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• Hydrous melts of the mantle above and below the transition zone have 40-50 mol% H 2 O. • Structure, density, and viscosity of hydrous melts are determined using ab initio MD. • Hydrous melts with Mg#s > 25 will be less dense than PREM mantle and are inviscid. • Hydrous melts are highly mobile and move upward above and below the transition zone. • Modelling shows the upper mantle and transition zone have similar H 2 O contents. We report ab initio atomistic simulations of hydrous silicate melts under deep upper mantle to shallow lower mantle conditions and use them to parameterise density and viscosity across the ternary system MgO-SiO 2 -H 2 O (MSH). On the basis of phase relations in the MSH system, primary hydrous partial melts of the mantle have 40-50 mol% H 2 O. Our results show that these melts will be positively buoyant at the upper and lower boundaries of the mantle transition zone except in very iron-rich compositions, where ≳ 75% Mg is substituted by Fe. Hydrous partial melts will also be highly inviscid. Our results indicate that if melting occurs when wadsleyite transforms to olivine at 410 km, melts will be buoyant and ponding of melts is unexpected. Box models of mantle circulation incorporating the upward mobility of partial melts above and below the transition zone suggest that the upper mantle becomes efficiently hydrated at the expense of the transition zone such that large differences in H 2 O concentration between the upper mantle, transition zone and lower mantle are difficult to maintain on timescales of mantle recycling. The MORB source mantle with ∼0.02-0.04 wt% H 2 O may be indicative of the H 2 O content of the transition zone and lower mantle, resulting in a bulk mantle H 2 O content of the order 0.5 to 1 ocean mass, which is consistent with geochemical constraints and estimates of subduction ingassing. [ABSTRACT FROM AUTHOR]

Eutectic melting curves in the system MgO–SiO 2 have been experimentally determined at lower mantle pressures using laser-heated diamond anvil cell (LH-DAC) techniques. We investigated eutectic melting of bridgmanite plus periclase in the MgO–MgSiO 3 binary, and melting of bridgmanite plus stishovite in the MgSiO 3 –SiO 2 binary, as analogues for natural peridotite and basalt, respectively. The melting curve of model basalt occurs at lower temperatures, has a shallower d T / d P slope and slightly less curvature than the model peridotitic melting curve. Overall, melting temperatures detected in this study are in good agreement with previous experiments and ab initio simulations at ∼25 GPa ( Liebske and Frost, 2012 ; de Koker et al., 2013 ). However, at higher pressures the measured eutectic melting curves are systematically lower in temperature than curves extrapolated on the basis of thermodynamic modelling of low-pressure experimental data, and those calculated from atomistic simulations. We find that our data are inconsistent with previously computed melting temperatures and melt thermodynamic properties of the SiO 2 endmember, and indicate a maximum in short-range ordering in MgO–SiO 2 melts close to Mg 2 SiO 4 composition. The curvature of the model peridotite eutectic relative to an MgSiO 3 melt adiabat indicates that crystallization in a global magma ocean would begin at ∼100 GPa rather than at the bottom of the mantle, allowing for an early basal melt layer. The model peridotite melting curve lies ∼ 500 K above the mantle geotherm at the core–mantle boundary, indicating that it will not be molten unless the addition of other components reduces the solidus sufficiently. The model basalt melting curve intersects the geotherm at the base of the mantle, and partial melting of subducted oceanic crust is expected. [ABSTRACT FROM AUTHOR]

The melting curve of Ni has been determined to 125 GPa using laser-heated diamond anvil cell (LH-DAC) experiments in which two melting criteria were used: firstly, the appearance of liquid diffuse scattering (LDS) during in situ X-ray diffraction (XRD) and secondly, plateaux in temperature vs. laser power functions in both in situ and off-line experiments. Our new melting curve, defined by a Simon–Glatzel fit to the data where T M ( K ) = [ ( P M 18.78 ± 10.20 + 1 ) ] 1 / 2.42 ± 0.66 × 1726 , is in good agreement with the majority of the theoretical studies on Ni melting and matches closely the available shock wave melting data. It is however dramatically steeper than the previous off-line LH-DAC studies in which determination of melting was based on the visual observation of motion aided by the laser speckle method. We estimate the melting point ( T M ) of Ni at the inner-core boundary (ICB) pressure of 330 GPa to be T M = 5800 ± 700 K ( 2 σ ) , within error of the value for Fe of T M = 6230 ± 500 K determined in a recent in situ LH-DAC study by similar methods to those employed here. This similarity suggests that the alloying of 5–10 wt.% Ni with the Fe-rich core alloy is unlikely to have any significant effect on the temperature of the ICB, though this is dependent on the details of the topology of the Fe–Ni binary phase diagram at core pressures. Our melting temperature for Ni at 330 GPa is ∼2500 K higher than that found in previous experimental studies employing the laser speckle method. We find that those earlier melting curves coincide with the onset of rapid sub-solidus recrystallization, suggesting that visual observations of motion may have misinterpreted dynamic recrystallization as convective motion of a melt. This finding has significant implications for our understanding of the high-pressure melting behaviour of a number of other transition metals. [ABSTRACT FROM AUTHOR]