were fitted to the Vinet equation of state with the Mie-Grüneisen-Debye model for thermal pressure. The thermal pressure of KCl was found to be as small as ~10 GPa even at 3000 K at any given volume, which is only half of that of common pressure markers (i.e. Pt, Au, or MgO). Such a low-thermal pressure validates the use of a KCl pressure medium as a pressure marker at high temperatures. [ABSTRACT FROM AUTHOR]

We have performed melting experiments on the MgO‐SiO2 binary system to 139 GPa in a diamond anvil cell. Both MgO‐MgSiO3 and MgSiO3‐SiO2 eutectic compositions were examined by textural/chemical characterizations of recovered samples. Results demonstrate that the MgO‐MgSiO3 eutectic becomes more enriched in MgO with increasing pressure and is close in composition to Mg2SiO4 at the core‐mantle boundary (CMB) pressure. On the other hand, the MgSiO3‐SiO2 eutectic is more SiO2‐rich at higher pressure and exhibits SiO2/(MgO + SiO2) = 0.66 at the CMB. These have profound implications for a large‐scale compositional stratification possibly formed upon solidification of a magma ocean. The MgO‐MgSiO3 eutectic liquid being more MgO‐rich than previously predicted suggests that only bridgmanite crystallizes up to 60 wt% solidification from a fully molten state of a pyrolitic lowermost mantle. In contrast, an SiO2 layer may develop when melting/crystallization occurs in rocky objects with silica‐rich enstatite‐chondritic compositions. Plain Language Summary: MgO‐SiO2 is the most important binary system to understand melting/crystallization processes in the Earth's crust and mantle as well as those in other rocky objects. Its melting phase relations, in particular, eutectic compositions, also have profound implications for a large‐scale compositional stratification possibly formed upon solidification of a magma ocean. However, the eutectic compositions in the MgO‐MgSiO3 and MgSiO3‐SiO2 systems have not been examined experimentally at the deep lower mantle conditions yet. Here we performed melting experiments and determined both of these eutectic compositions to 135 GPa, corresponding to the pressure at the bottom of the mantle, by a combination of high‐pressure‐temperature generation in a laser‐heated diamond anvil cell and textural/compositional characterization of recovered samples. The results demonstrate that the MgO‐MgSiO3 eutectic liquid is more MgO‐rich than previously predicted. It suggests that a large volume of MgSiO3 bridgmanite‐dominant‐layer may have formed during crystallization of a deep magma ocean and is possibly still preserved up to the present in the Earth's lower mantle. On the other hand, the MgSiO3‐SiO2 eutectic liquid implies that an SiO2‐phase‐rich layer could develop when extensive melting occurs in rocky objects with silica‐rich enstatite‐chondritic compositions. Key Points: Our DAC experiments determined the eutectic liquid compositions in both the MgO‐MgSiO3 and MgSiO3‐SiO2 systems up to the CMB pressureThe MgO‐MgSiO3 eutectic liquid composition is close to Mg2SiO4 at 135 GPaMgSiO3 bridgmanite crystallizes predominantly up to 60 wt% solidification from a fully molten mantle above the CMB [ABSTRACT FROM AUTHOR]