Melting of iron close to Earth's inner core boundary conditions detected by XANES spectroscopy in laser shock experiment

Abstract:

When modeling the Earth’s interior, essential features lie in our knowledge of iron and iron alloys physical properties at extreme pressures and temperatures. While the density profile of the Earth’s interior is rather well constrained from seismic data, the temperature at the boundary between the solid inner core and liquid outer core (ICB, Inner Core Boundary), where the pressure is estimated to be of 330GPa, remains up to now largely uncertain. It corresponds to the melting temperature of an iron alloy containing a small but unconstrained amount of impurities [1]. As a reference, the melting temperature of pure iron at ICB pressure condition is thus one of the most important parameters of earth and planetary interiors physics. For that reason, measuring the iron melting curve at conditions corresponding to the Earth Inner Core Boundary (ICB) under pressure of 330GPa has eluded scientists for several decades.

Here we used X-ray Absorption Near Edge Structure (XANES) spectroscopy with ultrafast X-ray Free Electron Laser (XFEL) sources coupled to a laser shock experiment, to detect the state of iron along the shock Hugoniot up to 420GPa (+/- 50) and 10800K (+/- 1390). Our results allows to put an upper constrain on the high pressure-melting curve of iron by detecting well beyond recent diamond-anvil cell measurements performed at 150GPa [2].

[1] J.P. Poirier, Phys. Earth Planet. Int. 85, 319 (1994).

[2] S. Anzellini et al., Science 340, 464 (2013).