Abstract Pressure-temperature-volume (P-T-V) data on liquid iron-sulfur (Fe-S) alloys at the Earth’s outer core conditions (~136 to 330 GPa, ~4000 to 7000 K) have been obtained by first-principles molecular dynamics simulations. We developed a thermal equation of state (EoS) composed of Murnaghan and Mie-Grüneisen-Debye expressions for liquid Fe-S alloys. The density and sound velocity are calculated and compared with Preliminary Reference Earth Model (PREM) to constrain the S concentration in the outer core. Since the temperature at the inner core boundary (TICB) has not been measured precisely (4850~7100 K), we deduce that the S concentration ranges from 10~14 wt% assuming S is the only light element. Our results also show that Fe-S alloys cannot satisfy the seismological density and sound velocity simultaneously and thus S element is not the only light element. Considering the geophysical and geochemical constraints, we propose that the outer core contains no more than 3.5 wt% S, 2.5 wt% O, or 3.8 wt% Si. In addition, the developed thermal EoS can be utilized to calculate the thermal properties of liquid Fe-S alloys, which may serve as the fundamental parameters to model the Earth’s outer core.

中文翻译：

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地核条件下液态铁硫合金的密度和声速

摘要 通过第一性原理分子动力学模拟获得了在地球外核条件（~136 至 330 GPa，~4000 至 7000 K）下液态铁硫 (Fe-S) 合金的压力-温度-体积 (PTV) 数据. 我们开发了由 Murnaghan 和 Mie-Grüneisen-Debye 表达式组成的液态 Fe-S 合金热状态方程 (EoS)。计算密度和声速并与初步参考地球模型 (PREM) 进行比较，以限制外核中的 S 浓度。由于内核边界 (TICB) 的温度尚未精确测量 (4850~7100 K)，我们推断 S 浓度范围为 10~14 wt%，假设 S 是唯一的轻元素。我们的结果还表明，Fe-S 合金不能同时满足地震密度和声速，因此 S 元素不是唯一的轻元素。考虑到地球物理和地球化学的限制，我们建议外核包含不超过 3.5 wt% S、2.5 wt% O 或 3.8 wt% Si。此外，开发的热 EoS 可用于计算液态 Fe-S 合金的热性能，可作为模拟地球外核的基本参数。