Melting of Iron Explored by Electrical Resistance Jump up to 135 GPa

The melting temperature (T-m) of iron at megabar pressures constrains the Earth's core temperature structure and dynamics. Previous experimental studies demonstrated large discrepancies in T-m at high pressures. We used the intrinsic resistance discontinuity across solid-liquid transition as a melting criterion to study the melting behavior of iron in laser-heated diamond anvil cells. The resistance jump is sensitive to the incipient melting, capable of detecting the emergence of less than 2 vol.% melts. We found a high melting curve of iron at 30-135 GPa, but a relatively low-transition temperature of the slow-fast recrystallization. The determined T-m of iron is 4306(300) K at the core-mantle boundary (CMB) pressure in good agreement with the static and shockwave experimental results by Anzellini et al. (2013, ) and Li et al. (2020, ). The high melting point of iron implies a high and steep geothermal gradient and influences a heat flow across the CMB. Plain Language Summary The melting of iron is fundamental to constrain the thermal structure, solidification, heat flux, and evolution of the Earth's core. Here, we propose a sensitive melting criterion, the resistance discontinuity across the phase transition, which is capable of detecting 2 vol.% melting of the bulk sample. Using this criterion, we determined the melting temperature of iron up to 135 GPa. The new melting curve of iron reconciles with the most recent shockwave and static experimental results, indicating a melting temperature of 4306(300) K at the Earth's core-mantle boundary. Large discrepancies existing in the melting of iron are unlikely caused by varying melting criteria, while chromatic aberrations should be one of the major reasons resulting from refractive optical lens in the temperature measurement system. Key Points The criterion of an electrical resistance jump is sensitive to detect 1%-2% melt fraction of bulk iron to 135 GPa in diamond anvil cells The slopes in the temperature-resistance profile are changed across the solid-solid and slow-fast recrystallization transitions of iron The melting temperature of iron is similar to 4306 K at core-mantle boundary pressure, consistent with latest static and shockwave experiments