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.
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Funding
The authors acknowledge support from the National Natural Science Foundation of China (NSFC) (Grant No. 11804175 and 11874033) and the K.C. Wong Magna Foundation at Ningbo University. A.B. is thankful to the Swedish Research Council (VR) for financial support (grant 2017-03744). J.F. thanks Yunguo Li from University College of London for discussions. Computations were performed using the facilities at the Swedish National Infrastructure for Computing (SNIC) located at NSC in Linköping. The data can be obtained from the Mendeley data repository (https://data.mendeley.com/datasets/b8t6j2yd3k/1).
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