4:15 PM - 4:30 PM
[SIT22-22] Ab initio prediction of potassium partitioning into the Earth's core
Keywords:Ab initio, Potassium, Earth's core
Ab-initio free energy simulations based on molecular dynamics conbined with thermodynamics integration[Taniuchi, 2014] are performed to investigate whether and how much potassium can enter the metal system. Potassium partition coefficient(Dk=Kwt%metal/Kwt%silicate) is determined as a function of pressure, temperature and composition by calculating the Gibbs free energy changes of its exchange reactions in different conditions. Helmholtz free energy is estimated with “thermodynamic integration” by computing the difference between two systems with different potential energy functions[Kirkwood, JCP, 1990].
Calculations performed from 3000 K to 5000 K suggest that temperature has no distinct effect in potassium incorporation into Fe-alloys. Results of Dk obtained from 20 GPa to 135 GPa at constant temperature and composition reveal that potassium partitioning behavior has a negligible pressure dependence. Besides, the potassium partial density of states (pDOS) shows its electronic structure remains to be alkaline metallic even at 135 GPa. Simulations show a limited effect of Al concentration in silicate composition to potassium solubility into metal system.
Influences of the light elements (O and S) proposed to be responsible for the density deficits of the core to potassium partitioning are also investigated in this study. Potassium solubility seems unchanged when the S content of the metal system increases. Simulations with oxygen free metal composition suggest that potassium will completely sequester into silicate system. However, with the presence of oxygen in metal, potassium will start its incorporation into metal system. Our results suggest that effects of temperature, pressure, silicate composition and S content are insignificant, while oxygen controls potassium partitioning between silicate and metal system.