Free Energies of Fe‐O‐Si Ternary Liquids at High Temperatures and Pressures: Implications for the Evolution of the Earth's Core Composition. Issue 4 (18th February 2022)

• Record Type: Journal Article
• Authors: Zhang, Zhigang; Csányi, Gábor; Alfè, Dario; Zhang, Yigang; Li, Juan; Liu, Jin
• Is Part Of: Geophysical research letters. Volume 49:Issue 4(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-18
• Subjects: Earth's core -- evolution -- free energy -- first‐principles -- iron‐alloying liquids -- machine learning; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL096749
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Solubility of oxygen and silicon in the iron‐alloying liquids is important for constraining the core composition over the Earth's history. In this study, we systematically simulated the free energies of Fe‐O‐Si ternary liquids from 3000 K, 55 GPa to 6000 K, and 330 GPa. We found that temperature and pressure have remarkable influences on the free energies, and the nonideality of mixing is important for the chemical potentials even at high temperatures. Equilibrating with SiO2 phase, Fe‐O‐Si liquids significantly enhance the solubility of Si and O simultaneously with increasing temperature. Considering the secular cooling of the Earth's core, this leads to high precipitation rates of SiO2 once it was saturated, which would efficiently drive ancient geodynamo. If the evolution of Earth's core started from an oxygen‐poor composition, later incorporations of oxygen seem to be needed to reach a core composition compatible with geophysical observations. Plain Language Summary: According to planetary formation models, the Earth's core experienced two stages in its history: during the early stage of accretion, the iron‐rich core gained light elements from the silicate mantle at high temperatures; in the subsequent stage of evolution, the core gradually cooled down and this might have led to changes in core composition. During these two stages, solubility of light elements in liquid iron determines their gain and loss limits and thus is of fundamental importance. In this study, we obtained the free energies of Fe‐O‐Si liquids and predicted the exsolution boundaries of SiO2 . The derived data show that Si and O would be precipitated out of the core as SiO2 crystals at the core‐mantle boundary with the secular cooling of the Earth's core, because the temperature effects are more significant than previous empirical extrapolations. With the predicted exsolution boundaries at various temperatures, we were able to provide new constraints on how the Earth's core evolved from the previous accretion stage to the present‐day status. Key Points: Free energies of Fe‐O‐Si ternary liquids have been predicted under Earth's core conditions Temperature effects on the solubility of SiO2 in Fe‐O‐Si liquids are significant Precipitation rate of SiO2 is high and efficient to drive magnetic field's dynamo