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Abstract Seismic and magnetotelluric studies suggest hydrous silicate melts atop the 410 km discontinuity form 30–100 km thick layers. Importantly, in some regions, two layers are observed. These stagnant layers are related to their comparable density to the surrounding mantle, but their formation mechanisms and detailed structures remain unclear. Here we report a large decrease of silicate melt viscosity at ~14 GPa, from 96(5) to 11.7(6) mPa⋅s, as water content increases from 15.5 to 31.8 mol% H₂O. Such low viscosities facilitate rapid segregation of melt, which would typically prevent thick layer accumulation. Our 1D finite element simulations show that continuous dehydration melting of upwelling mantle material produces a primary melt layer above 410 km and a secondary layer at the depth of equal mantle-melt densities. These layers can merge into a single thick layer under low density contrasts or high upwelling rates, explaining both melt doublets and thick single layers. 

Abstract With the advent of toroidal and double-stage diamond anvil cells (DACs), pressures between 4 and 10 Mbar can be achieved under static compression, however, the ability to explore diverse sample assemblies is limited on these micron-scale anvils. Adapting the toroidal DAC to support larger sample volumes offers expanded capabilities in physics, chemistry, and planetary science: including, characterizing materials in soft pressure media to multi-megabar pressures, synthesizing novel phases, and probing planetary assemblages at the interior pressures and temperatures of super-Earths and sub-Neptunes. Here we have continued the exploration of larger toroidal DAC profiles by iteratively testing various torus and shoulder depths with central culet diameters in the 30–50 µm range. We present a 30 µm culet profile that reached a maximum pressure of 414(1) GPa based on a Pt scale. The 300 K equations of state fit to ourP–Vdata collected on gold and rhenium are compatible with extrapolated hydrostatic equations of state within 1% up to 4 Mbar. This work validates the performance of these large-culet toroidal anvils to > 4 Mbar and provides a promising foundation to develop toroidal DACs for diverse sample loading and laser heating. 

Abstract The motion of liquid iron (Fe) alloy materials in the outer core drives the dynamo, which generates Mercury's magnetic field. The assessment of core models requires laboratory measurements of the melting temperature of Fe alloys at high pressure. Here, we experimentally determined the melting curve of Fe9wt%Si and Fe17wt%Si up to 17 GPa using in situ and ex situ measurements of intermetallic fast diffusion that serves as the melting criterion in a large‐volume press. Our determined melting slopes are comparable with previous studies up to about 17 GPa. However, when extrapolated, our melting slopes significantly deviate from previous studies at higher pressures. For Mercury's core with a model composition of Fe9wt%Si, the melting temperature‐depth profile determined in our study is lower by ∼150–250 K when compared with theoretical calculations. Using the new melting curve of Fe9wt%Si and the electrical resistivity values from a previous study of Fe8.5wt%Si, we estimate that the electronic thermal conductivity of liquid Fe9wt%Si is 30 Wm⁻¹K⁻¹at the Mercury'sCMBpressure of 5 GPa and 37 Wm⁻¹K⁻¹at an assumedICBof 21 GPa, corresponding to heat flux values of 23 mWm⁻²and 32 mWm⁻², respectively. These values provide new constraints on the core models. 

Abstract The heat extracted from the core by the overlying mantle across the core‐mantle boundary controls the thermal evolution of the core. This in turn leads to the solidification of the inner core in association with the exsolution of light alloying elements into the liquid outer core. Although the temperature (T) at the inner core boundary (ICB) would be adjusted to account for the effects of the light elements, the melting T of Fe places an upper bound at the ICB and it is a vital point in the thermal profile of the core. Here, we determine the melting T of Fe in the multi‐anvil press by characterizing the interface of Fe‐W interaction. Our data place a tighter constraint on the melting curve of Fe between 8 and 21 GPa, that is directly applicable to small planetary bodies and serves as an anchor for melting curve of Fe at higher pressure. 

The solidification of a deep magma ocean occurred early in Earth’s history. Although the initial amount of H2O in Earth’s magma ocean is predicted to be low (e.g., <3000 ppm), as an incompatible element it becomes highly enriched (e.g. >10 wt%) in the final few percent of crystallization. In order to understand how a hydrous magma ocean would crystallize at the top of the lower mantle, we determined liquidus phase relations in the MgO-FeOCaO-Al2O3-SiO2-H2O system at 24 GPa. We find that the bridgmanite (brg) + stishovite (st) + melt and bridgmanite (brg) + ferropericlase (fp) + melt cotectic boundary curves trend to Mg-rich melt compositions with decreasing temperature and extend to very high H2O contents (~80 mol% H2O). The brg+st+melt curve is a subtraction curve at < ~18 mol% H2O and a reaction curve at higher H2O contents, whereas the brg+fp+melt is a subtraction curve throughout its length. The density of melts along the two cotectics leads to neutral buoyancywith respect to shallow lower mantle and transition zone minerals at H2O contents up to ~25 mol%. A transient melt-rich layer can form at the top of the lower mantle during late-stage crystallization in a mushy magma ocean when melt percolation dominates. When crystallization exceeds ~98%, hydrous melts (>25 mol% H2O) become buoyant and can percolate into and hydrate the mantle transition zone.