Abstract
The high-pressure, high-temperature behavior of iron was investigated to 140 GPa and 3500 K with in situ synchrotron X-ray diffraction. Iron samples were compressed in diamond-anvil cells and heated up with the double-sided laser-heating system installed at the high-pressure ID27 of the European Synchrotron Radiation Facility (ESRF). Three different structures, namely α-bcc, γ-fcc or ε-hcp Fe were identified as a function of pressure and temperature in the domain we explored. At pressures above 90 GPa, it is clearly shown that ε-iron is the single stable solid phase up to 160 GPa at high temperatures. The analysis of the P-V-T relationship allows us to propose a reliable experimental thermal equation of state (EoS) for iron. We also show that the addition of low pressure points to our EoS refinement yields more robust constrain on the determination of the reference volume V0 of the ε-hcp structure, which has important implications on the final parametrization of the equation of state. The extrapolation of the proposed EoS to core pressure conditions indicates that a pure iron core would have an excess of density of 3% compared to the PREM density profile.
Highlights
Iron is considered to be the main constituent of the Earth’s core
The diffraction patterns recorded between 30 GPa and 140 GPa at temperatures varying from ambient temperature to 3400 K, display how the face centered cubic structure is stable at low pressure and high temperature and further transforms into the hexagonal closed packed structure above 60 GPa
The thermal equation of state of iron has a critical role in the evaluation of the chemical and physical state of the Earth’s solid inner core
Summary
Iron is considered to be the main constituent of the Earth’s core. Cosmochemical abundances and iron meteorites support the idea that iron is present in numbers of planetary cores. Light elements have been shown to be incorporated in the core (see [10] for a review), in order to match density and sound waves velocities obtained by seismological modelling. These light elements produce significant variations of density and elastic properties of the alloys. The nature and amount of light elements has been recently re-examined and models built on a comparison of elastic properties of alloys with those of the Earth’s core [11,12]. These studies indicate that silicon (with ca 2–3 wt %) is the main alloying element in the Earth’s inner core, and oxygen likely to be the main light element
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