International audienceEutectic melting temperatures in the Fe–FeO and Fe–Fe3C systems have been determined up to 150 GPa. Melting criteria include observation of a diffuse scattering signal by in situ X-Ray diffraction, and textural characterisation of recovered samples. In addition, compositions of eutectic liquids have been established by combining in situ Rietveld analyses with ex situ chemical analyses. Gathering these new results together with previous reports on Fe–S and Fe–Si systems allow us to discuss the specific effect of each light element (Si, S, O, C) on the melting properties of the outer core. Crystallization temperatures of Si-rich core compositional models are too high to be compatible with the absence of extensive mantle melting at the core–mantle boundary (CMB) and significant amounts of volatile elements such as S and/or C (>5 at%, corresponding to >2 wt%), or a large amount of O (>15 at% corresponding to ∼5 wt%) are required to reduce the crystallisation temperature of the core material below that of a peridotitic lower mantle

We performed laser-heated diamond anvil cell experiments on bulk compositions in the systems MgO–SiO2–H2O (MSH) and MgO–Al2O3–SiO2–H2O (MASH) that constrain the stability of hydrous phases in Earth's lower mantle. Phase identification by synchrotron powder diffraction reveals a consistent set of stability relations for the high-pressure, dense hydrous silicate phases D and H. In the MSH system phase D is stable to ~50GPa, independent of temperature from ~1300 to 1700K. Phase H becomes stable between 35 and 40GPa, and the phase H out reaction occurs at ~55GPa at 1600K with a negative dT/dP slope of ~−75K/GPa. Between ~30 and 50GPa dehydration melting occurs at ~1800K with a flat dT/dP slope. A cusp along the solidus at ~50GPa corresponds with the intersection of the subsolidus phase H out reaction, and the dT/dP melting slope steepens to ~15K/GPa up to ~85GPa.In the MASH system phase H is stable in experiments between ~45 and 115GPa in all bulk compositions studied, and we expect aluminous phase H to be stable throughout the lower mantle depth range beneath ~1200km in both peridotitic and basaltic lithologies. In the subsolidus, aluminous phase D is stable to ~55GPa, whereas at higher pressures aluminous phase H is the stable hydrous phase. The presence of hydrogen may sharpen the bridgmanite to post-perovskite transition. The ambient unit cell volume of bridgmanite increases systematically with pressure above ~55GPa, possibly representing an increase in alumina content, and potentially hydrogen content, with depth. Bridgmanite in equilibrium with phases D and H has a relatively low alumina content, and alumina partitions preferentially into the hydrous phases. The melting curves of MASH compositions are shallower than in the MSH system, with dT/dP of ~6K/GPa. Phase D and H solid solutions are stable in cold, hydrated subducting slabs and can deliver water to the deepest lower mantle. However, hydrated lithologies in the lower mantle are likely to be partially molten at all depths along an ambient mantle geotherm

International audienceThe 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