10:45 〜 12:15
[SIT18-P01] Sulfur partitioning between light element-bearing liquid iron and molten silicate
キーワード：硫黄、核-マントル相互作用、第一原理計算
It is known that the Earth’s core is 5~10% less dense than pure iron [1]. Since the sulfur in the mantle is depleted compared to the cosmic abundance [2], sulfur is thought to be one of the most promising elements to explain the density deficit of the core. To determine whether sulfur was absorbed into the Earth’s core through the core-mantle interactions, it is important to understand how much siderophile S is under high pressure and temperature. However, there are large discrepancies in the experimental results of the iron-silicate partitioning of S. Recently, experiments conducted at 45~65 GPa using a diamond anvil cell (DAC) [3] have reported less siderophile sulfur, denying the experimental results conducted at lower pressures (< 25 GPa) using a multi anvil (MA) and piston cylinders (PC) [4,5] and suggesting that sulfur cannot be a major light element in the Earth’s core. The experimental results are currently controversial and siderophility of sulfur at high pressure and temperature remains unclear. Therefore, we performed a theoretical verification.
In this study, we perform ab initio free energy simulations based on the thermodynamic integration method [6-8] to predict the partitioning behavior between liquid iron and molten silicate at high pressure and temperature (20~135 GPa, 4000~5000 K). The liquid states are reproduced by the constant-temperature ab initio molecular dynamics method based on the density functional theory [9,10]. We determine reaction free energies and partitioning coefficients for the equilibrium partitioning reactions of S [3,5], FeOsilicate + 1/2S2 ↔ FeSmetal + 1/2O2, which are often assumed in experimental studies.
The obtained partitioning coefficients of S range from logDS=1.5~3.5 at 20~135 GPa and 4000~5000 K. Being consistent with the previous experiments, the pressure dependence is found to be positive, while the temperature dependence is negative. DS is found to decrease with increasing the O content in the Fe liquid, while it does not change significantly with increasing the Fe content in the molten silicate. Compared to the previous studies, the siderophility of S is comparable or higher than that reported by the MA&PC experiments. However, the pressure dependence is relatively smaller, which is more similar to the DAC results. The temperature dependence is on the other hand intermediate between MA&PC and DAC. The decrease in the siderophility of S due to the incorporation of O in liquid iron might partially explain the discrepancy between the results of MA&PC and DAC. Other light elements such as Si, H, and C might also influence the S partitioning. The effects of other light elements are now being calculated to model the chemical evolutions of the mantle and core.
[1] F. Birch: J. Geophys. Res., 57, 227 (1952).
[2] J.P. Lorand, L. Ambre, and A. Olivier: Lithos 164, 2 (2013).
[3] T.A. Suer, et al.: Earth Planet. Sci. Lett., 469, 84 (2017).
[4] J. Li, and C.B. Agee: Geophys. Res. Lett., 28, 81 (2001).
[5] L. Rose-Weston, et al.: Geochim. Cosmochim. Acta, 73, 4598 (2009).
[6] T. Taniuchi and T. Tsuchiya: J. Phys.: Cond. Matt., 30, 114003 (2018).
[7] Z. Xiong, T. Tsuchiya, and T. Taniuchi: J. Geophys. Res., 123, 6451 (2018).
[8] Z. Xiong, T. Tsuchiya, and J.A. Van Orman: Geophys. Res. Lett., 48, e2020GL090769 (2021).
[9] P. Hohenberg and W. Kohn: Phys. Rev., 136, B864 (1964).
[10] W. Kohn and L.J. Sham: Phys. Rev., 140, A1133 (1965).
In this study, we perform ab initio free energy simulations based on the thermodynamic integration method [6-8] to predict the partitioning behavior between liquid iron and molten silicate at high pressure and temperature (20~135 GPa, 4000~5000 K). The liquid states are reproduced by the constant-temperature ab initio molecular dynamics method based on the density functional theory [9,10]. We determine reaction free energies and partitioning coefficients for the equilibrium partitioning reactions of S [3,5], FeOsilicate + 1/2S2 ↔ FeSmetal + 1/2O2, which are often assumed in experimental studies.
The obtained partitioning coefficients of S range from logDS=1.5~3.5 at 20~135 GPa and 4000~5000 K. Being consistent with the previous experiments, the pressure dependence is found to be positive, while the temperature dependence is negative. DS is found to decrease with increasing the O content in the Fe liquid, while it does not change significantly with increasing the Fe content in the molten silicate. Compared to the previous studies, the siderophility of S is comparable or higher than that reported by the MA&PC experiments. However, the pressure dependence is relatively smaller, which is more similar to the DAC results. The temperature dependence is on the other hand intermediate between MA&PC and DAC. The decrease in the siderophility of S due to the incorporation of O in liquid iron might partially explain the discrepancy between the results of MA&PC and DAC. Other light elements such as Si, H, and C might also influence the S partitioning. The effects of other light elements are now being calculated to model the chemical evolutions of the mantle and core.
[1] F. Birch: J. Geophys. Res., 57, 227 (1952).
[2] J.P. Lorand, L. Ambre, and A. Olivier: Lithos 164, 2 (2013).
[3] T.A. Suer, et al.: Earth Planet. Sci. Lett., 469, 84 (2017).
[4] J. Li, and C.B. Agee: Geophys. Res. Lett., 28, 81 (2001).
[5] L. Rose-Weston, et al.: Geochim. Cosmochim. Acta, 73, 4598 (2009).
[6] T. Taniuchi and T. Tsuchiya: J. Phys.: Cond. Matt., 30, 114003 (2018).
[7] Z. Xiong, T. Tsuchiya, and T. Taniuchi: J. Geophys. Res., 123, 6451 (2018).
[8] Z. Xiong, T. Tsuchiya, and J.A. Van Orman: Geophys. Res. Lett., 48, e2020GL090769 (2021).
[9] P. Hohenberg and W. Kohn: Phys. Rev., 136, B864 (1964).
[10] W. Kohn and L.J. Sham: Phys. Rev., 140, A1133 (1965).