Phase Relations of Earth’s Core-Forming Materials

Abstract

Recent updates on phase relations of Earth’s core-forming materials, Fe alloys, as a function of pressure (P), temperature (T), and composition (X) are reviewed for the Fe, Fe-Ni, Fe-O, Fe-Si, Fe-S, Fe-C, Fe-H, Fe-Ni-Si, and Fe-Si-O systems. Thermodynamic models for these systems are highlighted where available, starting with 1 bar to high-P-T conditions. For the Fe and binary systems, the longitudinal wave velocity and density of liquid alloys are discussed and compared with the seismological observations on Earth’s outer core. This review may serve as a guide for future research on the planetary cores.

Thermoelastic Properties of Liquid Fe‐Rich Alloys Under Martian Core Conditions

“…We conclude that in the investigated P‐T‐X range, that is, 19–35 GPa, 2100–2400 K with 0–14.8 mol% O, liquid Fe‐O alloys mix ideally regarding volume and density. In agreement with observations at 1 bar (Komabayashi, 2021) and melting experiments at higher pressures (Chen et al., 2008; Pease & Li, 2022; Pommier et al., 2018) that suggest non‐ideality for the Fe‐S system, we find that volume, density and compressibility cannot be accurately described by linear mixtures of liquid Fe and S (Figure 2 and Figure S3 in Supporting Information ). The excess molar volume of mixing for Fe‐S is taken into account in our mixing model and will be discussed below.…”

“…We conclude that in the investigated P-T-X range, that is, 19-35 GPa, 2100-2400 K with 0-14.8 mol% O, liquid Fe-O alloys mix ideally regarding volume and density. In agreement with observations at 1 bar (Komabayashi, 2021) and melting experiments at Results from our ab inito molecular dynamics simulations are shown as solid circles. Experimental data for liquid Fe (K20 (Kuwayama et al, 2020) and N20 (Nishida et al, 2020)), Fe-S (B03 (Balog et al, 2003), M13 (Morard et al, 2013), K17 (Kawaguchi et al, 2017), N20 (Nishida et al, 2020) and K22 (Kawaguchi et al, 2022)) and Fe-C (N15 (Nakajima et al, 2015)) are shown as solid or open angular markers.…”

“…Because of the challenging nature of high‐pressure experiments, on the one hand, and the ensuing scarcity of available data under extreme P‐T conditions relevant to the interior of Earth‐sized planets, on the other, ideal mixing of binary liquid Fe‐X alloys has been widely assumed (e.g., Badro et al., 2014; Komabayashi, 2021; Morard et al., 2018; Poirier, 1994) as a means of evaluating elastic properties of multi‐component Fe‐rich liquids. While ideal mixing has been confirmed at conditions of Earth’s core (D. Huang et al., 2019; Umemoto & Hirose, 2020), our low‐pressure simulations show non‐ideal (i.e., non‐linear) behavior of density and compressibility upon the mixing of Fe and S (Figure 2 and Figure S3 in Supporting Information ), which, when combined with the non‐ideal liquidus curve found for the Fe‐S system (Chen et al., 2008), clearly shows that the Fe‐S non‐ideality (in the Fe‐rich portion of the Fe‐S binary) persists across the pressure range covering Mars’ core.…”

High-pressure melting experiments of Fe3S and a thermodynamic model of the Fe–S liquids for the Earth’s core

“…Seismological, cosmochemical, and mineral physics studies indicate that the Earth's core is mainly composed of iron with some amounts of impurities to account for a 4%-7% density deficit compared to pure iron at the relevant pressure (P) and temperature (T) conditions [1,2]. Over 70 years since the first proposition by [1], the light element in the core is yet to be determined even among a small number of candidate elements: Si, S, O, C, and H [3][4][5]. Of these candidate elements, sulphur has been most extensively studied [3] and the binary system Fe-S is currently still of significant interest [6][7][8][9][10][11].…”

Properties of Transition Metals and Their Compounds at Extreme Conditions

“…In addition to the 10 original research articles described above, this Special Issue also include two review articles. In the first one, Tetsuya Komabayashi [11] shows an overview on the recent updates on the phase-relations in Fe-alloys as a function of pressure and temperature. Such phase-relations are of particular interest for the engineering and metallurgy fields-as they serve as recipes for creating specific phases with certain properties-and are of extreme importance for Earth science, as the major components of the Earth's core-subjected to extreme conditions of pressure and temperature-are Fe and a detailed characterisation of the phase domains of their (geophysically relevant) alloys at those extreme conditions can provide important information on the dynamics and evolution of our planet.…”