Melting of iron determined by X-ray absorption spectroscopy to 100 GPa

Abstract

Temperature, thermal history, and dynamics of Earth rely critically on the knowledge of the melting temperature of iron at the pressure conditions of the inner core boundary (ICB) where the geotherm crosses the melting curve. The literature on this subject is overwhelming, and no consensus has been reached, with a very large disagreement of the order of 2,000 K for the ICB temperature. Here we report new data on the melting temperature of iron in a laser-heated diamond anvil cell to 103 GPa obtained by X-ray absorption spectroscopy, a technique rarely used at such conditions. The modifications of the onset of the absorption spectra are used as a reliable melting criterion regardless of the solid phase from which the solid to liquid transition takes place. Our results show a melting temperature of iron in agreement with most previous studies up to 100 GPa, namely of 3,090 K at 103 GPa.iron melting curve | XAS | megabar range

Dynamic X-ray diffraction observation of shocked solid iron up to 170 GPa

“…1 (3,500 K, 170 GPa). All other studies are extrapolating data from lower pressures in the 80-to 100-GPa range (4,26,27,39). This is also in very good agreement with the results of our picosecond X-ray diffraction experiments made on single-crystal iron and shown in Supporting Information.…”

“…Therefore, because of this geophysical interest, its phase diagram under high pressure and temperature has been extensively studied by static (1)(2)(3)(4) or dynamic (5-9) compression techniques. In particular, the determination of its melting curve at the Earth's inner−outer core boundary constrains the thermodynamics that governs Earth's heat budget and dynamics such as the heat flux from the core to the mantle, the power available for the geodynamo, and the planetary cooling rate (10).…”

“…In addition to the study of the solid−solid phase transitions, like bcc−face-centered cubic (fcc) in temperature and bcc−hcp in pressure, and to the measurement of some properties of these phases (11,22,23), X-ray diffraction also allows the study of solid−liquid transitions (24) and measurement of liquid density (25). Static compression measurements of the melting curve of iron have led to over 1,000 K difference in melting temperatures between different studies (1,2,4,26,27). This ongoing controversy could be related to various problems of gradients and inhomogeneities in the studied material as well as of chemical reaction and thermal diffusion with the diamonds.…”

The Time-resolved and Extreme-conditions XAS (TEXAS) facility at the European Synchrotron Radiation Facility: the energy-dispersive X-ray absorption spectroscopy beamline ID24

“…Very little can be found on the structure of liquid 3d, 4d or 5d metals, although much attention is being devoted to the calculation or measurement of melting curves in these systems (Anzellini et al, 2013;Aquilanti et al, 2015). Due to strong directional bonding arising from an incomplete d-electron valence band, atoms are not 'spherical' and compression of liquid d-metals could produce interesting non-uniform effects on the local structure.…”

High-pressure studies with x-rays using diamond anvil cells

“…The mean square thermal displacement of atoms (Lamb-Mössbauer factor) can be measured as a function of temperature, providing a plot for determining the onset of melting [301,312]. Synchrotron x-ray absorption has also been successfully used for melting identification [141,473].…”