11:00 〜 13:00
[SIT21-P04] Effects of the core mantle interaction on the sulfur partitioning in the deep Earth
キーワード：硫黄、核-マントル相互作用、第一原理計算
1. Introduction
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, sulfur is 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 sulfur is at high pressure and temperature. However, there are large discrepancies in the experimental results of the iron-silicate partitioning of sulfur. Recently, experiments conducted at higher pressures (45~65 GPa) using a diamond anvil cell (DAC) [2] have reported less siderophile sulfur, denying the experimental results conducted at lower pressures (< 25 GPa) using a multi anvil etc. (MA etc.) [3] and suggesting that sulfur is not a major light element in the Earth's core. The experimental results are currently controversial and it is unclear the siderophility of sulfur at high pressure and temperature, and therefore we performed a theoretical verification.
2. Calculation methods and conditions
In this study, we perform first-principles free energy simulations based on the thermodynamic integration methods [4-6]. The equilibrium partitioning reaction free energies are calculated, and then the sulfur distribution between liquid iron and silicate melt at high pressure and temperature (60~135 GPa and 4000~5000 K) are predicted. We consider not only simple sulfur exchange and redox reactions but also more realistic reactions constructed according to experimental studies, in which sulfur is assumed to be bonded to iron in silicate melt and distributed to the metallic phase. The liquid states are simulated using the first-principles molecular dynamics methods based on the density functional theory [7,8]. In the thermodynamic integration methods, linear combinations of the potential energy of the reference system, whose free energy can be calculated analytically, and that of the first-principles system are generated, and then the Helmholtz free energy difference between the reference and first-principles systems is obtained by integrating them numerically. In this study, the ideal gas is applied to the reference system, and the second-order quadrature is used for numerical integrations.
3. Results and discussion
The present study shows that sulfur is 100~10000 times more partitioned to iron liquid at 60~135 GPa and 4000~5000 K (Fig.1). This result is intermediate between the experimental result of MA etc. and DAC. The high siderophility of sulfur obtained in this study suggests that sulfur is almost exclusively distributed to the core during the separation of the core and mantle in the magma ocean, and it can be considered that the sulfur concentration in the Earth's core is about three times higher than the average in the Earth's primitive materials. This means that if the building blocks of the Earth are C- or E-type chondrites, as generally assumed, sulfur could be a major light element in the core.
References
[1] F. Birch: Journal of Geophysical Research, 57, 227 (1952).
[2] T. A. Suer, et al. : Earth and Planetary Science Letters, 469, 84 (2017).
[3] L. Rose-Weston, et al. : Geochimica et Cosmochimica Acta, 73, 4598 (2009).
[4] T. Taniuchi, and T. Tsuchiya: Journal of Physics: Condensed Matter, 30, 114003 (2018).
[5] Z. Xiong, T. Tsuchiya, and T. Taniuchi: Journal of Geophysical Research: Solid Earth, 123, 6451 (2018).
[6] Z. Xiong, T. Tsuchiya, and J. A. Van Orman: Geophysical Research Letters, 48, e2020GL090769 (2021).
[7] P. Hohenberg, and W. Kohn: Physical Review, 136, B864 (1964).
[8] W. Kohn, and L. J. Sham: Physical Review, 140, A1133 (1965).
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, sulfur is 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 sulfur is at high pressure and temperature. However, there are large discrepancies in the experimental results of the iron-silicate partitioning of sulfur. Recently, experiments conducted at higher pressures (45~65 GPa) using a diamond anvil cell (DAC) [2] have reported less siderophile sulfur, denying the experimental results conducted at lower pressures (< 25 GPa) using a multi anvil etc. (MA etc.) [3] and suggesting that sulfur is not a major light element in the Earth's core. The experimental results are currently controversial and it is unclear the siderophility of sulfur at high pressure and temperature, and therefore we performed a theoretical verification.
2. Calculation methods and conditions
In this study, we perform first-principles free energy simulations based on the thermodynamic integration methods [4-6]. The equilibrium partitioning reaction free energies are calculated, and then the sulfur distribution between liquid iron and silicate melt at high pressure and temperature (60~135 GPa and 4000~5000 K) are predicted. We consider not only simple sulfur exchange and redox reactions but also more realistic reactions constructed according to experimental studies, in which sulfur is assumed to be bonded to iron in silicate melt and distributed to the metallic phase. The liquid states are simulated using the first-principles molecular dynamics methods based on the density functional theory [7,8]. In the thermodynamic integration methods, linear combinations of the potential energy of the reference system, whose free energy can be calculated analytically, and that of the first-principles system are generated, and then the Helmholtz free energy difference between the reference and first-principles systems is obtained by integrating them numerically. In this study, the ideal gas is applied to the reference system, and the second-order quadrature is used for numerical integrations.
3. Results and discussion
The present study shows that sulfur is 100~10000 times more partitioned to iron liquid at 60~135 GPa and 4000~5000 K (Fig.1). This result is intermediate between the experimental result of MA etc. and DAC. The high siderophility of sulfur obtained in this study suggests that sulfur is almost exclusively distributed to the core during the separation of the core and mantle in the magma ocean, and it can be considered that the sulfur concentration in the Earth's core is about three times higher than the average in the Earth's primitive materials. This means that if the building blocks of the Earth are C- or E-type chondrites, as generally assumed, sulfur could be a major light element in the core.
References
[1] F. Birch: Journal of Geophysical Research, 57, 227 (1952).
[2] T. A. Suer, et al. : Earth and Planetary Science Letters, 469, 84 (2017).
[3] L. Rose-Weston, et al. : Geochimica et Cosmochimica Acta, 73, 4598 (2009).
[4] T. Taniuchi, and T. Tsuchiya: Journal of Physics: Condensed Matter, 30, 114003 (2018).
[5] Z. Xiong, T. Tsuchiya, and T. Taniuchi: Journal of Geophysical Research: Solid Earth, 123, 6451 (2018).
[6] Z. Xiong, T. Tsuchiya, and J. A. Van Orman: Geophysical Research Letters, 48, e2020GL090769 (2021).
[7] P. Hohenberg, and W. Kohn: Physical Review, 136, B864 (1964).
[8] W. Kohn, and L. J. Sham: Physical Review, 140, A1133 (1965).