Melting curve of iron to 290 GPa determined in a resistance-heated diamond-anvil cell: implications for the Earth and planetary core

The Earth's magnetic field is generated by liquid iron core and protects the Earth's biosphere from loss of atmosphere. The melting temperature of iron is a key information to know whether liquid core exists in the rocky planets or not. Moreover, the melting point of iron at 330 GPa offers a key constraint on Earth's core temperatures, since the liquid core coexists with solid at the inner core boundary (ICB). However, previous results using a laser-heated diamond-anvil cell (DAC) have been largely inconsistent with each other, likely because of an intrinsic large temperature gradient and its temporal fluctuation. Here we employed an internal-resistance-heated DAC and determined the melting temperature of pure iron up to 290 GPa, for the first time above 200 GPa by static compression experiments. A small extrapolation of the present experimental results yields a melting point of 5500 ± 220 K at the ICB, higher than 4850 ± 200 K reported by previous laser-heated DAC by Boehler (1993) but is lower than 6230 ± 500 K by Anzellini et al. (2013). Accounting for the melting temperature depression due to core-alloying elements, the upper bounds for the temperature at the ICB and the core-mantle boundary (CMB) are estimated to be 5120 ± 390 K and 3760 ± 290 K, respectively. Such low present-day CMB temperature suggests that the lowermost mantle has avoided global melting, at least since early Proterozoic Eon. Obtained melting temperature of iron may also help to constrain the possible existence of the liquid iron core in the exoplanet.