  Several important geophysical features such as heat flux at the Core-Mantle
Boundary or geodynamo production are intimately related with the temperature
profile in the Earth's core. However, measuring the melting curve of iron at
conditions corresponding to the Earth inner core boundary under pressure of 330
GPa has eluded scientists for several decades. Significant discrepancies in
previously reported iron melting temperatures at high pressure have called into
question the validity of dynamic measurements. We report measurements made with
a novel approach using X-ray absorption spectroscopy using an X-ray free
electron laser source coupled to a laser shock experiment. We determine the
state of iron along the shock Hugoniot up to 420 GPa (+/- 50) and 10800 K (+/-
1390) and find an upper boundary for the melting curve of iron by detecting
solid iron at 130 GPa and molten at 260, 380 and 420 GPa along the shock
Hugoniot. Our result establishes unambiguous agreement between dynamic
measurement and recent extrapolations from static data thus resolving the
long-standing controversy over the reliability of using dynamic compression to
study the melting of iron at conditions close to the Earth's inner core
boundary and beyond.
