2:45 PM - 3:00 PM
[SIT18-04] The effects of O and Si incorporation to liquid iron on the high-P,T metal silicate partition of S obtained based on first-principles calculations
Keywords:Sulfur partitioning, First-principles calculations, Oxygen, Silicon, Local structure, Liquid
S is known to be depleted in the Earth’s mantle compared to its cosmic abundance [e.g., 1] and is one of the light elements promising for explaining the density deficit in the core [e.g., 2]. Experimental results of S partition between iron and silicate suggest that the siderophility of S decreases when O and Si are contained in liquid iron, and that they are mutually exclusive [3-5]. However, the microscopic mechanisms of such behavior have not yet been investigated. In this study, we reproduce the partitioning of S between liquid iron containing O and Si and molten silicate theoretically and investigate the chemical features of S, O, and Si atoms in liquid iron and their interactions under high-P,T (20-135 GPa, 3000-5000 K).
We perform S partitioning calculations using first-principles free energy simulation based on the thermodynamic integration method [e.g., 6]. The liquid states are simulated based on density functional molecular dynamics simulation [7,8]. Several different compositions with 6 combinations of Fe-light element binary liquids with varying O or Si concentrations of 0, 10, and 20 mol% iron and a ternary composition with 10 mol% of both O and Si. We then analyze the mechanisms of S partitioning behavior and S-O and S-Si interactions from the local structures and electronic structures of these liquids.
Under all P,T conditions considered in this study, the partition coefficient of S (DS) decreases with increasing the O content in liquid iron. In addition, it is confirmed that Si in liquid iron also tends to decrease DS. The results of liquid iron with both O and Si are comparable to the sum of the effects for O or Si alone. Therefore, as the experimental results, S-O and S-Si are found to be repulsive under high-P,T.
Local structure analyses based on pair radial distribution function show almost no short-range correlation between the S-O or S-Si pairs in liquid iron, indicating no mutual attractions. From the analyses of the electronic structure, a bonding-like interaction is found between Si-3p and Fe-3d orbitals in liquid iron, and this is thought to indirectly reduce the siderophility of S. In contrast, no such interaction is found between O and Fe, and the reduction in the volume of liquid iron accompanying the increase in the O content is rather thought to be the cause of the decrease in DS. In addition, when both O and Si are contained in the liquid iron, these effects work independently and then additively. These findings clarify that the exclusive behaviors between the S-O and S-Si pairs are due to different mechanisms. Nevertheless, since S likely has the strongest siderophility of the three elements, it might be difficult to assume an outer core composition that does not contain S.
[1] J.P. Lorand, L. Ambre, and A. Olivier: Lithos 164, 2 (2013). [2] F. Birch: J. Geophys. Res., 57, 227 (1952). [3] M.R. Kilburn, and B.J. Wood.: Earth and Planet. Sci. Lett., 152, 139 (1997). [4] L. Rose-Weston, et al.: Geochim. Cosmochim. Acta, 73, 4598 (2009). [5] T.A. Suer, et al.: Earth Planet. Sci. Lett., 469, 84 (2017). [6] T. Taniuchi and T. Tsuchiya: J. Phys.: Cond. Matt., 30, 114003 (2018). [7] P. Hohenberg, and W. Kohn: Phys. Rev., 136, B864 (1964). [8] W. Kohn, and L.J. Sham: Phys. Rev., 140, A1133 (1965).
We perform S partitioning calculations using first-principles free energy simulation based on the thermodynamic integration method [e.g., 6]. The liquid states are simulated based on density functional molecular dynamics simulation [7,8]. Several different compositions with 6 combinations of Fe-light element binary liquids with varying O or Si concentrations of 0, 10, and 20 mol% iron and a ternary composition with 10 mol% of both O and Si. We then analyze the mechanisms of S partitioning behavior and S-O and S-Si interactions from the local structures and electronic structures of these liquids.
Under all P,T conditions considered in this study, the partition coefficient of S (DS) decreases with increasing the O content in liquid iron. In addition, it is confirmed that Si in liquid iron also tends to decrease DS. The results of liquid iron with both O and Si are comparable to the sum of the effects for O or Si alone. Therefore, as the experimental results, S-O and S-Si are found to be repulsive under high-P,T.
Local structure analyses based on pair radial distribution function show almost no short-range correlation between the S-O or S-Si pairs in liquid iron, indicating no mutual attractions. From the analyses of the electronic structure, a bonding-like interaction is found between Si-3p and Fe-3d orbitals in liquid iron, and this is thought to indirectly reduce the siderophility of S. In contrast, no such interaction is found between O and Fe, and the reduction in the volume of liquid iron accompanying the increase in the O content is rather thought to be the cause of the decrease in DS. In addition, when both O and Si are contained in the liquid iron, these effects work independently and then additively. These findings clarify that the exclusive behaviors between the S-O and S-Si pairs are due to different mechanisms. Nevertheless, since S likely has the strongest siderophility of the three elements, it might be difficult to assume an outer core composition that does not contain S.
[1] J.P. Lorand, L. Ambre, and A. Olivier: Lithos 164, 2 (2013). [2] F. Birch: J. Geophys. Res., 57, 227 (1952). [3] M.R. Kilburn, and B.J. Wood.: Earth and Planet. Sci. Lett., 152, 139 (1997). [4] L. Rose-Weston, et al.: Geochim. Cosmochim. Acta, 73, 4598 (2009). [5] T.A. Suer, et al.: Earth Planet. Sci. Lett., 469, 84 (2017). [6] T. Taniuchi and T. Tsuchiya: J. Phys.: Cond. Matt., 30, 114003 (2018). [7] P. Hohenberg, and W. Kohn: Phys. Rev., 136, B864 (1964). [8] W. Kohn, and L.J. Sham: Phys. Rev., 140, A1133 (1965).