The temperature and viscosity of the Earth's inner core is fundamentally important to the understanding of the state and evolution of the Earth's interior. The melting temperature of iron at 330 GPa is the best first-order estimate of the temperature at the boundary between the solid inner core and the liquid outer core. Knowledge of the temperature of the inner core-outer core boundary provides a constraint on the thermal state and history of the core as well as clues to the powering of the Earth's magnetic dynamo. However, there is little agreement on the melting curve or even the solid phase at relatively low pressures between 50 and 100 GPa. Extrapolated to the pressure of the surface of the inner core, these discrepancies account for more than 3000 K uncertainty in the melting temperature. Much of the problem in these melting determinations stems from difficulty in detecting melt in available methods. Significant seismic anisotropy observed for the inner core suggests that it is undergoing a dynamic process of unknown origin. With no measurements of diffusion properties on hcp iron, existing hypotheses on the mechanism by which inner core anisotropy develops rely on extreme extrapolations of diffusion data on analogous materials. This experiment measures two key properties of iron at Earth's core conditions: its melting temperature and self-diffusion coefficient. A new approach has been developed to provide measurements of melting temperatures and self-diffusion coefficients of iron at high pressures. This new method combines high-precision temperature measurements at pressure in the laser-heated diamond anvil cell with ex-situ laser ablation inductively coupled mass spectrometry measurements to detect changes in isotopically marked iron foil samples recovered from the laser-heated diamond anvil cell. In this way, the diffusion or mixing of iron isotopes on the surface of the sample can precisely differentiate melting from diffusion in the sample, yielding high-precision melting temperatures and diffusion coefficients to high pressures.
