[S-IT14] Deep Earth Sciences

[SIT14-05] Ab Initio Calculations On The Mechanism Of The Iron Charge Disproportionation Reaction In Silicate Melts

*Taku Tsuchiya¹, Kazushi Kitaguchi¹ (1.Geodynamics Research Center, Ehime University)

Keywords:Ab initio free energy calculations, Iron charge disproportionation reaction, Magma ocean

It is widely believed that during the growth of the proto-Earth, accretion of building blocks caused a large-scale melting and the formation of a magma ocean (MO) and the metallic core formed inside the MO through the segregation of liquid iron. The microscopic mechanisms of this process are however yet to be fully revealed so far. Recently, high-pressure experiments showed that the charge disproportionation (CD) reaction of ferous iron, 2Fe²⁺ = Fe³⁺ +Fe⁰, could occur in silicate melts under high pressure and temperature with producing metallic iron (Armstrong et al., 2019; Kuwahara et al., 2023), suggesting that the iron CD could be one of the mechanism of core formation in the MO. Theoretical calculations also reported that the ferric iron content increases with increasing the size of MO. The experiments however indicate an unrealistic strong pressure dependence, which implies almost all ferous iron in the MO changes to ferric at 50~60 GPa, and the computational analyses are obtained based imply on indirect analyses on the volume contrast between ferous and ferric iron in silicate melts without considering the iron CD reaction (Deng et al., 2020). Since the mechanisms of the iron CD in the MO remain largely unclear, we have performed free energy calculations of iron bearing silicate melts using the thermodynamic integration (TI) molecular dynamics (MD) method (Taniuchi and Tsuchiya, 2018) and investigated the stability of the iron CD reaction with changing pressure, temperature, and the composition of silicate melts. Results show that its stability may depend strongly on the temperature and iron concentration. The microscopic mechanisms of the reaction will be discussed.