As the main constituent of planetary cores, pure iron phase diagram under high pressure and temperature is of fundamental importance in geophysics and planetary science. However, previously reported iron-melting curves show large discrepancies (up to 1000 K at the Earth's core-mantle boundary, 136 GPa), resulting in persisting high uncertainties on the solid-liquid phase boundary. Here we unambiguously show that the observed differences commonly attributed to the nature of the used melting diagnostic are due to a carbon contamination of the sample as well as pressure overestimation at high temperature. The high melting temperature of pure iron under core-mantle boundary (4250 ± 250 K), here determined by X-ray absorption experiments at the Fe K-edge, indicates that volatile light elements such as sulfur, carbon, or hydrogen are required to lower the crystallization temperature of the Earth's liquid outer core in order to prevent extended melting of the surrounding silicate mantle.Plain Language Summary Iron is the main constituent of planetary cores; however, there are still large controversies regarding its melting temperature and phase diagram under planetary interior conditions. The present study reconciles different experimental approaches using laser-heated diamond anvil cell with different in situ X-ray diagnostics (absorption, diffraction, and Mossbauer spectroscopy). The main reason of discrepancies (over 1000 K at core-mantle boundary conditions) is attributed to carbon contamination from the diamond anvils and metrology issues related to thermal pressure overestimation. A high-melting temperature for iron at core-mantle boundary pressure would imply the presence of volatile elements in the liquid outer core, such as sulfur, carbon, or hydrogen, in order to lower its crystallization temperature and avoid extended melting of the surrounding silicate mantle.
Precise knowledge of the melting temperatures of iron, nickel, and their alloys at pressures of the deep Earth would allow us to better constrain the parameters used for the Earth's heat budget and dynamics. However, melting curves of transition metals at pressures approaching 100 GPa and above are still controversial. To address this issue, we report new data on the melting temperature of nickel in a laser‐heated diamond anvil cell up to 100 GPa obtained by X‐ray absorption spectroscopy (XAS), a technique rarely used at such conditions. We couple this for the first time to ex situ analysis of the sample, providing a further validation of the melting criterion adopted here. Finally, a Simon‐Glatzel fit to the melting data obtained in this work, combined with those obtained in the most recent X‐ray diffraction experiments, gives TM(K)=1727×[]PM17±3+112.5±0.1, defining the most up‐to‐date X‐ray‐determined melting curve for Ni. This result confirms that Ni could be ignored in the discussion on melting properties and thermal profile of the Earth's core, as it should affect the Fe melting point by only 10–20 K at 90 GPa.
Investigation of the structural and electronic properties of solid and liquid FeO at high pressures and temperatures is of great interest in geophysics and planetary sciences, as well as in condensed-matter physics. FeO is a typical Mott insulator under ambient conditions and a prototypical highly correlated transition metal oxide. Different electronic and structural transitions occur in FeO under compression at ambient temperature. Along with other transition metal oxides, it undergoes a spin crossover and it metalizes under pressure (Ohta et al., 2012). One of the open questions is to understand if the metalization and the transition from high spin to low spin state in the solid state are simply concomitant or more intricately related (Greenberg et al., 2020;Leonov, 2015;. Similar to the case of other crystalline planetary materials, the spin crossover has received much attention in the solid state (see reviews by Badro [2014] and Lin et al. [2013]) but its effects in the liquid phase are still relatively poorly known (Holmstrom & Stixrude, 2016). An improved knowledge of physical properties of iron alloys in their liquid state under extreme conditions is required to understand the composition and properties of planetary cores, as well as to constrain scenarios of planetary differentiation.FeO represents an important end-member, as O is expected to be a major light element in Earth's core (Badro
In the past couple of decades, the laser-heated diamond anvil cell (combined with in situ techniques) has become an extensively used tool for studying pressure-temperature-induced evolution of various physical (and chemical) properties of materials. In this review, the general challenges associated with the use of the laser-heated diamond anvil cells are discussed together with the recent progress in the use of this tool combined with synchrotron X-ray diffraction and absorption spectroscopy.
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