Melting of Fe-Ni-Si and Fe-Ni-S alloys at megabar pressures: implications for the core-mantle boundary temperature

High pressure melting behavior of three Fe-alloys containing 5 wt% Ni and (1) 10 wt% Si, (2) 15 wt% Si or (3) 12 wt% S was investigated up to megabar pressures by in situ X-ray diffraction and laser-heated diamond anvil cell techniques. We observe a decrease in melting temperature with increasing Si content over the entire investigated pressure range. This trend is used to discuss the melting curve of pure Fe. Moreover, our measurements of eutectic melting in the Fe-Fe3S system show a change in slope around 50 GPa concomitant with the fcc-hcp phase transition in pure solid iron. Extrapolations of our melting curve up to the core-mantle boundary pressure yield values of 3,600-3,750 K for the freezing temperature of plausible outer core compositions.